Category Archives: Asteroids: Trans-Neptunian Objects

Satellites of large TNOs

Hi there! Today, we discuss about the Trans-Neptunian Objects, and more precisely objects orbiting in the Kuiper Belt, i.e. between the orbit of Neptune (~30 astronomical units) and ~50 AU (remember: the Earth orbits at 1 AU, i.e. 150 millions kilometers) for the classical Belt. Scattered objects orbit beyond that limit. During six decades, Pluto was the only known object of them, but the observation surveys with large telescope and adaptive optics has permitted the discovery of more than 100,000 of them! And among them are what we call dwarf planets, i.e. bodies which are large enough to have a pretty spherical shape.
Beside this, many Kuiper Belt objects have a companion, i.e. a satellite. When the satellite is large enough to compete with the primary body, the system is called a binary.
Today, we focus on the satellites of 2 large TNOs, named Eris and Orcus. The study I present, Medium-sized satellites of large Kuiper belt objects, by Michael E. Brown and Bryan J. Butler, tells us that their known satellites are pretty large and dark. This study has recently been published in The Astronomical Journal.

Many TNOs are not alone

Charon, i.e. the companion of Pluto, has been discovered in 1978. At that time, Pluto was the only known Trans-Neptunian Object. Since then, the discoveries of TNOs were often followed by the detection of a companion. There could be more than 20% of binary objects and multiple systems in the low-inclination populations, i.e. the objects which orbital plane is close to the mean one of the classical Kuiper Belt. This fraction seems to be much smaller (maybe 5%) for the inclined objects. But determining a frequency is a tough task, since the detection of such objects challenges the limitations of our observation facilities.

Anyway, there are many binaries among the Kuiper Belt, and this raises the question: how is that possible? How did they form? Does the companion result from an impact? Was it a single object, which have been trapped by the primary one?

Answering these questions requires to consider the properties of the two objects, i.e. their mass ratio, their distance, their composition (do they appear to be similar or not?). And this also raises other questions, related to the stability of these systems (how long can they survive as binaries)?

Two large guys: Eris and Orcus

Discoveries

Eris and Orcus were discovered in 2005 and 2004, respectively, by a team led by Michael Brown, with data from Palomar Observatory.
The discovery of a Solar System object usually happens during a systematic survey of the sky. You take several pictures of a given field of the sky. You first need to reduce them, i.e. you de-noise them to correct for the instrumental and atmospheric problems, then you make an astrometric correction in using the stars which appear on the image, to improve the reliability of the coordinates you use. Once this is done, you see small points on the images. From an image to another, most of the points are fixed. These are the stars. And sometimes, a point is slowly moving. This is a Solar System object. And if this object is not catalogued, it means you have discovered it.

Discovery of Orcus. © NASA
Discovery of Orcus. © NASA

Actually it is a little more complicated than that, you need to re-observe the object to validate the discovery and calculate orbital elements, i.e. determine its orbit. But once an object is observed, it is easier to re-observe it.

And the multiple observations of Eris and Orcus have permitted to infer some of their physical properties, and along with their orbital elements.

Interestingly, there were precoveries of Eris and Orcus. A precovery is an a posteriori identification of an object, after it is known, but on images taken before its discovery. In other words, its presence was on the images, but remained unnoticed. Precovery images have been identified back to September 3, 1954 for Eris, and to November 8, 1951 for Orcus. This means that we have observations over more than 60 years. Of course, these precoveries do not give us any clue on the physics of the object. However, they constrain its motion. If we consider the fact that its period is of the order of a few century, an observation arc of 60 years is highly valuable for determining its orbit.

Properties

You can find below some of their properties.

Eris Orcus
Discovery 2005 2004
Semimajor axis 67.781 AU 39.398 AU
Eccentricity 0.44 0.22
Inclination 44° 20.6°
Orbital period 558.04 yr 247.29 yr
Diameter 2326±12 km 910±50 km
Albedo 0.96±0.04 0.23±0.02
Apparent magnitude 18.7 19.1
Satellite Dysnomia Vanth

And it appears that these two objects are indeed very different. Orcus is a plutino, i.e. its orbit is close to the one of Pluto. It is in a 3:2 mean-motion resonance (MMR) with Neptune, i.e. it makes exactly 2 orbital revolutions around the Sun while Neptune makes 3. However, Eris is a scattered object. This means that its orbit does not make it a cold (i.e. unexcited) classical Kuiper Belt object, but it belongs to the objects, which have been somehow dynamically excited. As a consequence, its orbit is significantly inclined with respect to the classical Kuiper Belt, and it orbits far beyond.

Moreover, Eris is just the largest known TNO, even larger than Pluto. When it was discovered, Pluto was still called a planet. Its downgrading to a dwarf planet is the consequence of the discovery of these large TNOs, not only Eris and Orcus, but also Makemake, Haumea, Sedna…

And these two bodies appear to have at least one satellite each! Both were discovered in 2005 by teams led by Michael Brown, during observations of the main Kuiper Belt Objects. The satellite of Eris, Dysnomia, has been discovered thanks to the Keck Observatory, located on the Mauna Kea, Hawaii, while the satellite of Orcus, Vanth, was discovered thanks to the Hubble Space Telescope.

These objects appear so faint that we must use the best facilities to study them.

Observations with the Atacama Large Millimeter Array (ALMA)

The authors used data taken at the Atacama Large Millimeter Array (ALMA). This is an array made with a collection of 12m-antennae, in the Chilean Andes. They benefited from a recent upgrade of the instrument, to obtain spatial resolutions of 10s of milliarcseconds. Observations at this resolution at the frequency of 350 GHz, which is at the boundary between far infrared and sub-millimetric, permits to directly measure the thermal emission of satellites of Kuiper Belt Objects.

You can find in the video below some views of ALMA.

The authors disposed of 4 ALMA observations of the pair Orcus-Vanth, taken in October and November 2016, and 3 observations of Eris-Dysnomia, made in November and December 2015. The four observations of Orcus-Vanth gave an obvious resolution of the two bodies, while it is not that clear for Eris-Dysnomia. A combination of the 3 observations into a single image has anyway allowed a detection with a very good confidence, and using an a priori knowledge of the location of Dysnomia, due to previous studies.

The authors also supplemented their Orcus-Vanth dataset with unresolved data (i.e. on which you cannot separate the two objects) due to the infrared Spitzer Space Telescope and the Herschel Space Observatory.

Once they have these data, they should invert them to extract physical parameters. And this is not easy.

From the observations to the physical parameters

The difficulty comes from the accuracy of the observations. Remember that each of them is indeed a challenge.

If all of the observations had a perfect accuracy, you would just need a few images to get the position of the velocity of a planetary body, and then its orbital elements… But, if you try to do that on two different datasets related to the same object, you would get different numbers! And the reason is in the accuracy of the observations. Just an example: in using an orbital solution for Vanth resulting from a previous study, the authors got a difference of 11° in the longitude, i.e. Vanth is 11° in advance on its orbit on these observations, with respect to the predictions, which are derived from previous observations. This should give you an idea of the difficulty of the task.

And the authors should find a best fit between the models and the observations. They model Eris, Dysnomia, Orcus and Vanth as spherical bodies, which have an orbital motion and a thermal emission. These things depend on parameters, and you should find the numbers for these parameters, which give the best match between observations and models.

For that, they used the Markov Chain Monte Carlo scheme. This consists in testing a collection of parameters, which are distributed following a probability law. You can find below some of the results.

Dysnomia Vanth
Diameter 700±115 km 475±75 km
Albedo 0.04±0.02 0.08 ±0.02
Orbital period 15.79 days 9.54 days

I see two major elements from these results:

  1. The satellites are pretty large,
  2. they are much darker than their parent body (Eris for Dysnomia, Orcus for Vanth).

This last element suggests that they have a different composition.

On the origin of the satellites: impact of trapping?

An elegant scenario for the creation of a double system is an impact on the proto-primary body. This impact would have excavated a significant mass, which would have then formed the secondary. This is the most popular explanation for the formation of the Moon, and this seems to work for Pluto-Charon.

But the difference in albedo between the primary and the secondary, for these two pairs, could rule out this scenario, just because the surfaces seem to be too different. This could also mean that the secondary is essentially made of material, which initially belonged to the impactor. But this enforces another scenario as well, which is the trapping of the secondary by the binary. Originally there would have been two independent bodies, which would have met, and got gravitationally bound. Why not? That would be consistent with a difference in the composition.

The study and its authors

  • You can find the study here. The authors made it freely available on arXiv, thanks to them for sharing!
  • The homepage of Michael E. Brown. He discovered several Trans-Neptunian Objects, including Eris and Orcus, and is strongly involved in the quest for the Planet Nine. You cann see his blog here.
  • and the one of Bryan J. Butler.

And that’s it for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter, Facebook, Instagram, and Pinterest.

The rings of Haumea

Hi there! I guess you have heard, last year, of the discovery of rings around the Trans-Neptunian Object Haumea. If not, don’t worry, I speak about it. Rings around planets are known since the discovery of Saturn (in fact a little later, since we needed to understand that these were rings), and now we know that there are rings around the 4 giant planets, and some small objects, which orbit beyond Saturn.

Once such a ring is discovered, we should wonder about its origin, its lifetime, its properties… This is the opportunity for me to present a Hungarian study, Dynamics of Haumea’s dust ring, by T. Kovács and Zs. Regály. This study has recently been accepted for publication in The Monthly Notices of the Royal Astronomical Society.

The Trans-Neptunian Object (136108)Haumea

Discovery

The discovery of (136108) Haumea was announced in July 2005 by a Spanish team, led by José Luis Ortiz, observing from Sierra Nevada Observatory (Spain). This discovery was made after analysis of observations taken in March 2003. As a consequence, this new object received the provisional name 2003 EL61.

But meanwhile, this object was observed since several months by the American team of Michael Brown, from Cerro Tololo Inter-American Observatory, in Chile, who also observed Eris. This led to a controversy. Eventually, the Minor Planet Center, which depends on the International Astronomical Union, credited Ortiz’s team for the discovery of the object, since they were the first to announce it. However its final name, Haumea, has been proposed by the American team, while usually the final name is chosen by the discoverer. Haumea is the goddess of fertility and childbirth in Hawaiian mythology. The Spanish team wished to name it Ataecina, after a popular goddess worshipped by the ancient inhabitants of the Iberian Peninsula.

Reanalysis of past observations revealed the presence of Haumea on photographic plates taken in 1955 at Palomar Observatory (we call that a precovery).

Properties

You can find below some numbers regarding Haumea.

Semi-major axis 43.218 AU
Eccentricity 0.191
Inclination 28.19°
Orbital period 284.12 yr
Spin period 3.92 h
Dimensions 2,322 × 1,704 × 1,138 km
Apparent magnitude 17.3

As a massive Trans-Neptunian Object, i.e. massive enough to have a pretty spherical shape, it is classified as an ice dwarf, or plutoid. This shape is pretty regular, but not that spherical actually. As you can see from its 3 diameters (here I give the most recent numbers), this is a triaxial object, with a pretty elongated shape… and this will be important for the study.

It orbits in the 7:12 mean-motion resonance with Neptune, i.e. it performs exactly 7 revolutions around the Sun while Neptune makes 12. This is a 5th order resonance, i.e. a pretty weak one, but which anyway permits some stability of the objects, which are trapped inside. This is why we can find some!

We can also see that it has a rapid rotation (less then 4 hours!). Moreover, it is pretty bright, with a geometrical albedo close to 0.8. This probably reveals water ice at its surface.

And Haumea has two satellites, and even rings!

Two satellites, and rings

Haumea has two known satellites, Namaka and Hi’iaka, named after two daughters of the goddess Haumea. They were discovered by the team of Michael Brown in 2005, simultaneously with its observations of Haumea, i.e. before the announcement of its discovery. You can find below some of their characteristics.

Namaka Hi’iaka
Semi-major axis 25657 km 49880 km
Eccentricity 0.25 0.05
Orbital period 18.28 d 49.46 d
Mean diameter 170 km 310 km
Keck image of Haumea and its moons. Hi'iaka is above Haumea (center), and Namaka is directly below. © Californian Institute of Technology
Keck image of Haumea and its moons. Hi’iaka is above Haumea (center), and Namaka is directly below. © Californian Institute of Technology

Usually such systems are expected to present spin-orbit resonances, e.g. like our Moon which rotates synchronously with the Earth. Another example is Pluto-Charon, which is doubly synchronous: Pluto and Charon have the same spin (rotational) period, which is also the orbital period of Charon around Pluto. Here, we see nothing alike. The rotational period of Haumea is 4 hours, while its satellites orbit much slower. We do not dispose of enough data to determine their rotation periods, maybe they are synchronous, i.e. with spin periods of 18.28 and 49.46 days, respectively… maybe they are not.

This synchronous state is reached after tidal dissipation slowed the rotation enough. Future measurements of the rotation of the two satellites could tell us something on the age of this ternary system.

And last year, an international team led by José Luis Ortiz (the same one) announced the discovery of a ring around Haumea.

Rings beyond Jupiter

In the Solar System, rings are known from the orbit of Jupiter, and beyond:

  • Jupiter has a system of faint rings,
  • should I introduce the rings of Saturn?
  • Uranus has faint rings, which were discovered in 1977,
  • the rings of Neptune were discovered in 1984, before being imaged by Voyager 2 in 1989. Interestingly, one of these rings, the Adams ring, contains arcs, i.e. zones in which the ring is denser. These arcs seem to be very stable, and this stability is not fully understood by now.
Arcs in the Adams ring (left to right: Fraternité, Égalité, Liberté), plus the Le Verrier ring on the inside. © NASA
Arcs in the Adams ring (left to right: Fraternité, Égalité, Liberté), plus the Le Verrier ring on the inside. © NASA

Surprisingly, we know since 2014 that small bodies beyond the orbit of Jupiter may have rings:

  • An international team detected rings around the Centaur Chariklo in 2014 (remember: a Centaur is a body, which orbits between the orbits of Jupiter and Neptune),
  • another team (with some overlaps with the previous one), discovered rings around Haumea in 2017,
  • observations in 2015 are consistent with ring material around the Centaur Chiron, but the results are not that conclusive.

These last discoveries were made thanks to stellar occultations: the object should occult a star, then several teams observe it from several locations. While the planetary object is too faint to be observed from Earth with classical telescopes, the stars can be observed. If at some point no light from the star is being recorded while the sky is clear, this means that it is occulted. And the spatial and temporal distributions of the recorded occultations give clues on the shape of the body, and even on the rings when present.

Why rings around dwarf planets?

Rings around giant planets orbit inside the Roche limit. Below this limit, a planetary object cannot accrete, because the intense gravitational field of the giant planet nearby would induce too much tidal stress, and prevent the accretion. But how can we understand rings around dwarf planets? Chiron presents some cometary activity, so the rings, if they exist, could be constituted of this ejected material. But understanding the behavior of dust around such a small object is challenging (partly because it is a new challenge).

In 2015, the American planetologist Matthew Hedman noticed that dense planetary rings had been only found between 8 and 20 AU, and proposed that the temperature of water ice in that area, which is close to 70 K (-203°C, -333°F), made it very weak and likely to produce rings. In other words, rings would be favored by the properties of the material. I find this explanation particularly interesting, since no ring system has been discovered in the Asteroid Main Belt. That paper was published before the discovery of rings around Haumea, which is far below the limit of 20 UA. I wonder how the Haumea case would affect these theoretical results.

In the specific case of Haumea, the ring has a width of 70 kilometers and a radius of about 2,287 kilometers, which makes it close to the 3:1 ground-track resonance, i.e. the particles constituting the ring make one revolution around Haumea, while Haumea makes 3 rotations.

Numerical simulations

Let us now focus of our study. The authors aimed at understanding the dynamics and stability of the discovered rings around Haumea. For that, they took different particles, initially on circular orbits around Haumea, at different distances, and propagated their motions.
Propagating their motions consists in using a numerical integrator, which simulates the motion in the future. There are powerful numerical tools which perform this task reliably and efficiently. These tools are classified following their algorithm and order. The order is the magnitude of the approximation, which is made at each timestep. A high order means a highly accurate simulation. Here, the authors used a fourth order Runge-Kutta scheme. It is not uncommon to see higher-order tools (orders between 8 and 15) in such studies. The motions are propagated over 1 to 1,000 years.

A gravitational and thermal physical model

The authors assumed the particles to be affected by

  • the gravitational field of Haumea, including its triaxiality. This is particularly critical to consider the ground-track resonances, while the actually observed ring is close to the 3:1 resonance,
  • the gravitational perturbation by the two small moons, Namaka and Hi’iaka,
  • the Solar radiation pressure.

This last force is not a gravitational, but a thermal one. It is due to an exchange of angular momentum between the particle, and the electromagnetic field, which is due to the Solar radiation. For a given particle size, the Solar radiation pressure has pretty the same magnitude for all of the particles, while the gravitational field of Haumea decreases with the distance. As a consequence, the furthest particles are the most sensitive to the radiation pressure. Moreover, this influence is inversely proportional to the grain size, i.e. small particles are more affected than the large ones.

And now, the results!

A probable excess of small particles

The numerical simulations show that the smaller the grains size, the narrower the final ring structure. The reason is that smaller particles will be ejected by the radiation pressure, unless they are close enough to Haumea, where its gravity field dominates.

And this is where you should compare the simulations with the observations. The observations tell you that the ring system of Haumea is narrow, this would be consistent with an excess of particles with grain size of approximately 1 μm.

So, such a study may constrain the composition of the rings, and may help us to understand its origin. Another explanation could be that there was originally no particle that far, but in that case you should explain why. Let us say that we have an argument for a ring essentially made of small particles.

The study and its authors

And that’s it for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter, Facebook, Instagram, and Pinterest.

Locating Ultima Thule

Hi there! First I have to explain the title. Ultima Thule usually stands for any place beyond the known world… Actually, we will not leave the Solar System. Ultima Thule is an unofficial nickname for (486958) 2014 MU69, which is the next target of New Horizons spacecraft. Do you remember this spacecraft, which gave us outstanding images of Pluto and Charon in July 2015? That’s just the same one!
After having left Pluto, New Horizons changed its trajectory to 2014 MU69, which will be reached on January 1st, 2019. 6 months to wait then.
Interestingly, 2014 MU69 was unknown when New Horizons was launched in January 2006. Its primary mission was the binary Pluto-Charon and its satellites, and of course it was worth to extend the mission to another body. But choosing this second target was a difficult task, since the distant Solar System, here the Kuiper Belt, is very difficult to observe, and is pretty sparse. This is why observations programs of the Hubble Space Telescope (HST) were dedicated, and 2014 MU69 has been discovered in 2014.
Discovering an object is one thing, determining accurately its motion in view of a rendezvous with a spacecraft is another thing. This is the topic of the study I present today, High-precision orbit fitting and uncertainty analysis of (486958) 2014 MU69, by Simon Porter et al. This study has recently been published in The Astronomical Journal.

The New Horizons spacecraft

New Horizons is the first mission of NASA’s New Frontier program. It was launched in January 2006, and made its closest approach to Pluto in July 2015. Before that, it incidentally encountered the small asteroid (132524) APL at a distance of about 100,000 km in June 2006, and benefited from the gravitational assistance of Jupiter in February 2007.

The asteroid 132524 APL seen by New Horizons. © NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
The asteroid 132524 APL seen by New Horizons. © NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

It carries seven science instruments:

  • the Long-Range Reconnaissance Imager (LORRI), which images the encountered bodies,
  • the Solar Wind At Pluto (SWAP) instrument, which name is very explicit regarding its goal,
  • the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI), which supplements SWAP for the detection of high-energy particles,
  • Alice, which is an ultraviolet imaging spectrometer,
  • the Ralph telescope, which is a photographic instrument,
  • the Venetia Burney Student Dust Counter (VBSDC) measures dust. This instrument has been built by students of the University of Colorado,
  • and the Radioscience Experiment (REX), which measured the temperature and the atmospheric pressure of Pluto.

As you can see from some of the names of the instrument, Pluto-Charon was definitely the primary goal of New Horizons. Anyway, Pluto is now behind, and New Horizons is en route to 2014 MU69, also nicknamed Ultima Thule.

Ultima Thule

At this time, our knowledge of Ultima Thule is very limited. This body has been discovered in 2014, from a dedicated observation program on the Hubble Space Telescope, to identify potential targets for New Horizons. Finally, 2014 MU69 has been selected, partly for technical reasons, i.e. it is not so difficult to reach from Pluto.

It was discovered in June 2014, and has an apparent magnitude of nearly 27, and an absolute one of 11. We can guess its size from its magnitude, and its diameter should be smaller than 50 km. So, a very small body. A stellar occultation happening in 2017 has revealed that its diameter should be closer to 25-30 km, and its shape may be bilobal, or it could even be a contact binary.

Observations of its dynamics revealed that it is a cold, classical Kuiper Belt object. Its eccentricity and inclination are limited, since they are not excited by any resonance with the giant planets. So, it belongs to a region of the Solar System, which is quiet from a dynamical point of view.

As I previously said, discovering it is not enough if you want a spacecraft to reach it. You must know its motion accurately, and for that you need more data. And Ultima Thule can be observed only with the Hubble Space Telescope.

Hubble Space Telescope data

The authors disposed of 5 observations of 2014 MU69, by the Wide Field Camera 3 (WFC3) of the HST. Even with the HST, imaging 2014 MU69 requires 6 minutes of exposure, i.e. you need to accumulate photons reflected by 2014 MU69 during 6 minutes to have enough signal.

The detection of 2014 MU<sub>69</sub>. © NASA, ESA, SwRI, JHU/APL, and the New Horizons KBO Search Team
The detection of 2014 MU<sub>69</sub>. © NASA, ESA, SwRI, JHU/APL, and the New Horizons KBO Search Team

Another difficulty comes from the number of stars in that region of the sky. This is due to the galactic latitude of 2014 MU69, which is close to 0, i.e. close to the Galactic plane. The images have to be treated, to remove their incoming light. This is not just a pixel, but a diffraction spot, which needs to be modeled to be properly removed.

Once you have these data, you can start to determine the orbit of 2014 MU69, i.e. make ephemerides.

From astrometry to orbit

When you catch a body on a 2-dimensional image of the sky, you get two coordinates. Basically, these coordinates translate into a right ascension, and a declination. And to build ephemerides of a body, you need to integrate the equation ruling the orbital motion. This equations is a second-order 3-dimensional ordinary differential equation.

The motion is ruled by the gravitational perturbation of the Sun and the major planets, and for the problem to be solved, you need initial conditions. These are a position and a velocity of 2014 MU69 at a given date, which you derive from your astrometric observations, i.e. the 5 couples (right ascension, declination).

Easy, isn’t it? No, it’s not! Because of the uncertainties on the measurements, your 10 data, i.e. 5 right ascensions and 5 declinations, do not exactly correspond to an initial condition. So, you have to make a fit, i.e. determine the initial condition, which best fits the observations.

There are many potential sources of uncertainties: the accuracy of the positioning of the HST, the accuracy of the coordinates of 2014 MU69 (remember: this is not a pixel, but a diffraction spot), the duration of the exposure… and also the location of the stars surrounding 2014 MU69 in the field of view. To make absolute astrometry, you need to know precisely the location of these stars, and you get their locations from a star catalogue. Currently, the astrometric satellite Gaia is making such a survey, with a never reached accuracy and comprehensiveness. The Gaia Data Release 2 has been released in April 2018, and gives positions and proper motions (i.e. you can now consider that the stars move from date to date) of more than 1 billion stars! The authors had the chance to use that catalogue. This resulted in predictions, which were accurate enough, to predict a stellar occultation, which has been observed from the Earth.

Predicting stellar occultations

When a Solar System body occultates a star, you can indirectly observe it. You observe the star with your telescope, and during a few seconds, the star disappears, and then reappears, because of this object, which light is too faint for you. Multiple observations of a stellar occultation give information on the motion of the object, and on its dimensions. The rings around Chariklo and Haumea have been discovered that way.

For 2014 MU69 (or Ultima Thule), an occultation has been successfully predicted. It has been observed 5 times on 2017 July 17, in Argentina, giving 5 solid-body chords. This permitted us to infer that 2014 MU69 could be bilobal, or even a contact binary.

A stable orbit

And from these astrometric data, the authors propagated the orbit of 2014 MU69 over 100 million years, in considering the uncertainties on the initial positions. The outcomes of the simulations safely state that 2014 MU69 is on a very stable orbit, with a mean semimajor axis of 44.23 astronomical units (39.48 for Pluto), and an orbital eccentricity smaller than 0.04. This results in an orbital period of 294 years, during which the distance to the Sun barely varies.

We are looking forward for the encounter in 6 months!

The study and its authors

And that’s it for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter, Facebook, Instagram, and Pinterest.

OSSOS discovered 838 Trans-Neptunian Objects

Hi there! Today I will tell you of the survey OSSOS, which I already mentioned in the past. This survey made systematic observations of the sky to detect Trans-Neptunian Objects (TNOs), between 2013 and 2017. It was indeed a success, since it tripled the number of known TNOs. Its results are presented in OSSOS. VII. 800+ Trans-Neptunian Objects — The complete Data Release, led by Michele Bannister. This study is published in The Astrophysical Journal Supplement Series.

Previous surveys

The Trans-Neptunian Objects orbit beyond the orbit of Neptune. As such, observing them is a challenge. Pluto was the only known of them from its discovery in 1930, to the discovery of (15760) Albion in 1992. We now know 1,142 Trans-Neptunian Objects, essentially due to 4 surveys. The most prolific of them is the last one, i.e. OSSOS, but a survey cannot exist without its precursors, which were

  1. Deep Ecliptic Survey (DES),
  2. Canada-France Ecliptic Plane Survey (CFEPS),
  3. Pan-STARRS1.
The Deep Ecliptic Survey (DES)

The Deep Ecliptic Survey has been operating between 1998 and 2003, using two 4-m telescopes of the National Optical Astronomy Observatory: the Mayall telescope at Kitt Peak Observatory (Arizona, USA), and the Blanco telescope at Cerro Tololo Inter-American Observatory (Chile). It discovered 382 TNOs, including some Centaurs, which actually orbit inner to the orbit of Neptune. It covered 550 square degrees with sensitivity of 22.5.

The Canada-France Ecliptic Plane Survey (CFEPS)

This survey operated between early 2003 and early 2007, at the Canada-France-Hawaii Telescope (Hawaii, USA). It covered 321 square degrees with sensitivity of 24.4, and permitted to classify 169 TNOs. By classifying, I do not mean only discover, but also know their orbits with enough accuracy to determine to which dynamical group they belong. I will go back on this point later, but my meaning is that observing an object once is definitely not enough. This survey was limited to the detection of objects with a small inclination with respect to the ecliptic plane, i.e. the orbit of the Earth.

It was then extended by the High Ecliptic Latitude (HiLat) component, which looked for objects with significant inclinations. It examined 701 square degrees of sky ranging from 12° to 85° ecliptic latitude and discovered 24 TNOs, with inclinations between 15° and 104° (from Petit et al., 2017, The Canada-France Ecliptic Plane Survey (CFEPS) — High-latitude component, The Astronomical Journal, 153:5.

The Pan-STARRS1 survey

The Panoramic Survey Telescope and Rapid Response System (Pan-STARRS1) survey operates from Haleakala Observatory, Hawaii, USA since 2010. It is not specifically devoted for TNOs, but for moving objects (asteroids, stars,…), and is particularly known for the discovery of the first known interstellar object, i.e. 1I/’Oumuamua. It discovered 370 new TNOs, but without enough information to securely classify their orbits.

And now comes OSSOS!

The Outer Solar System Origins Survey (OSSOS)

OSSOS operated between 2013 and 2017 from the Canada-France-Hawaii Telescope, taking more than 8,000 images. It covered 155 square degrees with a sensitivity up to 25.2. This coverage has been split into 8 blocks, which avoided the Galactic plane. The study I present today is the complete data release, in which 838 objects are given without ambiguity on their orbital classification. This was an international collaboration, involving Canada, UK, France, Taiwan, USA, Finland, Japan, Slovakia,… but also involving different skills, like orbital characterization, astrometry, chemistry, cometary activity, data mining, etc. In other words, it not only aimed at discovering new objects, but also at understanding their orbital dynamics, their physics, and if possible their origin.

In the previous paragraphs I pointed out the difference between discovering an object, and classifying it following its orbit. Let us see that now.

Characterizing a new TNO

As we will see in the next paragraph, the Trans-Neptunian population is composed of different parts, following the orbits of the objects and the perturbations acting on them, i.e. the gravitational attraction of the giant planets. Classifying a newly discovered object requires some accuracy in the determination of its orbit. The following is a summary of how things work.

For an object to be discovered, it must appear on a triplet of images, which cover a timespan of about 2 hours. From it the relative motion of the object on the sky can be evaluated, which would permit to reobserve it. The new observations permit themselves to better constrain the orbit. The OSSOS team announces that an arc of observations of about 16 months is required to have enough confidence in the orbit. In many cases the arc is longer, actually the team tells us that for the 838 classified objects, astrometric measurements have been made over 2 to 5 oppositions. An opposition is the geometric alignment between the Sun, the Earth, and the object.

For an astrometric measurement to be accurate, you need to accurately know the positions of the other objects present on the image. These other objects are stars, which are referenced in astrometric catalogues. The astrometric satellite Gaia is currently performing such a survey. Its Data Release 2 has very recently (April 2018) been released, but this was too late for the present study. So, the authors used the Data Release 1, and the Pan-STARRS 1 catalogue when necessary.

In some cases, objects were lost, i.e. the authors were not able to reobserve it. This may have been due to the lack of accuracy of the orbital determination from the discovery arc, or just because the object left a covered zone.

Before giving you the results, I should tell you something on the structure of the outer Solar System. I mentioned orbital classification above, the classes are coming now.

Structure of the outer Solar System

First, we should make a distinction between resonant and non-resonant orbits.

Resonant orbits are in mean-motion resonance with a planet, which is mostly Neptune. For instance, the 2:1 resonance with Neptune means that Neptune accomplishes two revolutions around the Sun while the object makes exactly one. Such a ratio implies amplified dynamical effects on the object, which may excite its eccentricity or its inclination, destabilize or confine its orbit.

Besides these resonant objects are the non-resonant ones (you guessed it, didn’t you?). They are classified following their orbital elements:

  • Centaurs: they orbit inner to the orbit of Neptune, i.e. their semimajor axis is smaller than 30 AU. As such, they are not TNOs strictly speaking,
  • Inner-belt objects: here the belt is the Kuiper Belt, not to be confused with the Main Asteroid Belt between Mars and Jupiter. This objects orbit between the orbit of Neptune and the 3:2 resonance, i.e. the orbit of Pluto, at 39.4 AU.
  • Main-belt objects: between the 3:2 and the 2:1 resonance, i.e. between 39.4 and 47.7 AU.
  • Outer-belt objects: they orbit beyond the 2:1 resonance and have an eccentricity smaller than 0.24.
  • Detached objects: not only they orbit beyond the 2:1 resonance, but also have an eccentricity larger than 0.24. As a consequence, they may have very large semi-major axes, but could be detected since their perihelion distance, i.e. their closest distance to the Sun, is accessible to our terrestrial instruments. This is made possible by their high eccentricity. Among these objects are the eTNOs (e for extreme) mentioned here.

And now the results.

Key results

1,142 TNOs (including Centaurs) are now classified, 838 of them thanks to OSSOS. Among these 838 objects, 313 are resonant, including 132 in the 3:2 resonance, 39 in the 7:4 and 34 in the 2:1, and 525 are non-resonant. 421 of the non-resonant object are in the main belt, i.e. between the 3:2 and the 2:1 resonances.

Among the remarkable other results are

  • There should be about 90,000 detached objects with a diameter larger than 100 km, and probably less than 1,000 so large Centaurs,
  • the inner Kuiper Belt practically starts at 37 AU,
  • the population of low-inclination objects extends to at least 49 AU, but there is a huge concentration of them between 42.5 and 44.5 AU,
  • the inclinations are larger in the 3:2 resonance (the Plutinos) than in the 2:1,
  • securely occupied resonances exist at least up to 130 AU, which is the location of the 9:1 resonance.

The word origins appear in OSSOS. Actually, knowing the distribution of the Kuiper Belt Objects tells us something on the evolution of our Solar System.

Constraining the evolution of the Solar System

A TNO is a small body. This implies that, when perturbed by a giant planet, it just endures the orbital shacking. The consequence is that the giant planets have a strong enough gravitational potential to shape the Kuiper Belt. When perturbed, an object might get inclined, eccentric, be ejected, confined…

There are several competing models of the evolution of the Solar System, which implies migration of the giant planets. When a giant planet migrates, its perturbation migrates as well, and you should see the consequences on the Kuiper Belt. This is how an accurate snapshot of the Kuiper Belt might tell us something on the past of our Solar System, and if you constrain its evolution, then you can be tempted to transpose it to extrasolar systems. Moreover, this could give clues on the Planet Nine…

The OSSOS team provides software, which include a survey simulator, checking the relevance of a predicted model for the Kuiper Belt, when compared to the observations.

The study and its authors

And that’s it for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter, Facebook, Instagram, and Pinterest.

Forming Pluto’s satellites

Hi there! A team from the University of Hong Kong has recently explored a scenario of formation of the small satellites of Pluto. You know, there are 4 small bodies, named Styx, Nix, Kerberos, and Hydra, which orbit around the binary Trans-Neptunian Object Pluto-Charon. At this time, we don’t know yet how they were formed, and how they ended up at their present locations, despite the data that the spacecraft New Horizons sent us recently. The study I present you today, On the early in situ formation of Pluto’s small satellites, by Jason Man Yin Woo and Man Hoi Lee, simulates the early evolution of the Pluto-Charon system. It has recently been published in The Astronomical Journal.

The satellites of Pluto

The American Clyde W. Tombaugh discovered Pluto in 1930. He examined photographic plates taken at Lowell Observatory at Flagstaff, Arizona, USA, and detected a moving object, which thus could not be a star. The International Astronomical Union considered Pluto to be the ninth planet of the Solar System, until 2006. At that time, numerous discoveries of distant objects motivated the creation of the class of dwarf planets, Pluto being one of the largest of them.

The other American astronomer James W. Christy discovered a companion to Pluto, Charon, in June 1978. Still at Flagstaff.

The existence of far objects in our Solar System motivated the launch of the space missions New Horizons in 2006. New Horizons made a close approach of the system of Pluto in July 2015, and is currently en route to the Trans-Neptunian Object 2014MU69. The closest approach is scheduled for January, 1st 2019.

In parallel to the preparation of New Horizons, the scientific team performed observations of Pluto-Charon with the famous Hubble Space Telescope. And they discovered 4 small satellites: Nix, Hydra, Styx and Kerberos. You can find some of their characteristics below, which are due to New Horizons.

Charon Styx Nix Kerberos Hydra
Discovery 1978 2012 2005 2011 2005
Semimajor axis 17,181 km 42,656 km 48,694 km 57,783 km 64,738 km
Eccentricity 0 0.006 0 0.003 0.006
Inclination 0.8° 0.1° 0.4° 0.2°
Orbital period 6.39 d 20.16 d 24.85 d 32.17 d 38.20 d
Spin period 6.39 d 3.24 d 1.829 d 5.31 d 0.43 d
Mean diameter 1,214 km 10.5 km 39 km 12 km 42 km
Styx seen by New Horizons © NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute
Styx seen by New Horizons © NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute
Nix seen by New Horizons © NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute
Nix seen by New Horizons © NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute
Kerberos seen by New Horizons © NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute
Kerberos seen by New Horizons © NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute

Hydra seen by New Horizons © NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute
Hydra seen by New Horizons © NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute

We should compare these numbers to the ones of Pluto: a mean diameter of 2370 km, and a spin period of 6.39 d. This implies that:

  • Pluto and Charon are two large objects, with respect to the other satellites. So, Pluto-Charon should be seen as a binary TNO, and the other four objects are satellites of the binary.
  • Pluto and Charon are in a state of double synchronous spin-orbite resonance: their rotation rate is the same, and is the same that their mutual orbital motion. If you are on the surface of Pluto, facing a friend of yours on the surface of Charon, you will always face her. This is probably the most stable dynamical equilibrium, reached after dissipation of energy over the ages.

And the four small satellites orbit outside the mutual orbits of Pluto and Charon.

Proximity of Mean-Motion Resonances

We can notice that:

  • the orbital period of Styx is close to three times the one of Charon,
  • the orbital period of Nix is close to four times the one of Charon,
  • the orbital period of Kerberos is close to five times the one of Charon,
  • the orbital period of Hydra is close to six times the one of Charon.

Close to, but not exactly. This suggests the influence of mean-motion resonances of their orbital motion, i.e. the closest distance between Charon and Styx will happen every 3 orbits of Charon at the same place, so you can have a cumulative effect on the orbit. And the same thing would happen for the other objects. But this is actually not that clear whether that cumulative effect would be significant or not, and if yes, how it would affect the orbits. Previous studies suggest that it translates into a tiny zone of stability for Kerberos, provided that Nix and Hydra are not too massive.

Anyway, the authors wondered why these four satellites are currently at their present location.

Testing a scenario of formation

They addressed this question in testing the following scenario: Charon initially impacted Pluto, and the debris resulting from the impact created the four small satellites. To test this scenario, they ran long-term numerical simulations of small, test particles, perturbed by Pluto and Charon. Pluto and Charon were not in the current state, but in a presumed early one, before the establishment of the two synchronous rotations, and with and without a significant initial eccentricity for Charon. The authors simulated the orbital evolution, the system evolving over the action of gravitational mutual interactions, and tides.

The long-term evolution is ruled by tides

The tides are basically the dissipation of energy in a planetary body, due to the difference of force exerted at different points of the body. This results in stress, and is modeled as a tidal bulge, which points to the direction of the perturber. The dissipation of energy is due to the small angular shift between the orientation of the bulge and the direction of the perturber. The equilibrium configuration of Pluto-Charon, i.e. the two synchronous rotations, suggest that the binary is tidally evolved.

The authors applied tides only on Pluto and Charon, and considered two tidal models:

  1. A constant time delay between the tidal excitation and the response of the tidal bulge,
  2. A constant angular shift between the tidal bulge and the direction of the perturber.

The tidal models actually depend on the properties of the material, and the frequency of the excitation. In such a case, the frequency of the excitation depends on the two rotation rates of Pluto and Charon, and on their orbital motions. The properties of the material, in particular the rigidity and the viscosity, are ruled by the temperatures of the objects, which are not necessarily constant in space and in time, since tidal stress tend to heat the object. Here the authors did not consider a time variation of the tidal parameters.

Other models, which are probably more physically realistic but more complex, exist in the literature. Let me cite the Maxwell model, which assumes two regimes for the response of the planetary body: elastic for slow excitations, i.e. not dissipative, and dissipative for fast excitations. The limit between fast and slow is indicated by the Maxwell time, which depends on the viscosity and the rigidity of the object.

Anyway, the authors ran different numerical simulations, with the two tidal models (constant angular shift and constant time delay), with different numbers and different initial eccentricities for Charon. And in all of these simulations, they monitored the fate of independent test particles orbiting in the area.

Possible scenario, but…

The authors seem disappointed by their results. Actually, some of the particles are affected by mean-motion resonances, some other are ejected, but the simulations show that particles may end up at the current locations of Styx, Nix, Kerberos, and Hydra. However, their current locations, i.e. close to mean-motion resonances, do not appear to be preferred places for formation. This means that we still do not know why the satellites are where they currently are, and not somewhere else.

What’s next?

The next target of New Horizons is 2014MU69, which we will be the first object explored by a spacecraft, which had been launched before the object was known to us. We should expect many data.

The study and its authors

You can find here

  • The study, made freely available by the authors on arXiv, thanks to them for sharing!
  • and the homepage of Man Hoi Lee.

And that’s it for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter, Facebook, Instagram, and Pinterest.