1. Pluto, the Kuiper Belt, and Trans-Neptunian Objects

2. What about Pluto?

3. Discovery of Pluto
Calculations indicated that Neptune’s orbit was being
purturbed => another planet beyond Neptune.
Found in 1930 by Clyde Tombaugh. Too small to
account for the perturbations. They were later shown
not to exist – no need to invoke planet beyond Neptune!

4. Pluto: basic data Uranus Neptune Pluto
Semi-major axis 19.2AU
Orbital period 84 yrs
Inclination 0.8° 1.8° 17°
Axis tilt 98° 30° 120°
Eccentricity
Diameter 4.0 DEarth
Mass MEarth
Density 1.3 g/cm
Temp 55K 55K 44K

5. Orbit inclined 17º from Ecliptic, with a high eccentricity (sometimes inside Neptune's orbit)
Orbits don’t cross. When Pluto is at Neptune’s distance, it is above Neptune’s orbit.

6. HST hemispheric images Mean density: rock/ice
Spectroscopy: thin atmosphere of predominantly nitrogen, showing large pressure variations over the years. Thought to be seasonal.
Bright regions were speculated to be impact craters where ice has been exposed.
Atmos is ices evaporated from surface by sunlight. Seems to vary seasonally, as Pluto’s distance from Sun changes. May disappear in about 2015 for a long time.

7. Pluto and Charon
Angular separation here is only 0.9”
Smaller mass ratio than any planet/moon, same density.
Both objects tidally locked
Binary orbit gave precise mass estimates of both
Rare eclipses in gave diameters from timing measurements. Then could work out precise densities.
Orbital period 6.4 days. Slightly more than 90 deg inclination (retrograde spin). Charon 1/7th mass of Pluto.

8. Now five moons found Charon (New Horizons)
Pluto and Charon almost same density. Suggests a shattering collision as origin. Same collision may have produced Nix and Hydra.
Nix and Hydra found in 2005, Kerberos 2011, Styx 2012, all with Hubble
Hubble image showing Pluto, Charon, Nix and Hydra

9. New Horizons Launched January 2006, flew past in July 2015.
Goals are to understand surface composition, temperature, thin atmosphere of Pluto, study Charon, look for more satellites, examine one other Kuiper Belt object.

10. Plain of frozen nitrogen (“Sputnik Planum”), no impact craters found, age <10 million years.

11. Flyover Sputnik Planum may be impact basin, possibly 1-3 km lower than
surroundings. Ice may
be from cryo-volcanoes or
atmospheric freeze-out
At northern edge there appear to be flowing glaciers.
Flyover

12. Other areas show a range of ages, up to >4 billion years old. A long
history of geologic activity. But crater-dating uncertain because of
uncertainty in population of impactors in outer Solar System.

13. Elevation maps of
the two candidate
ice volcanoes
50 km
One of two possible ice volcanoes. If so, internal heat source is unknown!
Although nitrogen-dominated, frozen hydrocarbons give thin
atmosphere blue color. Atmosphere loss rate is low, so it’s permanent, but should still vary seasonally. May be fed by ice volcanoes.

14. Charon
Long fractures. Also may have ice volcanoes. IR spectra show ice crystals of water and ammonia. Sunlight would soon break these down if not replenished. Surface has age variations. Charon flyover.

15. Moon system Moon orbits all prograde. Only Charon is tidally locked.
Others spin rapidly, Nix retrograde. Spins are slower closer to Pluto.
Hydra spin period only 10.3 hours. Nix and Hydra spin chaotically, due to combined gravity of Pluto and Charon.
Charon’s gravity may be preventing Pluto from tidally locking the other moons.
All presumably formed by fragmenting collision with Pluto? Although why not more moons? Speculation that some merged.
Moons’ orbit animation. Nix chaotic spin animation

16. What is Pluto…?
Pluto and Charon are similar to Triton in density (mixtures of rock and ice)
Characteristics of objects formed in the outer solar system.
The discovery of more, similar objects beginning in early 1990’s means that Pluto is just one of the “Trans-Neptunian Objects – icy bodies orbiting beyond Neptune. The “Kuiper Belt” is a zone from 30 to 50 AU containing great majority of TNOs.
Orbit inclinations can be typically 20°.

17. If orbit determined and distance from Sun and Earth known, then measurement of amount of sunlight reflected (optical) and reradiated (infrared) allows size and albedo (fraction of light reflected) to be found.
Other sizes from stellar occultations, again if orbit is known.
Some are binaries and masses can be found, thus densities.
May be 35,000 TNOs larger than D=100 km. >1200 known. Probably many more smaller ones. Smallest found has R about 1 km.
Pluto probably wouldn’t have been called a planet if discovered now!

18. Kuiper Belt and TNOs are leftover planetesimals thought to be scattered outward by the late outward migration of Neptune. Total mass of Kuiper Belt Earth masses.

19. Eris and other large TNOs
27% more massive than Pluto, but diam slightly smaller. Mass found from moon’s orbit
Orbit of Eris – the most massive known TNO
Eris and Dysnomia from HST

20. Other large Trans-Neptunian Objects
Makemake Haumea
All drawn as spherical but so far suspected that the bottom four may not be large enough.

21. Reminder of definitions
A planet is a spherical object orbiting a star, is not a
star itself, and has swept out its path
A dwarf planet is a spherical object orbiting a star that has not
swept out its path (Pluto, Eris, Ceres, a few other TNOs), and is
not a satellite. Note Pluto and Eris are also TNOs, Pluto is also
a KBO, and Ceres is an asteroid.
Haumea and Makemake are probably dwarf planets. Their diameters are estimated at 800 km, which is probably enough to be spherical.

22. Mass and Size Functions
Whenever you have a large number of objects with various
masses, useful to describe the number as a function of mass,
N(M), or size, N(R). Constrains theories of their origin. Useful for Kuiper Belt, Asteroids, impact craters, Saturn’s ring particles, stars, gas clouds, galaxies.
Often have many small objects and a few large ones, which
we can try to describe with a “power law” mass function:
N(M) α M-β
Gives relative importance of large and small objects (not total
numbers – this depends on constant of proportionality)

23. A Stellar Mass Function

24. For KBOs with measured radii, find N(R) α R-4
If they all have the same density then M/R3 = constant, so
M α R3, so R-4 α M-4/3, and
N(M) α M-4/3
Now can ask, for example, how many are there of mass M1
vs. 10xM1?
N(M1)/N(10xM1) = M1-4/3/(10xM1)-4/3 = 104/3 = 21.5
Albedo directly measured for Pluto and Eris, whose sizes can be measured with HST. Then, knowing how much sunlight the reflect, can work out how much they should have reflected if 100% albedo. One KBO measured by stellar occultation with many telescopes simultaneously. Transit has different lengths at each due to parallax. Knew angular speed of star. Max transit time gives diameter.

25. Can also ask: is most of the mass in larger KBO objects or smaller ones? For example, how much mass in objects of mass M1 vs. objects of mass 10xM1?
If you have N objects of mass M, the total mass is MxN. So for N=N(M), total mass is MxN(M) α MxM-4/3 or M-1/3. So relative mass is
M1-1/3/(10xM1)-1/3 = 101/3 = 2.2
So somewhat more mass in lower mass objects. Recall β was -4/3. Note if β > -1, more mass in higher mass objects.

26. The mass function is a constraint on our understanding of the origin of the population. KBOs represent leftovers from planet building process. Sizes affected by
growth through collisions (and gravitational focusing when they get massive enough),
shattering by collisions, which depends on speed and composition (held together by material strength, or by gravity (rubble piles)?),
ejection from Solar System, etc.
Mass function helps us understand the importance of each process.

28. True color, HST Animation
Brownish. Due to frozen methane being cooked by feeble sunlight. Mapped out during eclipses by Charon in Albedo and color variations were measured.
Animation