9.2 Comets Our Goals for Learning How do comets get their tails? Where do comets come from?

How do comets get their tails?

Comet Facts Formed beyond the frostline, comets are icy counterparts to asteroids. “Dirty snowballs” = the nucleus Most comets do not have tails. Most comets remain perpetually frozen in the outer solar system. Only a few enter the inner solar system, where they can grow tails.

When a comet nears the Sun, its ices can sublimate into gas and carry off dust, creating a coma and long tails.

Comets eject small particles that follow the comet around in its orbit and cause meteor showers when Earth crosses the comet’s orbit.

Meteors in a shower appear to emanate from the same area of sky because of Earth’s motion through space

Where do comets come from?

Kuiper belt: On orderly orbits from AU in disk of solar system Oort cloud: On random orbits extending to about 50,000 AU Only a tiny number of comets enter the inner solar system - most stay far from the Sun

How did they get there? Kuiper belt comets formed in the Kuiper belt: flat plane, aligned with the plane of planetary orbits, orbiting in the same direction as the planets. Oort cloud comets were once closer to the Sun, but they were kicked out there by gravitational interactions with jovian planets: spherical distribution, orbits in any direction.

What have we learned? How do comets get their tails? The vast majority of comets do not have tails. Only those few comets that enter the solar system grow tails. As the comet approaches the Sun its nucleus heats up. Some of the comet’s ice sublimates into gas, and the escaping gases carry along some dust. The gas and dust form a coma and two tails: a plasma tail of ionized gas and a dust tail. Larger particles can also escape, becoming the particles that cause meteors and meteor showers on Earth.

What have we learned? Where do comets come from? Comets that enter the solar system come from one of two reservoirs in the outer solar system: the Kuiper belt and the Oort cloud. The Kuiper belt comets still reside in the region beyond Neptune in which they formed during the birth of the solar system. The Oort cloud comets are thought to have formed in the region of the jovian planets, and were kicked out to the great distance of the Oort cloud by gravitational encounters with the planets.

9.3 Pluto: Lone Dog or Part of a Pack? Our Goals for Learning What is Pluto like? Is Pluto a planet or a Kuiper belt object?

Pluto: the exception Not a gas giant like the other outer planets. Has a very elliptical, inclined orbit. By far the smallest planet, and smaller than several moons. Has a surprisingly large moon Charon, probably formed by a huge collision of another object with Pluto when the solar system was forming. Also has two smaller moons: Nix and Hydra.

Pluto will never collide with Neptune because of a 3:2 orbital resonance.

What is Pluto like? 1978 discovery of Pluto’s moon Charon: Pluto’s mass from Newton’s version of Kepler's Third Law (like Fall computer lab). It has a thin nitrogen atmosphere that will refreeze onto the surface as Pluto’s orbit takes it farther from the Sun discovery of small moons Nix & Hydra. Pluto will be visited by the New Horizons spacecraft in 2015 (launched January 2006).

Hubble Space Telescope’s direct view of Pluto & Charon

Brightness variations during eclipses of Pluto by Charon (or vice versa) show dirty ice on surfaces - like comets.

Is Pluto a planet? a Kuiper Belt object? something else?

Come to think of it... what is a planet?

Is Pluto a planet or a Kuiper Belt object? Pluto is well beyond Neptune, in the Kuiper Belt. Inclined orbit is typical of Kuiper Belt comets. Composition is typical of Kuiper Belt comets, but not any of the other planets.

Is Pluto a planet or a Kuiper Belt object? Kuiper Belt objects have similar orbital resonances with Neptune. Kuiper Belt comets can have moons. Kuiper Belt objects Triton (captured moon of Neptune) and Eris (formerly code-named Xena) are even larger than Pluto. Even larger Kuiper Belt objects may be discovered in the future, which may lead to confusion in calling them `dwarf planets'.

International Astronomical Union definition of a planet (as of 2006) Planets must be big enough to be “nearly round” due to self-gravity, and must have “cleared the neighbourhood around their orbit” of similarly sized objects (8 planets) Dwarf planets are big enough to be spherical due to self-gravity, but have not cleared their orbits of similarly sized objects [think belts], and are not moons of larger objects (Ceres, Pluto, Eris) All other objects directly orbiting the Sun are “small solar system bodies”.

What have we learned? What is Pluto like? Pluto is much smaller than any other planet, with an orbit more elliptical and more inclined to the ecliptic plane than that of any other planet. It is made mostly of ices and has a very thin atmosphere of gases that are expected to freeze onto the surface as Pluto moves farther from the Sun in its 248-year orbit. It has a large moon, Charon, with a slightly lower density than Pluto, suggesting that Charon may have been formed in a giant impact. Is Pluto a planet, dwarf planet, or Kuiper belt object? Whether Pluto should be called a “planet” (dwarf or regular) is a matter of opinion, but its properties suggest that it is a Kuiper belt object. Its composition and orbital properties match those of other Kuiper belt objects and do not fit in with the other planets. It is among the largest known Kuiper belt objects today, but there may be larger ones still awaiting discovery.