Astro Seminar: Exoplanet basics

08Feb07

This is the second half of the Astrophysics Seminar described in the previous post. It follows the same general pattern, only with extrasolar planets instead of brown dwarfs.

Ready, world? Here’s what Josh said:

Exoplanet research has its roots in two fundamentally philosophical questions: Where do we come from? and Are we alone? Answering the former leads you into planet formation, and answering the latter gets you into exobiology. Either of them gets you into some fascinating physics.

The idea of “other worlds” goes back to Old Man Epicurus in 300 BC. In opposition to Aristotle, Epicurus believed that there were an infinite number of atoms, and that, consequently, “there [would] be nothing to hinder an infinity of worlds”. (I’ve never understood this argument. You can quite easily have an infinity with a hole in it, can’t you? Somebody explain this to me.) Some centuries later, Giordano Bruno favored a universe that was infinite, homogeneous, and isotropic, and contained many stars and planets. He was burned at the stake. Later, people put up a statue of him to make up for it. Sorry, Giordano Bruno! Here’s you in bronze. And we’ll quit killing planetary scientists, maybe.

Back in the day a.k.a. before 1995, scientists worked under the assumption that extrasolar planetary systems would be characteristically very similar to our solar system. They’d noted some salient features of ours:

• The orbits of planets are coplanar to within 10 degrees;
• The orbits of planets are nearly circular;
• The type of planet are correlated to both the distance from the sun and the planet’s mass. Rocky planets are small and close; gaseous giant are huge and at a medium distance; icy giant planets are big and far away. And Pluto…well, you know what happened to Pluto.

For reference, here are some ratios:

• Mass of Earth: Mass of Neptune : Mass of Jupiter : Mass of Sun :: 1 : 15 : 300 : 300,000
• Radius of Earth: Radius of Neptune: Radius of Jupiter: Radius of Sun :: 1 : 4 : 10 : 100

The traditional hypothesis about the formation of the solar system was that it had formed when a spinning disk of matter congealed. The sun was at the center, of course, and stayed there; since they’d all been in a disk to begin with, the planets remained roughly coplanar; since the disk was circular, the resulting orbits were roughly circular.

The modern theory, called core-nucleated growth, goes something like this:

1. Some stars have disks around them made of gas and dust.
2. The dust settles into the midplane of the disk.
3. Somehow (handwave, handwave) the dust clumps together.
4. Eventually, the clumps grow large enough that gravitational attraction can cause further collisions and clumping.
5. When a clump has approximately the mass of the earth, it begins to accrete the surrounding gas into an atmosphere.
6. At approximately 10 times the mass of the earth, the gas accretion goes out of control and the planet puffs up. POOF.

Rocky planets are cores that were too small to poof out. Maybe they weren’t sticky enough; maybe the dynamics were unfavorable. Gas giants made it all the way through and puffed up properly. Ice giants grew too slowly and were too far away. Reasonable? Reasonable.

With a decent theory in place, it remained only to find some planets. Like the brown dwarfs, exoplanets had a series of false starts. The most famous involved Barnard’s Star, which is the second-closest stellar system to us. As a consequence of its nearness, it appears to be whipping by on the sky. (Adam broke in here to point out that by “whipping by” Josh meant “at the speed of a turtle walking across the moon.” Astronomers develop a strange sense of scale.) In 1963, somebody called Peter van de Kamp found that the star had just a little extra wobble, which could be accounted for by a Jupiter-mass planet with a 25-year period, or by two planets with 12- and 26-year periods respectively. In the first scenario, the orbit was highly eccentric (i.e. more elliptical than circular); in the second, the orbits could be “nice” and circular — just like our solar system. Unfortunately, the “wobble” was actually a flaw in the archival plates van de Kamp was using. Ouch.

A really good start came, at last, in 1992, when a planetary system was found around a millisecond pulsar. With masses comparable to Earth’s, these planets are still the lowest-mass exoplanets known. But the pulsar business is just weird. A pulsar is pretty much a dead star, the leftovers from a supernova, and nobody knows whether these planets formed before or after all the explosions. Anyway, WEIRD. But finally, in 1995, a really nice system was found. It had a Jupiter-mass (or possibly larger) planet orbiting a fairly ordinary non-dead star, and exoplanetologists rejoiced.

As of yesterday afternoon, 210 exoplanets had been found. They don’t all conform to the patterns of our solar system, either: there are huge gas giants close in; there are highly eccentric orbits that whip the planet around its star on one end and send it out to freeze, slowly, on the other. But the important thing is THERE ARE PLANETS OUT THERE. And if planets are not uncommon, and Sun-like stars are no rarity, then it’s very possible that Earth-like planets exist. Somewhere.

Sweet.

Next week: Ways of finding brown dwarfs & exoplanets.

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