The Situation in the Solar System

• Known things to compare: Eight planets (4 terrestrial, 4 jovian); Asteroid belt; Kuiper belt (including Pluto and other dwarf planets) & Oort cloud.
• Is there a planet X? Occasionally, new evidence is announced, but it would have to be way beyond the Kuiper belt, 30,000 AU away (Why?); Closer than Neptune, we can't have missed an object as big as the planets (although there was a search for Vulcan inside Mercury's orbit), but (i) The asteroid belt might have assembled into one, were it not for Jupiter's influence; and (ii) There are large asteroids, even in the Kuiper belt between Neptune and Pluto.
• Questions: How common is this type of situation around other stars? How can we find out?

Outside the Solar System

• Current approaches: Mostly ground-based, relying on indirect observations.
• Main teams: Initially, many planets were found by Michel Mayor's Geneva Extrasolar Planet Search team and the Marcy-Butler team at UC Berkeley/SFSU; There are now more teams.
• The challenge: Planets are much smaller and dimmer than their host stars!

• Other projects: Space missions including NASA's Kepler space observatory, launched in 2009, the Origins Program, including the Keck Interferometer, and the ESA's GAIA mission, to be launched soon.

  How Do They Find Planets?

• Direct imaging: Very few have been seen, but disks around stars are easier to detect.
• Doppler shifts: Star radial motion shows up as periodic wavelength variation; It is very effective [the speed is now measured to about 1 m/s!]; It has given us most of the candidates and thousands of stars are being monitored.
• Astrometry: Based on sideways wobbling; Very hard, but an example has been seen (a star with 3 planets–How do we know?).
• Gravitational microlensing: Uses light focusing by the curvature of space, gives a time variation in the amount of light seen; Several examples are known.
• Transit: The second most productive method is to look for planets that eclipse their star and cause a temporary dip in the light.

What Do We Know?

• Examples: 51 Pegasi, 70 Virginis, 47 Ursae Majoris, Fomalhaut, Vega, ..., the closest one is Alpha Centauri Bb, 4.37 ly away.
• Number of planets known: A total of 1000+ around almost 800 stars (mostly main sequence ones, some neutron stars) as of Nov 2013, and including 170+ multi-planet systems (55 Cancri has at least 5!).
• Sizes: Most planets are larger than Jupiter, and/or in short orbits close to their stars (so close that the star may roast them and rip off their atmosphere); The most similar system to ours is the one around 55 Cancri; A few small planets have now been detected, but are unlikely to have life on them.
• Easier things to find out: Planet mass, size of orbit, period of revolution.
• Harder things to find out: Density; Composition (but can perhaps detect IR radiation or radio waves); Rotation (but can perhaps detect diurnal variations); Atmosphere (but one has been detected from absorption lines in starlight!).
• Things to watch out for: Some stars have starspots, which can also cause rhythmic shifts in the spectrum from the star's rotation!
• Other objects: A few stars are known to have asteroids (for example, zeta Leporis) and comets around them (tau Ceti seems to have 10 times as much as the Sun!); Not good for life on possible planets.

  Peculiar Cases, Puzzles

• More than one star: There are at least 19 known examples of planets orbiting a star in a binary or multiple system; They may not be stable for long times.
• Starless planets: Several sightings reported in the past few years; Were they ejected by binary stars?
• Highly eccentric orbits: How can orbits end up having eccentricities as high as 0.71? (vs 0.25 for Pluto!).
• Question: Could it be that many of the planets we think we found are actually brown dwarf stars?

Interesting Questions

• Earth-like planets: Do we expect Earth-size planets to be rare out there? No. We are probably seeing so many large ones just because they're easier to find. Question: What is the range of sizes for terrestrial planets? Are they like the solar ones, or can some of them have, say, a diamond core?
• Multi-planet systems: Do we expect multi-planet systems to be rare? No. We are seeing relatively few probably because they are harder to identify, but we still don't know how common protoplanetary disks are, and how often they actually produce planets.
• Around which stars? Probably ones that have not evolved beyond the main sequence, although Jovian-type planets could possibly survive late stages of stellar evolution; Stars with more heavy elements are more likely to have planets (no planets formed in the first few billion years), and those that have a dust disk seem to be most promising (the Sun has one).
• When? Probably the peak planet-formation age for a star is when it is 1-3 million years old.
• In what parts of the galaxy? In the disk, but not close to the core, or in the thick parts of the spiral arms (too many close encounters with other stars), and not very far out (too few heavy elements); we are in a thin portion of the disk, 2/3 of the way out [but a 12.7-billion year old planet is known in the globular cluster M4!].
• In what galaxies? Terrestrial planets probably form predominantly in large galaxies, that can retain the hot gas ejected by stellar winds and explosions.