• We have discovered planets around more than fifty solar type stars, and one (or possibly two) neutron stars. Planets are very common (see this web site ) A table of the planets observed by Marcy, Butler, and coworkers is at this link. These surveys find planets around roughly 5% of the stars examined.
• All the planets so far found around solar type stars are similar to Jupiter or Saturn in mass.
• Currently, the only way to detect a planet is by its gravitational tug on the primary star .
• Using the best available instruments, we could not possibly detect Earth around even the nearest extra-solar star.
• More than twenty of these planets have orbits smaller than that of Mercury!
• More than forty have eccentricities in excess of 0.1.
• Ten have very low eccentricities, but are in such short period orbits that tides in the planets may have circularized the orbits.
• At least two of the planets are well outside the tidal circularization limit, yet have small eccentricities. The implication is that 1) some non-tidal dissipation acted efficiently in these two systems, but not in the forty systems that have planets on high eccentricity orbits, OR 2) whatever mechanism(s) produced the eccentricities in the  high eccentricity systems did not operate in these two.

The case against in situ formation

• The planet orbiting tau Bootis at a distance of 0.0462AU has an equilibrium T=1550K, higher than the condensation temperature of iron.
• Even a hydrodynamic collapse is difficult, since the proto-planet must be much larger than it is currently. If the planet is Jupiter-size as well as Jupiter mass, it is currently stable against evaporation. If it were much larger than Jupiter size in the past, it would have been unstable if it also had its current (probable) temperature. During a collapse of course, the planet is larger than its current size. The fact that the protoplanet's atmosphere would be unstable to evaporation makes it difficult to understand how the collapse can occur.

Migration Scenarios

• Interaction with another massive planet---scattering off planets.


 Pluses

• Simple to understand
Minuses
• How common are the initial conditions?
• 25/1000 stars have Jupiter mass planets inside 0.25AU ---Simulations show that scattering cannot produce so many short period systems
• Planets such as those orbiting rho Corona Borealis and HD 195019 are well outside the tidal circularization radius, yet have small eccentricities

 
• Interaction with a gas disk.


 Pluses

• The gravity of the planet launches waves in the disk.
• Seen to operate in our solar system (Rings)
Minuses
• Migration times much shorter than disk lifetime; how do planets form?
• Stopping by interactions with star: \rho Corona Borealis is well outside the tidal circularization radius, yet has e=0.028
• No evidence that large scale migration occurred in Solar System.

 
• Interaction with a planetesimal disk.


 Pluses

• Recall that Jupiter flings comets out of the solar system (some end up in the Oort cloud). The result is that Jupiter must move closer to the sun.
• Evidence that this occurred: depletion of asteroid belt, capture of Kuiper belt objects in resonances.
• A similar process should happen in other solar systems
• If there are enough planetesimals, the process is unstable, and the planet migrates toward its primary.
• Stopping assured.
Minuses
• How common are the initial conditions?

Observational Consequences

Planetesimal Migration

• Effects in the solar system
• Depletion of the outer asteroid belt due to inward migration of Jupiter.
• Outward migration of Saturn, Uranus, and Neptune; capture of Pluto into resonance.
• Jupiter mass planets at a_p=0.05 AU; larger masses correlate with larger a_p and e_p.
• There will be a correlation between metallicity and short period Jupiters:
• High metallicity gas will produce a more massive planetesimal disk, making migration to small a more likely.
• The migration will alter metallicity in the surface layers of the star by dropping rocks onto it.
• The stars harboring extra solar planets are now known to have accreted roughly 5 Earth masses of iron
• The migration will produce interstellar comets/planetesimals.
• The migration may produce Oort clouds.
• There will be a correlation between infrared excesses (``Vega-like disks'') and short period planets. The outer parts (>10's of AU) of the massive planetesimal disk will produce and maintain large amounts of dust over the lifetime of the system.

Future Prospects

• The observational situation will change radically in the next decade.
• Radial velocity techniques will improve. For example, new searches are underway at the Keck telescope, and in the southern hemisphere. This will increase the number of stars being observed by a factor of about three, which will in turn increase the number of known extra solar planets by a similar factor.
• Space based networks of large telescopes are being planned. This will allow astrometric detection of planets around nearby stars see this web site.
• NASA has already budgeted nearly $1,000,000,000 to build the NGST (next generation space telescope) and SIM (space interferometry mission)!
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