Descriptive Astronomy 101
Extrasolar Planets

Extrasolar Planets

Worlds in orbit around stars other than the Sun.
Detection Bias: Until recently, the great majority of extrasolar planets detected had small orbital radii and masses similar to or greaterthan Jupiter.
Recently, lower mass planets have been detected.

Extrasolar Detection

Problems with detection:
Direct imaging is not easy but may be possible in the near future.
Indirect detection techniques:

1) Radial Velocity Method

Small, regular variations in the line-of-sight velocity or position of its host star

2) Transit Method

Dimming of light from the host star as a planet passes in front of the star (eclipsing)

3) Astrometry

The precise measurement of the position of astronomical objects.
The gravitational influence of the unseen planet causes the star to appear to wobble slightly.

Close In Jovians

717 extrasolar planets have been discovered.
What has been found so far is mostly Jovian planets
Many are in sub-Mercurian orbits: Closer than 60 million kilometers
With highly eccentric orbits
Closest in: 0.00585 AU (877,500 km) (Mercury: 0.387 AU)
Furthest out: 113 AU from star (Neptune: 30 AU)
Biggest mass: 11.68 Jupiters

Lots of Planets or Few?

Extrasolar planets have been detected in orbit around roughly 12% of the stars searched
A planetary system has been detected in orbit around a star in the Andromeda constellation (upsilon Andromedae)
Accretionary disks have been found around many stars
All of this indicates that planets and planetary systems may be common around stars.

“Metal-rich stars are much more likely to harbor gas giant planets…planets with radii less than four Earth radii form around host stars with a wide range of metallicities (but on average a metallicity close to that of the Sun), whereas large planets preferentially form around stars with higher metallicities. This observation suggests that terrestrial planets may be widespread in the disk of the Galaxy, with no special requirement of enhanced metallicity for their formation.
- Buchhave et al., 2012. An abundance of small exoplanets around stars with a wide range of metallicities. in Nature 486(7403):375-7.

Life in the Universe

Life as we know it occurs on terrestrial planets.
These are much harder to detect.
Life may be common - bacterial life.
Intelligent life may be very rare.

Planetary Life Factors

Galaxy type
Galactic position
Distance from star
Mass of the star
Stable planetary orbit
Planetary mass
Jupiter-like neighbor
Plate tectonics
Large moon
Axial tilt
Giant Impacts
Atmospheric properties
Biologic evolution
Oxygen generation