Extrasolar Planets

Nebular theory leads us to believe planets should be associated with most stars .

Why is it so hard to detect planets around other stars?

Planets are really small If the Sun is a grapefruit on the West coast Earth is a pinhead about 15 meters away Jupiter is a marble 80 meters away the nearest star is on the East coast If things are similar…you are trying to find a pinhead on a grapefruit about 2500 miles away And the star is a billion times brighter than its planet

How can we find Extrasolar Planets?

Direct Detection Special techniques, like using a coronograph, can eliminate light from brighter objects, enabling direct planet detection

Captured using a 1. 5-meter-diameter (4 Captured using a 1.5-meter-diameter (4.9-foot) portion of the Hale telescope's mirror. IR light The astronomy team combined two techniques -- adaptive optics and a coronagraph -- to minimize the glare from the star and reveal the dim glow of the much fainter planets. 

How else can we find Extra solar Planets? Indirect Detection: Measure properties of the star and reveal the presence of extra solar planets

How does a Planet influence its Star? All objects in the solar system orbit a common center of mass Sun is so large that the center of mass is inside the Sun

Orbiting planet causes its star to “wobble” http://www.daviddarling.info/encyclopedia/R/radial_velocity_method.html

Astrometric Technique We make careful positions of the position of a star planets cause a change in a star’s position on sky These tiny motions are very difficult to measure (~0.001 arcsecond)

Doppler/ Radial Velocity Technique See Doppler shift in spectrum of star (motion toward and away from us) Current techniques can measure motions as small as 1 m/s (walking speed!)

Transit Method A planet passing it front of its star produces a change in brightness of 1/10,000 lasting for 2 to 16 hours Credit: NASA

What have we learned about extrasolar planets?

First Extrasolar Planet First extrasolar planet to be discovered (1995) Doppler shifts of star 51 Pegasi indirectly reveal a planet with 4-day orbital period Short period means small orbital distance

First Extrasolar Planet Planet around 51 Pegasi has a mass similar to Jupiter’s, despite its small orbital distance

Revisiting the Nebular Theory Nebular theory predicts massive Jupiter-like planets form outside the frost line (at << 5 AU) “hot Jupiters” have forced reexamination of nebular theory

Planetary Migration A young planet’s motion can create waves in a planet-forming disk Models show that matter in these waves can tug on a planet, causing its orbit to migrate inward

Planets: Common or Rare?

Extrasolar Planets Most of the early detected planets had greater masses than Jupiter Reason: planets with smaller masses are harder to detect with Doppler technique After Kepler we know that there’s a huge range in mass

Orbits of Extrasolar Planets Most of the detected planets have orbits smaller than Earth’s

Orbits of Extrasolar Planets Orbits of most extrasolar planets are much more elongated (greater eccentricity) than those in our solar system

Interesting systems

Tau Ceti

Gliese 581

Alpha Cen B Proxima b’s host star, is a red dwarf that is quite a bit smaller than our yellow dwarf sun. It’s found in the Centaurus constellation. Proxima b itself is 1.3 times more massive than Earth, and it takes 11.2 days to completely orbit its star. Scientists think only one side of the exoplanet is exposed to light from Proxima Centauri (similar to our view of the moon).

Alpha Cen B Proxima b lies well within the habitable zone around the star and has an estimated surface temperature that would allow the presence of liquid water.