Lecture 34 ExoPlanets Astronomy 1143 – Spring 2014

Key Ideas: Many planets around other stars found Two methods: Transits of host star Gravitational microlensing Finding Earth-mass planets is tough! All methods more sensitive to bigger planets Not all solar systems look like ours Hot Jupiters – massive planets near star Planets on highly elliptical orbits More familiar situations Multiplanet systems Rocky Planets in habitable zones (not yet around Sun- like star)

Are we alone? The question of the existence of other planets beyond the Solar System is an old one in Astronomy. Are there solar systems around other stars? Are such solar systems like ours or different? Are any of the planets like the Earth? Has life arisen on other planets? Has intelligent life arisen on other planets?

Scientific Questions Finding other solar systems test our ideas of how solar systems form Do all have terrestrial and Jovian planets? What are the periods and distances of the planets? Does the kind of solar system depend on the type of star? What about systems with multiple stars?

What is a planet? Star: massive enough to ignite 1 H fusion in its center. Mass > 0.08 M sun Brown Dwarf: no 1 H fusion, but is massive enough to ignite 2 H fusion. Mass between 0.05 M sun and 0.08 M sun Planet: No nuclear fusion Orbits star Shape determined by gravity Dominates its local gravity

Searches for ExoPlanets There are two basic search strategies: Light Detection: Images of planets orbiting other stars. See planets transit their parent star, causing a characteristic drop in brightness. Gravitational Detection: Orbital motions of the star because of the planet's gravity. Gravitational microlensing by the planet.

Direct Imaging: Not this easy!

Problems Star is very bright (at all wavelengths!!!), planet is very dim A million to 10 billion times brighter Planet is very close to star on the sky because of distance Planet usually lost in the glare/diffraction spikes/Airy disk of star Not a planet

Direct Imaging

Planetary Transits Planet's orbital plane along the line of sight: The planet periodically crosses (transits) the face of its parent star. Star dims slightly (fraction of a %) during transit. Biased towards close-in Jupiter-sized planets.

Earth in Transit We are on a distant planet, looking back at the Earth-Sun system. If the Earth transits the star from our perspective, what fraction of the surface area of the Sun will it block?

Earth in Transit of the Sun will be blocked. Therefore the Sun will appear less bright when the Earth transits it. This is a very small change, and difficult to detect.

CoRoT and Kepler Missions Measuring changes in the brightness of stars is much easier above the Earth’s atmosphere No blurring/extinction from atmosphere Can always be looking (no bright blue sky) In recent years two spacecraft took repeated observations of nearby stars, looking to detect Earth-sized planets in transit

Transiting Planet HD b Mass of ~0.7 Jupiters ~0.045 AU from its star Orbital Period of ~3.5 days

Smaller Planets, Harder to Find!

Some Planets Discovered by Kepler

Gravitational Microlensing Two stars line up: Light from the background star is amplified by the gravity of the foreground "lensing" star. Brief brightening of the background star as they pass

Microlensing by Planets If there is a planet around the lensing star, it will amplify the light as well. 22 planets have been found this way by the MicroFUN team led by OSU!

Jupiter-mass planet discovered in 2005 by the MicroFUN Collaboration

Detected Exoplanets As of 17 April 2014, planets had been confirmed Planetary Transits Radial velocity variations Microlensing Direct Imaging Transits are the most effective method 1132 planets so far Kepler has many more candidates Other upcoming missions

We can’t “see” all planets Planetary transits Larger radius = better Closer in =better Edge-on orientation = necessary Gravitational microlensing More massive = better More distant = better Alignment = necessary Bigger (in mass or radius) planets are always easier to find, regardless of technique. Most techniques favor detection of close-in planets as well

Exoplanets compared to Solar System Exoplanets as of 1 year ago S. Seager, Science

Exoplanet radius & mass

Earth Mass Earth Radius

Exoplanet Semi-major Axis & Mass

Earth’s Semi-major Axis Earth Mass Lots of massive planets Not very many distant planets

Picture of Our Solar System

Planets to Scale

Upsilon Andromedae – Hot Jupiters

Upsilon Andromedae Comparison of our Solar System and Upsilon Andromeda o Extremely close inner planet o More elliptical orbits

HD 80606b – Very Elliptical Planet mass = 4 Jupiter masses Originally discovered by Doppler wobble e=0.95 periastron=0.03 AU apastron=0.88 AU When star eclipsed it, we could learn about the light from the planet

Every 111 days, things get interesting When the planet is at periastron T spikes by 682 K in six hours Goes from 980 o F to 2240 o F Then drops by same amount Creates weather on planet

Habitable Zone

Planetary Properties

Kepler 37 – small planets!

Kepler 186f Planet about the size of the Earth Likely to be rocky, but don’t know its mass yet Orbital period – 130 days In a system with 4 inner planets Host star is ½ the size and mass of the Sun Habitable zone much closer than 1 AU Receives about 1/3 as much energy as Earth Very exciting to discover a rocky planet in the habitable zone around a star

Kepler 186f

Kepler 186

Comparison with 55Cnc 5 Planets detected so far 1.~ mass of Uranus with a period=3 Earth days 2.~ mass of Jupiter with a period=14.7 Earth days 3.~ mass of Saturn with a period=44 days 4.~ ½ mass of Saturn with a period=260 days The first four planets are closer to the star than 1 AU 5. ~ 4 times the mass of Jupiter with a period=14 years, at a distance of 5 AU

OGLE System Our Solar System

The Present & Future Continuing search for other systems Find systems more like our own How common are planetary systems? Future Goals: Find Earth-sized planets. Find Earth-sized planets in orbits where liquid water is possible Search for Life Markers like O 2 & O 3 in their atmospheres