Lecture 34

ExoPlanets

Astronomy 1143 – Spring 2014

Key Ideas:Many planets around other stars foundTwo 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 anold 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 1H fusion in its center. Mass > 0.08 Msun

Brown Dwarf: no 1H fusion, but is massive enough to ignite 2H fusion. Mass between 0.05 Msun and 0.08 Msun

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!

ProblemsStar 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

0.000084 of the Sun will be blocked. Therefore the Sun will appear 0.000084 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 209458b• 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 MicrolensingTwo stars line up:

Light from the background star is amplified by the gravity of the foreground"lensing" star.

• Brief brightening of thebackground star asthey 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 planetdiscovered in 2005by the MicroFUNCollaboration

Detected ExoplanetsAs of 17 April 2014, --1782 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

Exoplanet radius & mass

Earth Mass

Earth Radius

Exoplanet Semi-major Axis & Mass

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 EllipticalPlanet 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 980oF to 2240oF

• 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 far1. ~ 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-2006-109 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 O2 & O3 in their atmospheres