Exoplanets

Exoplanets

• Formation• Detection• Habitability• Implications

Reference:The Exoplanet Handbook, M. Perryman, Cambridge University

Press, 2011Since this is a very dynamic field, there are constantly new findings

and publications are outdated at a fast pace!

History

“This space we declare to be infinite... In it are an infinity of worlds of the same kind as our own.”

Giordano Bruno, 16th century

• Isaac Newton was speculating on the possibility of exoplanets based on similarity and comparisons between our sun and other stars

• Claims for observations since the 19th century but not confirmed• 1988 radial velocity observations of Cephei indicated a planet but

was confirmed only in 2003• 1992 first confirmed exoplanets around a pulsar• 1995 first planet around a main-sequence star (51 Pegasi)

(Current) Number of Exoplanets

Date Confirmed Planetary systems Multi-planet Candidates

(Kepler)

2012 725 591 87 1235

Jan 2013 854 673 126 1800

Dec 2013 1054 800 175 3568

Jan 2015 1855 1164 473 >4000 ↑

Planets are everywhere

• One or more bound planets per Milky Way star from microlensing observations, A. Cassan et al., Nature, 481, 167–169, (12 January 2012), doi:10.1038/nature10684

• Today we assume on average at least one planet per star (~100 – 400 Billion Exoplanets)

• Indication for free-floating objects – planets ejected from their original star system

• Most known exoplanets so far resemble planets like gas (Jupiter) or ice giants (Neptune)

• Statistical studies however indicate that “terrestrial” planets might be the majority (observational bias)

Image: NASA/JPL

Detection methods

• Radial velocity– Measuring the Doppler shift

• Photometry and transits– Planets crossing in front of host star

• Direct imaging– Coronagraphie, space based imaging, polarimetry, (no definitive confirmation yet)

• Astrometry– Obtaining precise Star/Planet position changes (< 1 marcsec)

• Timing– Using pulsating features modulated by planets

• Microlensing– Can detect only fast orbiting planets, but also free floating planets

Planet distribution (so far @ 2014)

• radial velocity (dark blue)• transit (dark green)• timing (dark purple)• astrometry (dark yellow)• direct imaging (dark red)• microlensing (dark orange)• pulsar timing (purple)• Solar System planets for

reference

Current planet distribution biased by observational effects!

Distance/Orbital Period

Mas

s

Definition of a planet

• IAU 2006 Resolution B5:1. A planet is a celestial body that:

a) Is in orbit around the sun,b) Has sufficient mass to assume through self gravitation a

hydrodynamic equilibrium (round) shape,c) Has cleared the neighbourhood around its orbit

2. A dwarf planet is a celestial body that:a) Is in orbit around the sunb) Has sufficient mass to assume through self gravitation a

hydrodynamic equilibrium (round) shapec) Has NOT cleared the neighbourhood around its orbit

3. Other objects except satellites are referred to as small solar system bodies

Definition extension for exoplanets

1. Objects with true mass below the limiting mass for thermonuclear fusion (<13 MJupiter) that orbit stars or star remnants; Minimum mass limit is equivalent to solar system bodies (not yet detected)

2. Substellar objects above thermonuclear mass limit are brown dwarfs

3. Free-floating objects in young star clusters with masses below thermonuclear limit are NOT planets but sub-brown dwarfs

4. Free floating objects are only planets if evidence points to planetary formation process with subsequent ejection from the original host system

Short amended unofficial version (2006): A planet is an end product of disc accretion around a primary star or substar

Classification of planets

• Terrestrial planets – rocky (metallic + silicates)– Mercury, Venus, Earth, Mars,

Ceres + asteroids• Icy planets

– rocky/icy– Pluto, Eris, Haumea,

Makemake + (most KBO)• Giant (ice) planets

– rocky/icy + H2/He < 50%– Uranus, Neptune

• Giant planets– gas rich H2/He > 50%– Jupiter, Saturn

Image: Lodders K., 2010 Exoplanet chemistry in: Formation and Evolution of Exoplanets, R. Barnes (ed.), Wiley, Berlin

Black = metallix, light grey = silicate, dark grey = various ices, second light grey on top = H2 + He gas

Large planets: Hot Jupiters

• Among the first known exoplanets– Large m > 1.9˟1027 MJupiter

– Close orbit 0.015 – 0.5 AU– Formed further out beyond the

frost line and migrated inwards during early system formation

– Bound/tidally locked rotation and nearly circular orbits

– Similar types: Hot Neptune, hot Saturns

– Solar wind can strip away the atmopshere resulting in a “Chtonian planet” (Corot 7b) similar to a terrestrial planet

Image: NASA/JPL Hot Jupiter artists expression

Habitable zone

Image: NASA/Ames/Caltech

Conditions for habitability

• Environment conditions necessary to sustain carbon-based life forms

• Temperature range where liquid water is stable over geologic timescales– Orbit radius– Eccentricity (low and stable preferable)– Planet rotation (no tidal locking preferable)– Other heat sources– Atmospheric properties (Temperature distribution, Greenhouse

effect, big enough to keep atmosphere)– Stable star conditions (luminosity, bursts, x-rays)

• This might include larger moons of gas giant planets• Red dwarfs (70 -90% of all stars), long lived stable stars

Super earth

• MGas planet > M > MEarth • Typically (< 10 MEarth)• Most of them are close to the host star

(observational bias!)• Due to larger size the physical

characteristics may differ from our known terrestrial planets

• Similar types: Ocean planets (GJ 1214)– Oceans deeper >100 km– High pressure ice at bottom– Supercritical hot top layer (surface

not exactly defined)– Oceans composed of liquid

mixtures• Hydrocarbon lakes (Titan)• High-density fluids (Uranus?,

Neptun?) below cloud layer• Liquid hydrogen (Jupiter,

Saturn)

Earth size planets

• Earth analogue or “Twin Earth” is a planet with similar physical conditions

• Measured by Earth Similarity Index (ESI) from SETI Insitute– Mass– Radius– Surface temperature

• Candiates*: Kepler 22b, HD 85512 b, Gliese 581 d, Mars, Gliese 581 c• Actual size range (January 2012):

– KOI-961.03 0.57 RE

– KOI-961.02 0.73 RE

– Kepler-452b (G2V*)1.6 RE

– 55 Cancri e 2 RE

– Kepler 22b 2.4 RE

* Updates on almost daily base Image: NASA/Ames/Caltech