Extrasolar Planets Susan Cartwright

The Exoplanet Zoo 25 years ago we had no direct evidence of planets around other Sun-like stars As of 12/07/2017 the list includes 3630 planets in 2721 systems it is now clear that planetary systems are common What do extrasolar planets and planetary systems look like? is our solar system “typical”, or an oddball? Kepler data releases

Finding Exoplanets Planets are close to their stars the distance between the Sun and the Earth corresponds to an angle of 0.77" as seen from the nearest star very faint compared to their stars Therefore, observing planets round other stars directly is very difficult most planets are found indirectly, by their effect on the observed properties of their star directly imaged planets are not typical: they tend to be very large and a very long way from their stars

Finding Exoplanets: Radial Velocity The first planets detected around other stars were found by detecting the orbit of the star around the system centre of mass, using the Doppler shift of its light very small effect: less than one part per million typically

Finding Exoplanets: Transits If the system is edge-on to us, planets crossing in front of the star cause a temporary dip in the star’s brightness ~1% drop for a Jupiter-sized planet crossing the face of a Sun-like star only ~0.01% for an Earth-sized planet This is now the most successful technique, because of the Kepler spacecraft, which has detected 2330 planets to date but the information we get is limited: size, but not mass, of planet; period, but not radius, of orbit

Finding Exoplanets: Microlensing In a few cases gravitational bending of starlight around an intervening mass shows the presence of a planet Note that the planet is orbiting the lensing star, not the background star you are detecting a planet you can’t see, orbiting a star you can’t see! Information obtained: mass and position relative to star one-off event, never repeated for any given star

Finding Exoplanets: Other Methods Gaps in the dusty disc around the young star HL Tau suggest the presence of planets This suggests that planets form very quickly: HL Tau is only about a million years old Circumbinary planets and planets round pulsars can be found by variation in the observed timing of the pulses/orbit (similar to Doppler method) These planets may be secondary systems, formed from debris ejected by the evolving star(s) certainly unlikely that planets round pulsars are “original”! ALMA

The Known Exoplanets (with known masses)

The Known Exoplanets (with known radii)

Planets with known mass AND radius

Planets with known mass AND radius We observe a wide range of mean densities, probably corresponding to a wide variation in atmosphere mass Bigger point = better data, not bigger planet Inamdar and Schlichting 2016

Orbits of Exoplanets

Planetary Systems 608/2721 systems contain more than one planet 4 have 7 planets, 4 have 6, and 15 have 5 the solar system

What Does This Tell Us? Planets are common Note that our detection methods favour larger planets closer to their stars! Planets are common about half of all Sun-like stars have smallish planets with orbital periods < 1 year or so about 10% have giant planets with orbital periods less than a few years Planets are more varied than we thought there are surprisingly large high-density planets, and surprisingly small low-density planets the big gap in size between “terrestrial” and “giant” planets in the solar system is not typical, but there is a difference between giant planets and smaller ones at about 0.1 MJup or 0.4 RJup many planets, especially giants, have non-circular orbits

Planetary Oddballs Some planets orbit binary stars like Tatooine in Star Wars e.g. Kepler-453b There are rocky planets as massive as Neptune… Kepler-10c, 17 M⊕, 2.3 R⊕ …and lightweight fluffballs Kepler-11f, only 2 Earth masses, which would float on water

Habitable Worlds? Liquid water is probably essential for life, but the range over which water is liquid depends on atmospheric pressure, and the surface temperature of a planet also depends on its atmosphere.

Proxima Centauri b Detected by Doppler shift measurements minimum mass 1.3±0.2 M⊕; distance from star 0.05 AU; orbital period 11.2 days Proxima is an M6Ve red dwarf mass 0.12 M⊙; luminosity 0.017 L⊙ (mostly in IR) like many red dwarfs, it is a flare star, with occasional short bursts of higher luminosity (and X-rays)

Habitability of Proxima Centauri b It is likely but not certain that the planet is rocky exact mass depends on orbital inclination, 90% likely to be <3 Earth masses; radius is unknown note that Kepler-11f, 2.0±0.8 M⊕, has a density of 0.71±0.28 g/cc and is certainly not Earth-like! It is more or less certain that the planet will be locked in a tidal resonance either 1:1 like the Moon or 3:2 like Mercury which one depends on eccentricity of orbit: currently the only estimate is <0.35 1:1 resonance would imply that one side of the planet always faces the star

Habitability of Proxima Centauri b Turbet et al. 2016 Habitability of Proxima Centauri b Can it maintain an atmosphere? liquid water? This is very difficult to assess depends on planet mass composition and pressure of atmosphere 1:1 vs 3:2 tidal locking (which in turn depends on eccentricity) topography (the terrain at the substellar/antistellar points can make a significant difference) It does appear that plausible scenarios exist

The Future GAIA (launched Dec 2013) is designed to make high-precision measurements of star positions to determine their parallax (and hence distance) this will also allow it to detect the presence of planets by observing the star’s orbit around the system centre of mass in 5 years it could detect ~20000 giant planets (Jupiter or larger) in orbits from the Earth’s out to Jupiter’s because GAIA observes the same stars repeatedly, it can also detect transits in this mode it will see mostly short-period giant planets (hot Jupiters and hot Neptunes) first data release published in late 2016 with data from over a billion stars—but no exoplanet analysis yet

The Future James Webb Space Telescope (due for launch 2018) 6.5 m space telescope E-ELT (under construction in Chile) 39.3 m telescope Both have plans to study the chemical composition of exoplanet atmospheres by spectroscopy a few have already been measured by HST This has the potential to detect signs of alien life— oxygen in Earth’s atmosphere is totally biogenic

Summary Planets are common, but very varied in size, mass and composition Our planetary system may be quite unusual in the contrast between terrestrial and giant planets mid-sized planets seem to be rather common the “natural” gap seems to be between Uranus/Neptune and Saturn/Jupiter Orbits of planets also vary widely from much closer than Mercury to much more distant than Neptune Prospects for alien life appear quite good many planets orbit at about the right distance, and water seems to be common in their atmospheres