1. Extrasolar Planets
Susan Cartwright

2. 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 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

3. 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

4. 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

5. 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 planets to date
but the information we get is limited: size, but not mass, of planet; period, but not radius, of orbit

6. 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

7. 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

8. The Known Exoplanets (with known masses)

9. The Known Exoplanets (with known radii)

10. Planets with known mass AND radius

11. 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

12. Orbits of Exoplanets

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

14. 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

15. 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

16. 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.

17. 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 L⊙ (mostly in IR)
like many red dwarfs, it is a flare star, with occasional short bursts of higher luminosity (and X-rays)

18. 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

19. 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

20. 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

21. 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

22. 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