2 1.0 Introduction There are a lot of slides in this lecture. Much of this should be familiar from PHY104 (Introduction to Astrophysics) and PHY106 (The Solar System). The key point is to understand the basic methods behind the Doppler, Transit, and Direct Detection methods. I have included some maths, but you are not required to learn this, it is just useful to make some points (such as the Doppler method gives you the minimum mass of a planet mp sin i). The list of questions I can ask you will tell you the sort of material that you need to know.
3 1.1 What is a planet? What isn't a planet: STARS: STARS a object that will, is, or has produced energy through hydrogen fusion: Upper limit of ~300+ Msun (Eddington limit) Lower limit of ~0.075 Msun (H-burning limit) BROWN DWARFS: DWARFS an object whose interior is too cool to ever fuse hydrogen, but are capable of fusing deuterium Upper limit of ~0.075 Msun, Lower limit of ~0.02 Msun (~20 MJup).
4 1.1 What is a planet? Working definition of a planet: 1. A planet orbits a star. 2. A planet is too small to fuse deuterium: < 2 x 1028 kg (~20 MJup) 3. A planet is large enough to a) be spherical (in hydrostatic equilibirum) b) clear its own orbit of debris > 1023 kg (~Mercury)
5 1.1 What is a planet? In exoplanet studies planets are divided into four basic categories: 1. Gas Giant planets (>20 Earth masses) Mostly H and He (e.g. Jupiter & Saturn), no solid surface, probably not good places to look for life. However, they may have life on moons (e.g. Europa or Titan) 2. Ice Giant Planets (10-20 Earth masses) Mostly volatiles (water, methane, ammonia) with a H-He atmosphere. Again, no solid surface, but might have suitable moons (less likely as the moons are probably smaller, and tidal heating is less).
6 1.1 What is a planet? 3. Super-Earths (2-10 Earth masses) Solid, but massive planets (nothing like them in the Solar System). Possibly the most common type of planet. Probably a rock-volatile mix (ocean worlds?). 4. Terrestrial planets/earth-like planets (<2 Earth masses) Planets like the Earth, solid, maybe largely rocky, and a place we know life can exist. Difficult to find, but we're starting to find them...
7 1.2 Detecting extrasolar planets As of Feb 2016 we have confirmed detections of over 2000 exoplanets. See for latest numbers and data. The most successful methods so far are Doppler (radial velocity) variations (~630 planets) Transits (~1300 planets) Direct imaging (~65 planets) Microlensing (~40 planets) Pulsar timing (~25 planets) Transit timing variation (~5 planets) Note that many transits are also Doppler planets and vice-versa.
8 1.3 Doppler method This looks for line-of-sight variations in lines due to the influence of a planet (see PHY104).
9 1.3 Doppler method Both the star and planet orbit a common centre of mass with position: therefore the star and planet will both have an orbital velocity which depends on their distance from the centre of mass. We measure the line-of-sight component of the star's velocity and the period of the orbit. V i V sin i
10 1.3 Doppler method First planet detected in 1995: 51 Peg: Peg maximum v sin i = 60 m s-1 period = 4.23 days => planet of mass ~0.5 MJup
11 1.3 Doppler method We can use the generic form of Kepler's third law: and knowing the period find the semi-major axis of the planet's orbit: the period is related to the velocity so: and so the minimum mass of the planet is:
12 1.3 Doppler method Things become more complex with elliptical orbits as there is no simple analytic fit to the velocity curve.
13 1.3 Doppler method The fit also depends on eccentricity and viewing angle. To deal with this we use computers to find a best fit.
14 1.3 Doppler method When there are multiple planets then we have several radial velocity Variations imposed on each-other. The outer 2 planets of UpAnd:
15 1.3 Doppler method summary Planets can be detected indirectly by looking for periodic radial velocity variations of the parent star. This method gives the most information. The period of the variation gives the semi-major axis of the orbit. The shape of the radial velocity curve gives the eccentricity of the orbit (a sine curve indicating a circular orbit). The line-of-sight velocity then gives a minimum planetary mass M sin i, that depends on the inclination.
16 1.4 Transits Attempt to detect planets when they pass in front of a star: Variations are the same in all bands so can be distinguished from pulsations or star spots. Transit of HD209458
17 1.4 Transits In order for a planet to be seen in transit, the inclination of the system must be low (how low depends on the orbital distance), but is around 1-2o for planets at ~1 au. This means a few % of systems should be inclined such that they transit. ap (rstar~7 x108 m, ap~1011 m) i rstar
18 1.4 Transits Required sensitivity: what is the decrease in light due to a planet? This depends on the size of the planet and the size of the star. For a Sun-like star: An Earth-like planet (r~107 m) will block ~0.01% of the starlight, a Jupiter-like planet (r~108 m) will block ~1% of the starlight. For an M-dwarf (only about a Jupiter radius): An Earth-like planet could block 1% of the starlight easy to see! But transits will only last a few hours, depending on the size of star and the orbital velocity, and so stars need to be monitored constantly. Transits allow atmospheric transmission spectra to be taken to look for life... we'll come back to this later!
19 1.4 Transits Kepler and CoROT are space-based transit searches. Also superwasp and MEarth are ground-based searches (MEarth is looking around M-dwarfs). Kepler 'died' in May 2013 but the huge amounts of data are still being analysed (it sort-of-ish still works).
20 1.4 Transit method summary Planets can be detected indirectly by looking for periodic luminosity variations of the parent star. This method gives the most information. The period of the variation gives the period of the orbit (gives a rough semi-major axis, but without the eccentricity information). The depth of the variation gives the radius of the planet. If we have Doppler and Transit then this sets a limit on i of close to 90o which gives a true mass (sin i ~ 1) and hence a density.
21 1.5 Direct detection: imaging So far directly imaged planets tend to be wide and large (most are probably more properly described as brown dwarfs).
22 1.5 Direct detection: imaging With current instruments we need to remove the star and Adaptive Optics to reach the diffraction limit of ~10m telescopes. We need bigger telescopes...
23 1.6 Future prospects We are starting to reach the limits of what we can do with what we have. Modern instruments can get down to velocity resolutions of <1 m/s, this can be pushed lower but stellar oscillations and variability start to become important. The big problem is timescales of distant planets (decades). To push transits we really need space-based missions for constant monitoring. We would really like to directly image more (and closer) planets.
24 1.6 Astrometry: GAIA GAIA has 10-5 arcsec resolution mapping the positions of the 109 brightest objects (V magnitudes < 20) in the sky will find close massive planets out to a kpc! First data release in 2016 (final in 2022). Should find significant numbers of close-in (<5 au) giant planets.
25 1.6 Future transit missions CHEOPS (ESA, 2017 launch?): targeting known bright planet host stars to get accurate radii (and densities) of Ice Giants and superearths in particular. TESS (NASA, 2017 launch?): targeting all-sky nearby bright stars looking for transits.
26 1.6 Huge telescopes The big game-changers will be the next generation of telescopes E-ELT (ESO, 40m, 2024 first light?) TMT (US/China/Japan+, 30m, 2022 first light?) GMT (various, 25m, 2020 first light?). All will have advanced adaptive optics to reach the diffraction limit.
27 1.6 E-ELT The reasons for going big are: Collecting area = sensitivity and speed. Angular resolution down to arcsec (instrument and wavelength dependent). (Picture below for 100m OWL not E-ELT.)
28 1.6 Planet finding Current technology: Ground-based doppler method (~few m s-1), ground and space-based transit searches. With longer time baselines we should find more massive planets in distant orbits, and possibly the first Earth-like planets. Transits are finding more planets and direct imaging is starting to become useful. Near future: Space-based astrometry should find most close-in gas giants within a few kpc! Medium-term: Extremely large telescopes (30+m-class) to find Earthlike planets and image giant planets. Will also be able to do spectroscopy on the planets atmospheres!
29 1.7 Summary It is important to know the basics of how we find planets. At the moment we mostly rely on the Doppler and Transit methods, but the next generation of telescopes will push direct detection down to 1AU. Each method has its limitations and biases. What we find is limited by how we can search. This is crucial in interpreting the observations. As you can see in the list of questions you are never asked to derive an equation. But you often need to know something about what is detected and what the limitations are.