3/2/2020

Topics in Observational astrophysics Ian Parry, Lent 2020 Lecture 19 ‐ 1

• Our Solar System as a comparison reference. • Early discoveries: PSR1257+12 and 51 Peg. • Statistics of known exoplanets. • Discovery methods for exoplanets. • Radial velocities • Transits

Extra‐solar planets (exoplanets) • Very exciting, relatively new field of . • Exoplanets are, of course, planets not in orbit around the sun. • No strict definition to distinguish very low mass stars (i.e. brown dwarfs) from planets but basically they have low mass (<20 MJ) and are probably formed in the disk of debris left over from the formation of the star they orbit. • Over 4100 confirmed exoplanets now known. • There is at least one planet per star on average. • 10‐20% of solar‐type stars have a potentially habitable Earth‐sized planet. • We would like to understand the diversity of exoplanets (e.g. sizes, masses, structure and composition) and how they formed. • Ultimately, we would like to know how many Earth‐like planets there are and how common life is in the universe.

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Our solar system

• For comparison purposes let’s consider the basic facts about our own solar system. (Before 1991 we had no observational evidence of any planets outside our solar system). • 99% of the mass of the solar system is in our Sun with a mass of 2x1030 kg. Our Sun is a main sequence star of type G2V with an effective temperature, T ~ 6000K. • The mass in our planetary system is very much less than the mass in our central star, the Sun. There are eight planets (Pluto was demoted). • The planetary system is concentrated in a thin disk with orbits that range from 0.4 to 30 AU. The orbital sizes follow Bode’s law (each orbit is ~1.71 times bigger than the one inside it). • The angular momentum of the solar system is almost entirely contained in the orbits of the planets (mostly in Jupiter’s and Saturn’s). • The orbits are very close to being circular. • At distances < 2 AU there are four high‐density rocky planets whereas at distances > 5 AU the other four planets are low‐density gas giants. • Do we believe that our solar system is unique? Do we have any evidence that what happened to create our solar system might have happened elsewhere and created other planetary systems?

The Planets in our Solar System

8 light-minutes from Earth

4 light-hours from Sun

40 light-minutes Rsun ~ 10Rjup ~ 100REarth from Sun

Planet sequence shows increasing size (not distance from Sun). Sizes are to scale

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The 10 Largest Moons in our Solar System

Name Radius (km) Host Planet notes Ganymede 2634 Jupiter Bigger than Mercury (R=2440km) Titan 2575 Saturn Bigger than Mercury (R=2440km), has an atmosphere and surface liquid hydrocarbon lakes Calisto 2410 Jupiter Io 1822 Jupiter Our Moon 1737 Earth Europa 1561 Jupiter Probably has a sub‐surface water ocean Triton 1353 Neptune Titania 788 Uranus Rhea 764 Saturn Oberon 761 Uranus

Enceladus 252 Saturn Sub‐surface water ocean, ~28km deep Pluto 1188 ‐ Trans‐Neptunian Object (TNO) or dwarf planet

First exoplanets discovered • In 1991 Alexander Wolszczan discovered planets orbiting the pulsar PSR1257+12. • This pulsar gives off ~160 pulses per second and their arrival times can be measured with great accuracy. It’s a spun‐ up milli‐sec pulsar. • Periodic departures from the expected arrival times revealed 3 planets • 2 are as massive as the Earth and one is as massive as the moon. • 7 pulsar planets have now been detected (orbiting 4 of the ~3000 known pulsars).

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A pulsar planet fluorescing

The harsh radiation environment means the rocks of a pulsar planet will fluoresce if there’s no atmosphere.

On the other hand there could be surface water (life even?) if there is a substantial, shielding atmosphere.

Credit LYNETTE COOK / SCIENCE PHOTO LIBRARY

Exo‐planets orbiting normal stars • The first one was discovered in 1995 by Queloz and Major and it orbits the star 51 Peg. It was discovered using the radial velocity technique. • Today nearly 800 confirmed exoplanets are known via the radial velocity technique. • The Kepler spacecraft has discovered most known exoplanets using the method. • Searches find many Jupiter‐sized planets in small orbits. (“Hot Jupiters”) • Queloz and Major were awarded the Nobel prize for Physics in Oct 2019.

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Radial velocity curve for 51 Peg

Msini = 0.46 MJup M = 0.48 MJup T=1284K a=0.053 AU P=4.23 days I = 80 deg

51 Peg is a G2 IV star

51 Peg b is a “”

From NASA archive 1st March 2020

Most of the transit detections have come from Kepler

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From NASA exoplanet archive 1st March 2020

RNep ~ 3.8 REarth RSat ~ 9.4 REarth RJup ~ 11.2 REarth

MNep ~ 17 MEarth MSat ~ 95 MEarth MJup ~ 318 MEarth

Exoplanet Detection Methods • Dynamical perturbations (see the star wobble). • Radial velocities (spectroscopy) • Astrometry • Pulsar timing • Variations in total system light (photometry). • Transits • Phase curves (reflected light) • Polarised Flare echoes • Direct high resolution imaging (take a picture). Includes interferometry. • All direct imaging detections to date are from self‐luminous exoplanets • Micro‐lensing (photometry). • Cross‐correlation high resolution spectroscopy. • Finding gas disk gaps and warps due to planets. • Finding spectroscopic evidence of planetary material in the atmospheres of WDs. • Search for radio signals from exo‐civilizations (SETI). Highly unlikely to work.

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Exoplanet Detection Methods

Credit: Michael Perryman

• Planet causes star to orbit around the system’s barycentre. • This can be detected as a Doppler shift unless the system is perfectly face-on to our line of sight (the radial velocity technique). • This reflex motion can also potentially be detected as a very small astrometric shift.

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Radial velocities

• An exoplanet in orbit around a star causes the star to orbit around the barycentre of the system. • Measure Doppler shifts in the star’s spectral features, which track the line‐ of‐sight gravitational accelerations of a star caused by the planets orbiting it. • The motion of the telescope must be subtracted, the instrument must be calibrated, and spurious Doppler shifts “jitter” must be mitigated or corrected. • Two requirements for accurate instrumental calibration are: a) the stable spectrograph and b) absorption cell methods. • Spurious, apparent Doppler shifts due to non‐center‐of‐mass motion (jitter) can be the result of stellar magnetic activity or photospheric motions and granulation. • Several avoidance, mitigation, and correction strategies exist, including careful analysis of line shapes and radial velocity wavelength dependence.

Radial velocities Credit: Jason Wright, https://arxiv.org/pdf/1707.07983.pdf • K is the (semi-)amplitude of the radial velocity variations over the course of an orbit with period P. • f is the mass function • e is the eccentricity of the orbit • i is the inclination of the plane of the orbit with respect to the plane of the sky (i = 0 is face-on) • G is Newton’s gravitational constant

• Mseen and Munseen are the masses of the seen component (i.e. the star) and the unseen component (i.e the exoplanet).

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Measuring Radial Velocities

• Require a precision of ~1 m/s or better. • For example, the reflex motion of the Sun due to Saturn is about 3 m/s. • Use an Iodine cell (or a laser comb) in the light path to provide an ultra precise wavelength reference. • Use a spectrometer with a very high spectral resolution R=100,000=λ/Δλ. • Need stability over many years (>10).

Measuring Radial Velocities • The main limit to the precision of differential Doppler shift measurements is the wavelength calibration of the spectrograph. • Typical RV spectrographs resolve starlight with a power of R = λ/∆ λ ∼ 50,000 – 100,000, meaning that a shift of a single pixel corresponds to a change in radial velocity of 1 km/s. • Since giant planets change their host stars’s velocities by of order tens of m/s, one must measure shifts to a precision of 10−2 pixel for giant planets and two or three orders of magnitude better than that to detect Earth‐mass planets. • Since typical pixels on astronomical detectors are of order 15 µm across, one is measuring the “motions” of stellar lines to a fraction of a nanometer. • Need a combination of high stability and very accurate wavelength calibration to achieve this.

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Stability and Wavelength Calibration

• The RV instrument HARPS (Queloz et al 2001b) on ESO’s 3.6m telescope is stable to better than 1 m/s. • HARPS‐N is a copy of HARPS on the TNG in La Palma. • ESPRESSO (Pepe et al 2010) on ESO’s VLT is stable to 10 cm/s precision. • In the early days, Iodine cells were used for wavelength calibration. • More recently laser combs have been employed. • The basic measurement is the differential shift between the stellar spectral lines and the spectral lines of the calibrator.

Analog of Jupiter

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Detecting planets via transits

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For R*=Rsun and Rp=Rearth Depth = 10-4 Need a measurement precision of at least 3x10-4 (33 parts per million) or more like 10-5 (10 parts per million).

For R*=0.2 Rsun and Rp=RJup Depth = 0.25 and minimum measurement precision is 2.5x10-2 (one part in 40). These numbers show that transit searches are biased towards large planets and small stars. The bias is even more so when transit probability is taken in to account.

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Slide credit: Geoff Marcy

HD209458 • One of the systems (HD209458), that was first discovered through the Doppler technique, was later found to exhibit transits. • This must therefore have sini =1 giving an accurate determination of the mass. (0.69 MJ) • The size of the planet can be determined from the light curve. (1.347RJ). • Na I has been detected in the atmosphere of the planet (transit spectroscopy – see later). • The star is 2.3210‐4 times dimmer at the Na D wavelength (589.3nm) than at an adjacent wavelength.

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HST data

Light curve of HD209458 extra-solar planet transit

Exoplanet transit

• Transit measurements from ground-based telescopes are not so precise due to systematics. • Signal varies with atmospheric conditions. • Photon noise is not so much of an issue so small telescopes can be used. • They need a wide field of view to find transits.

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Extra Solar Planets: Indirect Detection: Transits

(Graphics from Suzanne Aigrain)

• We can work out the effects of different sizes of planets, the effects of the distance of the planet from the star and the effect of the spectral type on our ability to detect the transit. • The other problem is that the probability of a transit occurring is very low because it needs good alignment between the orbital inclination and the line of sight. There is a 1 in 1142 probability of a Jupiter transit being observed and for an earth transit this becomes a probability of 1 in 229. • Variations in transit timings can reveal the presence of Earth‐size planets in the so‐called “habitable zone”, even if the smaller planets do not transit.

Extra Solar Planets: Stellar variability: Transits

• Stellar variability is a serious problem when detecting transits. • It is due to the stellar revolution and the evolution of structures on the stellar disk. • We have not detected these small‐scale variations from Earth transits the ground except for young stars: for the Sun they have only been detected by spacecraft such as SOHO. ‐3 • The amplitude of these variations is approximately 10 (SoHO/VIRGO/PMO6) in the sun. • An earth transit would only produce a variation of Solar variability 8x10‐5. • The spectrum of the variability is complex and has a non‐white power spectrum.

• We need to know how it varies with mass, age, etc of Both... the stars being looked at. • It will affect all space‐based transit searches.

(Graphics from Suzanne Aigrain)

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