3677 Life in the Universe: Extra-solar planets

1. 3677 Life in the Universe:Extra-solar planets Dr. Matt Burleigh www.star.le.ac.uk/mrb1/lectures.html

2. Course outline • Lecture 1 • Definition of a planet • A little history • Pulsar planets • Doppler “wobble” (radial velocity) technique • Lecture 2 • Transiting planets • Transit search projects • Detecting the atmospheres of transiting planets: secondary eclipses & transmission spectroscopy • Transit timing variations

3. Course outline • Lecture 3 • Microlensing • Direct Imaging • Other methods: astrometry, eclipse timing • Planets around evolved stars • Lecture 4 • Statistics: mass and orbital distributions, incidence of solar systems, etc. • Hot Jupiters • Super-Earths • Planetary formation • Planetary atmospheres • The host stars

4. Course outline • Lecture 5 • The quest for an Earth-like planet • Habitable zones • Results from the Kepler mission • How common are rocky planets? • Amazing solar systems • Biomarkers • Future telescopes and space missions

5. Useful numbers • RSun = 6.995x108m • Rjup= 6.9961x107m ~ 0.1RSun • Rnep= 2.4622x107m ~ 4Rearth • Rearth= 6.371x106m ~ 0.1Rjup ~ 0.01RSun • MSun= 1.989x1030kg • Mjup= 1.898x1027kg ~ 0.001MSun = 317.8Mearth • Mnep= 1.02x1026kg ~ 5x10-5MSun~ 0.05Mjup = 17.15Mearth • Mearth= 5.97x1024kg = 3x10-6MSun = 3.14x10-3Mjup • 1AU = 1.496x1011m • 1 day = 86400s

6. Blue: radial velocity, Green: transiting, Red: microlensing, Orange: direct imaging, Yellow: pulsar timing

8. Gravitational microlensing • When a foreground star passes in front of a background star, light from the background star is bent by the gravitational field of a foreground lens to create distorted, multiple and/or brightened images • Consequence of general relativity • The milli-arcsecond separation between multiple images is too small to be resolved by modern telescopes. The combined light of all images is instead observed as a single image of the source • The brightness of the combined image is a function of the projected separation of the source and lens on the sky, and changes as the source, lens and observer move relative to one another

9. Gravitational microlensing • If the lens is a single, isolated object, the lightcurve of the background source is simple, smooth and symmetric. • The background star appears to brighten and then dim as the projected separation between the source and lens first decreases and then increases. • For sources and microlenses in our own Galaxy, a typical timescale for the detectable rise and fall of the apparent brightness of the source star is weeks to months. • The basic shape is the same regardless of the relative path the source takes on the sky; the amplitude of the the lightcurve is determined by the minimum angular separation between the lens and source in units of the Einstein radius, ie θLS/θE .

10. Gravitational microlensing • If the star has planets, the magnification pattern experienced by a background source is no longer circularly symmetric on the sky • The combined gravitational field of the star and planet can create strong deviations in the lensing pattern, called caustics • This means that the changes in the lightcurve of the background source can be quite dramatic if it does happen to cross the planet-affected area, even for Earth-sized planets. • In the diagram, the red patches are the caustics and P indicates the position of the planet • Because the planet has a gravitational mass that is much smaller than that of the lensing star, the percentage of the lensing pattern area influenced by the planet will be relatively small. • This means that the probability that the source will cross the planet-affected area is low, and thus the chance of detecting a planet by microlensing is also low,

11. Gravitational microlensing • Beginning in the 1990s, millions of stars have been monitored every night in search of the few that are microlensed at that time • 30 planets found • Microlensing gives the mass ratio between the planet and its parent star, q=Mp/M∗ , and the angular separation between the planet and star on the sky at the time of the lensing event, θ∗,p/θE , in units of the Einstein ring radius • M* is obtained from the spectral type of the lensing star • The star’s proper motion gives the time to cross the Einstein ring

12. Gravitational microlensing • Advantages • Can detect Earth-size planets across Galaxy • Can detect planets in other galaxies • Disadvantages • Must monitor millions of stars constantly (eg in galactic bulge) • Lensing event never repeats • Star too far away to study planet again

13. Astrometry • The motion of a star around the centre of mass of a star-planet system can be detected by repeatedly measuring the position of the star on the sky • The amplitude of the motion in micro arseconds (10-6 arcsec) is given by: • Where q=(Mpl/M*), a = semi-major axis of the orbit in AU and d = distance to star in pc • For Jupiter at 5AU, the amplitude of the Sun’s motion as seen from another star is ~5x10-4 arcsec (right) • GAIA will discover ~10,000 Jupiters at 1-4AU around stars up to 200pc away

14. Other methods • Pulsation timing: • Many stars, like white dwarfs, have very stable pulsation modes. The presence of a planet will be revealed in anomalous timings, just as with pulsar planets • Eclipse timing: • Close, eclipsing binary systems can also reveal the presence of planets through anomalous timings of the expected eclipses • A good example is the close eclipsing white dwarf + red dwarfs binary NN Ser, which appears to have 2 Jupiter mass planets orbiting it

15. Direct detection • Imaging = spectroscopy = physics: composition & structure • Difficult • Why? • Stars like the Sun are billions of times brighter than planets • Planets and stars lie very close together on the sky • At 10pc Jupiter and the Sun are separated by 0.5”

16. Direct detection • Problem 1: • Stars bright, planets faint • Solution: • Block starlight with a coronagraph • Problem 2: • Earth’s atmosphere distorts starlight, reduces resolution • Solution: • Adaptive optics, Interferometry – difficult, expensive • Or look around very young and/or intrinsically faint stars (not Sun-like)

17. First directly imaged planet? • 2M1207 in TW Hya association • Discovered at ESO VLT in Chile • 25Mjup Brown dwarf + 5Mjup “planet” • Distance ~55pc • Very young cluster ~10M years • Physical separation ~55AU • A brown dwarf is a failed star • Can this really be called a planet? • Formation mechanism may be crucial!

18. First directly imaged planetary system • In 2008 3 planets imaged around the star HR8799 • 130 light years away (40pc) • Three planets at 24, 38 and 68AU separation • In comparison, Jupiter is at 5AU and Neptune at 30AU • Masses of 7Mjup, 10Mjup and 10Mjup • Young: 60Myr • Earth is ~4.5Gyr

19. Fomalhaut (alpha Piscis Austrini) • One of the brightest stars in the southern sky • Long known to have a dusty debris disk • Shape of disk suggested presence of planet • 2Mjup planet imaged by HST inside disk • 200Myr old • Like early solar system

20. Direct detection: White Dwarfs • White dwarfs are the end state of stars like the Sun • What will happen to the solar system in the future? • WDs are 1,000-10,000 times fainter than Sun-like stars • contrast problem reduced • Over 100 WD within 20pc • At 10pc a separation of 100AU = 10” on sky • Planets should be located well away from the host white dwarf • At Leicester we are searching for planets around nearby WD with 8m telescopes and the Spitzer space telescope

22. The end of our solar system -4000 -2000 0 Solar Radii

23. The end of our solar system • The inner planets, Mercury, Venus and probably Earth, will be destroyed by the expanding red giant • As a red giant (actually, asymptotic giant), the Sun’s radius will be ~1AU • Mars, the asteroids and outer gas giants will survive • As the red giant loses mass when it evolves to the planetary nebula stage, the outer planets orbits evolve outwards by factor: • Jeans (1924): • Where MMS and MWDare the main sequence and white dwarf masses in solar mass units (Msun) • Note: the relationship between a star’s mass and a white dwarf’s mass is given by: • This is called the “initial-final mass relation”, and is derived from observations of white dwarfs in clusters (Casewell et al., 2009, MNRAS, 395, 1795)

24. Spitzer 4.5micron imageGJ3483 (LTT3059 / WD0806-661) I am the white dwarf 130” / 2500AU I maybe a planet… or a brown dwarf

27. Proper motion • PM error +/-25mas/yr WD Companion

28. Calculating the planet’s mass • How can we estimate the mass of a directly imaged planet? • Planets of identical mass are assumed to be born with identical temperatures, & cool with age (no nuclear burning in core) • Thus by measuring their brightness, and estimating the host star’s age & distance, we can use a theoretical “evolutionary model” to convert the brightness to a mass! • Method: • (1) measure it’s brightness from the image • (2) determine the star’s distance (eg from it’s spectral type if its main sequence, or better still from its parallax) • (3) convert the star’s apparent mag to absolute mag • (4) estimate the star’s age (eg from it’s rotation period, or if it belongs to an open cluster or coeval moving group) • (5) compare the absolute mag to evolutionary model predicted masses and luminosities for the correct age • Caveats: • Ages of main sequence stars are notoriously difficult to measure • There is no guarantee that two planets of the same age and mass will have the same atmospheric chemistry, structure and temperature • Evolutionary models are only as good as the input physics and assumptions, and are particularly poor at predicting masses at very young ages (few million years)

29. Calculating the planet’s mass: example: GJ3483b • Measure apparent magnitude of object in Spitzer’s 4.5micron filter (“Band 2”) • Find m = 16.75 • We know the distance d to the white dwarf star from its parallax (it’s 19.2parsecs away) • So we can convert the apparent mag to an absolute mag • Absolute mag M is magnitude at 10pc • Use m – M = 5 log d – 5 (Pogson’s equation) • Find M = 15.33 • We also know how old the white dwarf is from its temperature (white dwarfs cool steadily with time) – 2Gyr • Look up a theoretical model which predicts brightness of gas giant planets at different ages • t (Gyr) = 2.00 • -------------------------------------------------------------------------------- • M/Ms Teff L/Ls gIRS Blue IRS red Band1 Band2 Band3 Band4 • 0.0080 370. -6.73 4.29 18.25 18.74 18.93 15.44 16.99 18.16 • 0.0090 391. -6.64 4.35 17.99 18.55 18.65 15.20 16.76 17.87 • Jupiter is about 0.001M/Ms, so our planet GJ3483b is between 8-9 times the mass of Jupiter!

30. What did the original system look like? • The white dwarf GJ3483 has a mass of 0.58MSun • From the intial-final mass relation • The progenitor main sequence star had a mass of 1.83MSun (a late A or early F star) • The white dwarf and planet are separated by 130” • At 19.2pc, 130” = 2500AU • (Note 1AU = 1” at a distance of 1pc) • Using the Jean’s relation between the initial and final separations the planet was originally located at a separation: 2500AU / (MMS/MWD) = 790AU • Still a very large solar system!