Planets and Brown Dwarfs

Transcription

1 Extra Solar Planets

2 Extra Solar Planets We have estimated there may be billion stars in Milky Way with Earth like planets, hospitable for life. But what evidence do we have that such planets even exist? 11 years ago we knew of no planets outside our own Solar System (aside from 1 or 2 planets around pulsars, not interesting in terms of life). We now know of 185 extrasolar planets (Mar 2006)

3 Planets and Brown Dwarfs We need to have a clear definition of what a planet is. We need to consider low mass stars and brown dwarfs (failed stars) (1) Stars: we define a star as an object massive enough to burn H in its core. This requires a mass > 0.08 solar masses (2) Brown Dwarfs: These are objects which formed similar to stars, but not big enough to fuse H. They can burn deuterium (D). Not clear how small brown dwarfs could be; could they be as small as planets? How do we distinguish planets and BDs? They form differently but this is hard to tell

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5 Decision: BDs are those objects big enough to burn D. BDs thus have masses > 13 Mjup and planets have masses < 13 Mjup

6 Ways to Detect Extra Solar Planets There are several possible ways to find ESPs: Detection of radio messages from intelligent civilizations: we will discuss this later (nothing so far!) Observe the planets directly (i.e. take a picture) Planetary transits: planet blocks off some of star's light Gravitational microlensing Wobbles in star's position or velocity caused by planet

7 Extra Solar Planets: The Doppler Technique Nearly all ESPs discovered using the Doppler technique (173) A star and giant planet both orbit together around their common center of mass. Star is much more massive, so star's orbit much smaller than planet's. Orbital Wobbles

8 So a giant planet will cause the star's orbit to wobble. The more massive the planet, the larger the wobbles. We will talk later about astrometric detections of such wobbles, which have not yielded results to date.

9 Doppler Technique Look for wobbles in radial velocity of the star, by Doppler shifting of its light (compare spectroscopic binary stars).

10 Doppler Technique Doppler technique only measures that part of star's velocity along the line of sight The changes in velocity are small and hard to measure, between few m/sec to ~100 m/sec Requires special spectroscopic setup: high spectroscopic resolution to measure such small velocities 1995: Swiss team (Michel Mayor & Didier Queloz) announced the first extra solar planet (51 Pegasus). An American team (Marcy, Butler & co) have found most extrasolar planets so far

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13 What Doppler Technique Tells Us (1) Period of the Doppler shifting gives us the orbital period (of star and planet)

14 (2) Shape of the lightcurve gives us the orbital eccentricity Nearly Circular Orbit

15 Very Elliptical/Eccentric Orbit

16 (3) Period + Star Velocity + Star Type gives: Planetary orbital radius Planet velocity Planet's Mass (nearly!): Msini, where i = inclination

17 (4) Combined with transit data also get: definitive planet mass, plus planet size and density

18 Gravitational Lensing Gravity bends light. Spectacular gravitational lensing seen in galaxy clusters

19 Gravitational Microlensing Less spectacular microlensing can be seen when objects such as brown dwarfs or planets pass in front of background stars

20 Duration depends on planet's mass and speed. So by measuring duration we find out the planet's mass Durations of a few hours for Jupiter sized planets Each event is one time only: you don't get a second chance! No information about planet's orbit (just its size) But microlensing is sensitive to Earth mass objects (unlike Doppler technique), with distances from 1 5 AU

21 A variation of this method is to search for secondary spikes in microlensed stars, caused by planets orbiting them lots of searches underway around the world, e.g. PLANET PLANET found that no more than 1/3 of all sunlike stars have Jupiter sized planets with orbits between AU (2001)

22 First Success!!! Probably a planet ~2 MJup a distance of ~3 AU from its star

23 More Microlensing Discoveries... 4 microlensed planets now known Two of these (below) are several Earth masses, orbits of a few AU such planets should be common, and detectable via microlensing Planets common around fainter M stars? OGLE Lb: Mp = 3 10 ME OGLE 2005 BLG 169b: Mp = 13 ME a=2 4 AU D=6.6 kpc Ms = 0.22 Msun a=3 AU Tp ~ 70K (cold!) Tp ~ 50K (like Pluto!)

24 Planetary Transits: A planet passing in front of a star can block and dim some of the star's light, something like 1 2% for a hot Jupiter, with durations of typically hours This is similar to studies of eclipsing binary stars: can get planet's size, distance from star, and orbital period. With velocity measurements, could then get planet's mass and hence density (rocky, gas giant?) Transit Animation

25 Advantages of Transits: Sensitive to Earth sized planets, unlike most other methods. Better than microlensing, because you can followup The geometry is known (edge on), simplifying things Disadvantages of Transits: Planet orbit has to be edge on to us to see transit. This will be rare, so lots of stars have to be monitored The brightness dip is small, so difficult to measure

26 Transit Results to Date: First transit seen (1991) was that in HD209458, a planet found earlier using the Doppler technique. With the velocity data, the planet's radius, mass, and density could be determined: it is definitely a gas giant HST spectra found Na, H, O, and C in the planet's upper atmosphere, which is escaping from the star (because the planet is so near its star, and thus so hot)

27 HD209458

28 Further Transit Discoveries 5 found by Optical Gravitational Lensing Experiment (OGLE) These have masses MJ, sizes ~ Jupiter, a= AU, periods 1 4 days, T: deg Hot Jupiters Many transit experiments in progress, and planned space missions (later)

29 TrES 1 Found in 2003 by STARE project using 10cm telescope! M = 0.75 MJ, a = 0.04 AU, P = 3 days (Hot Jupiter) Spitzer Space Telescope detected IR photons from this planet

30 Transit Searches in Globular Clusters 47 Tuc has been searched for planets by HST (Gilliland et al. 2000). None found! But they only searched a small region near the cluster center I'm involved in a search for planets over a much larger area in two GCs: in progress but no planets so far! Looks like stellar metallicity is very important for planet formation

31 Proper Motion (astrometry): Stars that are close enough to us to have observable proper motions are candidates. Idea is to look for wobbles in their motions across the sky caused by a massive planet orbiting them

32 Proper Motion Very difficult to measure because wobbles are small. Most sensitive to massive planets But planned for ground based interferometers. E.g. Keck hopes to detect wobbles < 3000 km at the distances of the nearest stars. Could detect Uranus size planets around stars up to 60 light years away. Also VLT But no planets discovered yet... Space Interferometry Mission (SIM) should be able to detect Earth size planets! Also GAIA.

33 Imaging Extra Solar Planets Observing planets directly is hard! Planets shine mostly by reflected light Planets are ~1 billion times fainter than star Planets are very close to their stars (1 AU at 1 pc is 1 in angular size (atmospheric resolution limit). Need very high angular resolution and blocking of light from star. Possible from the ground with extreme adaptive optics and a coronagraph (e.g. Gemini), or interferometery. Even better in space (e.g. JWST, Terrestrial Planet Finder, SIM) Works best in the IR, where the contrast between star and planet is lowest, and with smaller, fainter stars

34 Success: 2M1207b Imaged with the VLT in 2004 and confirmed in Star is (faint) Brown Dwarf 5± 1 M ;1.5 R, 41 AU, D=53 pc J J Spectrum shows evidence for water absorption

35 Two Other Possibilities AB Pictoris 13.5 ± 0.5 MJ, a = 275 AU GQ Lupi 22 ±21 MJ a = 103 AU R = 2RJ

36 A niche for each technique C. Lineweaver

37 Properties of Extrasolar Planets As of Mar 2006, 185 planets have been found around ~150 stars, 149 of these planets using the Doppler technique At least 10% of stars surveyed have detected planets (fraction depends on stellar metallicity see below) Orbital periods from few to thousands of days! 18 stars have multiple planets (2 or more planets) Almost all giant planets: most techniques are sensitive to massive planets close to their stars (Earth mass planets difficult at present time). This is an important selection effect we have to bear in mind.

38 Host Star Properties Stars richer in heavy elements are more likely to host planets Most planets have been found around Sun like stars (F, G, K) but this is partly a selection bias: planets are now found around fainter M stars 18 systems are multiple star (2 or more stars): it is possible to have planets in such systems (but are these planets habitable?)

39 ESP Masses

40 ESP Sizes and Densities Density: 0.1 to 2.5 times that of Jupiter: Not silicate iron composition But not many ESPs have measured radii (transits, microlensing)

41 ESP Semi Major Axes

42 ESP Semi Major Axes Most ESPs are very close to their host stars! Compare Mercury at 0.39 AU Hot Jupiters!

43 ESP Orbital Eccentricities Many ESPs are on very eccentric orbits! Unlike our solar system Not good for life: extreme hot/cold cycles

44 Puzzles Our planet formation model predicts near circular orbits: planets form from condensations in rotating disk of gas and dust Compare our solar system: gas giants much further out partly a selection effect: most sensitive to massive, inner planets; but will improve with time Still, this doesn't explain everything: how did these planets get so close? Unlikely they could have formed so close. just too hot for material to condense to form gas giants

45 Current thinking: They formed out at several AU, then migrated inward due to tidal/friction effects in solar nebula Type I migration: interaction between giant planet and circumstellar gas/dust disk pushes planet inwards Type II migration: Gap in disk opens and migration slows Have to halt the process: removal of disk; tidal/magnetic interactions between planet/star/disk Multiple giant cases can explain high eccentricity orbits by resonances or close encounters between giants Not clear how difficult it is for Earth mass planets to form and survive under giant planet migration Quite likely every planetary system has lost planets... Type II Migration

46 Implications For finding Earth like Planets Having Jupiter sized planets in elliptical orbits, near their host stars really decreases chances of forming Earth sized planets We need: Jupiter sized planets in Jupiter orbits (circular and >5 AU from their stars)! Only then can we be (more) secure that Earth sized planets can form within the habitable zone Doppler technique biased against finding such planets: ones closer to their stars having faster periods and easier to detect However, ~12+ years on, we are now starting to find such Jupiter analogues!

47 55 Cancri: a 3 planet system! (1) 15 days, 0.84 MJ, AU (2) 44 days, 0.21 MJ, 0.24 AU (3) 14 yrs!, 4 MJ, 5.9 AU Compare Jupiter at 5.2 AU, e = 0.04, P = 11.9 years

48 55 Cancri (AAT Planet Search Program)

49 Some Thoughts Planets could still exist outside giant planets (but far from star) Earth like planets may be captured by giant planets (like Trojan asteroids), and go along for the ride to inner solar system Moons of the giant planets may be suitable for life We have always to be aware of the selection techniques in the Doppler technique: we are discovering more planets as time goes on. At least ~10% of stars studied so far have planets, and this fraction will only increase with time So it's impossible to estimate number of Earth like planets in our Galaxy from the Doppler data

50 Ways to Find Earth Mass Planets Doppler technique (on current telescopes) will never find Earth like planets: their velocity wobbles are simply too low We need different techniques: we have discussed two already that should yield Earth size planets: transits and microlensing lots of ground based programs at the moment Space based missions hold much promise: Transit: COROT, Eddington, Kepler (within next few years) Interferometry: SIM, TPF, Darwin (within next years) The interferometry instruments and/or large ground based telescopes should be able to directly detect extra solar planets and life gases in their atmospheres!

51 Kepler (2007)

52 SIM (2009)

53 Terrestrial Planet Finder (TPF) and Darwin (Next Decade)

54 Detecting Life on Exoplanets? Direct sampling: send a probe! Pretty damn hard... IR spectrum of exoplanet gives temperature of surface and/or atmosphere, and atmospheric composition If H2O vapour and CO2 found, and if temp right for liquid water and carbon compounds conditions good for LAWKI Strong O3 (ozone) would indicate O2 and a biosphere (e.g. oxygenic photosynthesis). Even stronger evidence: existence of redox pairs such as O2 and CH4 (i.e. out of chemical equilibrium) lack of O2 does not mean there's no biosphere! Spectra might also detect atmospheric gases or effects of chlorophyll, or by changes in the light curve with time

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