Finding exoplanets (planets orbi2ng other stars)

Transcription

1 Finding exoplanets (planets orbi2ng other stars) Why is this hard? Well, take the Earth and the Sun? The Sun is bright, Earth is faint; Earth doesn t weigh much 1/300,000 th of Sun. As seen from nearest star, Earth is very close to the Sun Earth is 0.7 arcsec away So Earth is faint and near a bright object, taking a photo is very hard. Earth doesn t weigh much, it can t push the Sun around much gravita2onally.

2 Nonetheless, we have succeeded, how did we do it? Technique 1: the radial velocity method center of mass of earth-sun system is only 300 miles from center of sun. Sun only moves at a speed of 10 cm/s (5 inches/sec) due to Earth s pull on it. This measurement is beyond current instrumental capabili2es. But Jupiter would move Sun 13 meters/sec, and if Jupiter were where Earth is, it would move Sun 70 m/s. The later is easy, the former was hard 2ll a few years ago.

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5 First discovery, 1995 by a Swiss group/ The star was a innocuous star called 51 Peg. The planet, called 51Peg B, is 0.05 AU from the parent star (Merucry is 0.4 AU from Sun), orbits in 4 days, and weighs 0.5 Jupiter masses (150 Earth masses). So it s a Jupiter inside the orbit of Mercury. Well, that was unexpected. Similar searches over the next few years found hundreds Og=f such Hot Jupiters. The Solar System has no hot Jupiters. OK, but does 51 Peg have other planets, maybe an Earth, maybe a Jupiter at 5AU where it belongs?

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7 A litle out of date, But you get the idea

8 Why is it easy to find hot Jupiters and hard to find Earths at 1AU or Jupiters at 5AU? How are we going to understand the existence of these hot Jupiters? How do you make a gas giant interior to Mercury? Well, you move it there! So our simplis2c Astro 100 model of the Solar System, as presented in class, is deficient. There was a disk of gas and dust out of which the planets form. When a planet orbits, it slams into that stuff, and it creates waves in the distribu2on of dust.

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11 Both of these effects cause the planet to lose orbital energy and spiral in. So whether you get hot Jupiters or not depends on how quickly the planetary disk cleared out. Yes, some planets can spiral all the way into the star. So planetary migra2on is one addi2on to the old, simple picture. Another addi2on is the understanding that planets can be ejected from the solar system en2rely. Or, planets can migrate inward by flinging smaller bodies outward. So Jupiter may not be in its birth place, but Circumstances weren t right for it to become a hot Jupiter.

12 Technique 2: the transit technique. This is exemplified (but is not the only example) by the Kepler satellite. The idea is that some small frac2on (a few percent at best) of stars and their planets are lined up so that, as seen by us, the planets pass in front of the star, causing a small (1% or less) dimming of the star. If we measure the brightness of that star oien enough and if our tool is stable enough, we can derive the size of the orbit and the size of the planet. One can get the mass from Technique 1 followup measurements. One problem is that there are false detec2ons, namely, variable stars, sunspots, and binary stars. But that can be dealt with aier some prac2ce.

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14 htp://phl.upr.edu/library/notes/currentnumberokabitableexoplanets16

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16 The Kepler Satellite Orrery htp:// v=qrj30nyiu4 Another visualiza2on htps://vimeo.com/ /

17 Technique 3: Direct Imaging Well, at the beginning we pointed out that planets are faint and stars are bright. We also pointed out a couple of classes ago that Jupiter is s2ll forming. Young, massive Jupiters (so bigger than Jupiter) give off a lot of light. So if you can suppress the light from that really bright star (even young massive Jupiters are faint compared to their parent star) you can actually take a picture. Main problem aier that is to show that the star and the alleged planet are physically related. But over 2me you can watch the planet orbit the star.

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22 The nice thing about these techniques is that they re complimentary. Radial velocity and transits find inner planets more easily, while imaging finds outer planets more easily. But imaging is s2ll hard because the stars are so darn bright. Transi2ng planets allow you to actually measure the atmosphere of the planets.

23 Holy grail is an earth-mass planet in the habitable zone, namely, at the distance from the star where water would be liquid. We re gepng close. We have found a few super earths, and the Kepler spacecrai should deliver a few earths. We have found a few planets in the habitable zone, Most as gas giants, but some are super-earths. Other techniques, in concert with these, allow you to deduce that the typical star actually has at least one planet orbi2ng it. And about 10% of sun-like stars studied to date have a fairly massive planet orbi2ng them. We have also found several stars with mul2ple planets. So planets are not rare (even though Earth-like planets are s2ll tough/impossible to detect). Our solar system is not unique, though we had to learn about hot Jupiters first.

24 The next genera2on of missions With specialized techniques we can image some planets now. But again, the Holy Grail is to image and study other Earths. If we could put up several satellites and combine their light (remember interferometry from telescopes lecture?) we can get very high resolu2on and block starlight. Steward Observatory (Dr Phil Hinz and others) have developed and demonstrated a technique called NULLING INTERFEROMETRY where you combine the light from 2 telescopes in such a way that the image of the star disappears (destruc2ve interference), allowing you to see faint stuff around the star. You have to make the star disappear at a really exac2ng level, nothing can be lei behind. One such mission, SIM, was funded for years and then un-funded. TPF is not funded except for perhaps a litle technology development money.

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26 Gravita2onal lensing: One of the predic2ons, shown to be correct, of Einstein s General Rela2vity, is that light is bent by gravity. There are 2 regimes, strong lensing and weak lensing. We ll start (and we ll return to this later in the course) with strong lensing. The cluster of galaxies Abell 1689 is bending light from more distant objects.

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28 But here we want to talk about microlensing. Any object, a planet, a star, a black hole, etc, bends light. A star doesn t bend it very much. But, it magnifies the image by diver2ng some of the light rays that would have passed us by, much as a megaphone diverts sound waves and makes your voice louder. Unlike strong lensing, the magnifica2on is all you get. And the lineup of us/lens/source has to be perfect. Well, microlensing was detected by the MACHO project 20-ish years ago. But wait, there s more

29 It turns out that if the lens has a planet, the planet can cause lensing of its own. And even beter, the equa2ons of lensing Show there are special places called caus2cs, where the lensing is large. So you can have an object gepng brighter because of lensing, and then brighter again as the planet crosses these special places.

30 This is a prety cool link, you can go there yourself: htp://planetquest.jpl.nasa.gov/system/interactable/11/index.html

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