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Extrasolar Planets Dimitar D Vol 451|3 January 2008 Q&A ESA/C. CARREAU ESA/C. ASTRONOMY Extrasolar planets Dimitar D. Sasselov Hundreds of planets are known to orbit stars other than the Sun, and unprecedented observations of their atmospheres and structures are being made. It’s an invaluable opening for understanding the planets’ diverse natures, the formation of our Solar System, and the possibility of habitable planets beyond our home. 2 How many planets outside our Solar for at least one complete orbit of the planet. A will dim the star’s light by a fraction (Rp/Rs) , System have we found so far? wobble can also be detected directly by care- where Rp and Rs are the respective radii of the The number is going up week by week, but at fully observing the position of the star with planet and star. For a planet the size of Jupiter the moment there are about 270 confirmed respect to other stars (a technique known as and a star the size of the Sun, this dimming extrasolar planets — exoplanets, as they’re astrometry), but this is technically even more effect would be roughly 1%, which is easily known in the jargon. Not bad, considering the demanding. detectable even with amateur equipment. The first one was discovered just 13 years ago, and catch is that a transit requires the planet’s orbit they’re not that easy to spot. What other ways are there of finding to be almost exactly edge-on as we look at the exoplanets, besides the Doppler effect? star, which is extremely unlikely. Tens of thou- What makes exoplanets difficult to find? Gravitational lensing is another common sands of stars must be monitored with patience They are, by definition, far away and orbiting method, and also exploits the star–planet mass to detect periodic dimming in just a handful. close to a star that is far bigger and brighter ratio. For this, a source of light is needed, usu- than them. Light contrast ratios of 1010 (at ally another star, behind the star being inves- What can these crude remote-sensing visible wavelengths) to 107 (in the infrared) tigated. As we look at it, the light from the techniques tell us about exoplanets? between a star and planet make direct detec- background star will be bent by the gravity of A surprising amount. The Doppler technique tion by imaging extremely hard. Exploiting the intervening star. If the intervening star has gives the size, period and eccentricity (devia- mass ratios, which are generally in the region an attendant planet, this will alter the lensing tion from a perfect circle) of the planet’s orbit, 3 5 of 10 –10 , is slightly less daunting. Indeed, the effect in a noticeable way. as well as a quantity Mpsin(i) that contains most popular way of spotting an exoplanet is both the planet’s mass Mp and the inclination, through a star’s ‘wobble’, which is caused by the And what about the transiting method? i, of its orbit to our line of sight. The transit gravitational pull of an orbiting planet. This Transiting is perhaps the most fruitful method technique contributes the planet’s radius and a is measured through the Doppler effect — a of observation, because of the information we direct value for i. Combining these two param- shift in wavelength — in the spectrum of vis- can glean about the planet. It exploits the least eters can thus provide the actual value of Mp, ible light from the star. It’s a tiny effect, propor- daunting inequality between star and planet and also — a big prize, this, because it tells us tional to the mass ratio. Measuring it requires — the factor 10 to 100 difference in size. As it something about what the planet is made of patience, because the wobble must be followed passes across the disk of its parent star, a planet — the planet’s mean density, calculated from 29 NEWS & VIEWS Q&A NATURE|Vol 451|3 January 2008 is easy — it probably has a larger core, with log planetary mass (Jupiter masses) more heavy elements in general. But for exo- a Rocky planet 0.01 0.1 1 planets that are less dense, there comes a point Crust 12 TrES-4 when even a planet made just of hydrogen isn’t 1.6 enough to explain its low density. How can we get around this difficulty? 10 HD 209458b 1.4 A possible explanation is additional heating, either because of some persistent heat source Silicate Iron HD 189733b 1.2 or a seriously delayed cooling of the planet. mantle core /He 8 ure H P adii) One obvious source of heat is the parent star km) 4 b Ocean planet 1 itself: the exoplanets in Figure 1 are mainly Water ocean 10 Jupiter × ‘hot Jupiters’ that orbit extremely close to their ( Jupiter r 6 e stars. But then it’s unclear why some planets dius H Saturn % H/ 0.8 0 dius ( ra 5 would absorb much of that heat and others ra would not. The same problem — accounting Gliese 436b 0.6 for the entire range of observed planetary sizes 4 H/He Planetary Neptune 10% — also bedevils other proposed solutions. Planetary s Uranus lanet n p ea 0.4 c d iron What more can remote sensing tell us Water O k an 2 Roc about a planet, besides basic features ice Earth such as its size and density? Rich in iron 0.2 This is where transiting exoplanets come Figure 2 | Journey to the centre of a super- Venus into their own. Their well-constrained basic 0 0 Earth. Super-Earth planets, with masses 1 10 100 1,000 parameters and natural ‘on–off switch’ allow between two and ten times that of Earth, come log planetary mass (Earth masses) their weak signal to be calibrated accurately. in at least two varieties: a, rocky; b, ocean. Both That means we can perform basic spectros- types originate in a well-mixed structure of Figure 1 | The gamut of transiting copy: atomic sodium and hydrogen have solids (silicates) and volatiles (such as water and exoplanets. The composition of a planet will already been detected in exoplanets’ upper ammonia), with trace amounts of hydrogen and determine its average density and where it will atmospheres. Unprecedented spectroscopy noble gases. The structure differentiates very lie on a plot of radius against mass. Here, various and thermal mapping of the atmosphere of the quickly and in a strictly predictable way: iron and planets within (red circles) and beyond (grey transiting hot Jupiter HD 189733 b has been siderophile elements (high-density transition circles) the Solar System are compared with metals that like to bond with iron) precipitate possible at the point where it is seen ‘side-on’, into a core, and excess water, ammonia and so models of different interior composition: from just as it disappears behind its star. Here, the an interior made of rock and iron alone (Venus on precipitate above a silicate mantle. If water is infrared light flux from the planet can be dis- present in significant quantities, most of it will and Earth approximate to this state) to a gaseous –4 composition of just hydrogen and helium (of cerned at a level just 10 that of the star’s flux. be under pressures in excess of 10 gigapascals, which Jupiter is the closest example in our Solar NASA’s infrared Spitzer space telescope has and thus in a solid state — ice — despite the high System). Owing to difficulties in spotting such proved perfect for this task. temperatures. An ocean planet will be larger small bodies, few exoplanets at the small, dense than a rocky one owing to the lower density of end of the scale have so far been found. Many What have we learnt about the water, and the size of a planet of a given mass will giant planets have been detected that have very atmosphere of HD 189733 b? depend on the proportion of core, mantle and low densities above the upper range of pure First, we have a measure of the temperature dif- water. Models show that an uncertainty of less than 5% in radius and less than 10% in mass are hydrogen–helium planets (dotted line) — the ference between the day and night side, which exoplanet TrES-4 is the most extreme example needed to distinguish ocean from rocky planets. — and these are a serious challenge to theory. is about 230 °C. In such an extremely close orbit — HD 189733 b circulates around its star at just 3% of the Earth–Sun distance — a and oxygen. Absence of water might indicate the radius and the mass. More than 20 transit- planet’s rotation and orbital revolution become thermal and radiative loss, as seems to have ing planets are already known, and the radius synchronized, with one side permanently facing been the case with our small and hot planetary and mass of several have been determined to the star. (In the same way, the Moon always neighbour Venus. Hot Jupiters should be mas- better than 4% and 8% accuracy, respectively. presents the same side to Earth.) During tran- sive enough to stave off complete loss, but then But there’s a sting in the tail of this simple, sit, we thus see the night side, and the heat flux again we know little about the extreme condi- surprisingly accurate method — most of the we see coming from this side indicates that the tions in their upper atmospheres. By measur- planets found so far are much less dense than planet’s atmosphere must transport heat very ing the ‘depth’ (the loss or gain in intensity) our theories predict (Fig.
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