15.6 Planets Beyond the Solar SystemPlanets orbiting other stars are called “extrasolar planets.”Until 1995, whether or not extrasolar planets existed was unknown. Sincethen more than 300 have been discovered.We want to know: What methods are available for detection ofextrasolar planets? How common are extrasolar planets? Are they similar to planets in our solar system? Are other planetary systems like ours, withterrestrial and Jovian (gas giant) planets? How common are Earth-mass planets?Earth-like planets? Do they have water? Life?Direct detection of extrasolar planets is too difficultMost extrasolar planets have beendiscovered indirectly, through theirgravitational or optical effects, andcannot be seen directly due to the glareof their star.This are a couple of exceptions: Thisstar is a brown dwarf (ch.21), and theplanet is clearly visible.If you wanted to directly detect anextrasolar planet, what wavelengthregion should you look at?This would be called a “direct detection” andwill continue to be rare, and only possible formassive planets, for (probably) decades.Planets around other stars can be detected if they are large enough tocause the star to “wobble” as the planet and star orbit around theircommon center of mass. The magnitude of the wobble of the stardepends on the gravitational tug, hence the mass, of the planet. Noticethat all you obtain is the mass of the planet.Detection based on “wobble” of star produced bygravitational tug of the planet on the starIf the “wobble” is partly in our line of sight (i.e. there is a component awayand toward us), it can also be detected through the Doppler shift as the star'smotion changes. But the shifts should be tiny, and very difficult to detect,even for massive gas giants like Jupiter.1995: Surprise! “Close-in” Jupiters, periods of days! Hot!!The radial velocity method:Wobbles detected using spectral line Doppler shiftThe first extrasolar planetdiscovered: 51 PegasiA multiple planet system:Upsilon AndromedaeThese two graphs show radial velocityas a function of time: these stars areapproaching, then receding from us.Orbits of the three planetsin Upsilon AndromedaeA different method for detecting planets: TransitsAn extrasolar planet may also be detected if its orbit lies in the plane of the line of sight to us. Theplanet will then eclipse the star, and if the planet is large enough, a (very small!) decrease inluminosity may be observed as a function of time. This is called a transit.Notice a transit requires a planetary orbit almost perfectly in the line of sight of an observer onEarth. So transits will be found in only a tiny fraction of stars, even if all have planets.The time to enter and exit the “dip” in the light curve is a measure of the planet’s diameter, sothe transit method gives you the mass and the density of the planet! (Radial velocity methodonly gives the mass.)Only a few planets have been discovered this way so far, but the transit monitor satellites COROT(European) and Kepler (U.S., launch in March) will change that by a huge margin, within a fewyears.Brightness as function of time is called a “light curve.”For most stars it would be constant, but with planets itmay show variation of about 0.01to 0.1 percent:Difficult observational challenge. Nearly all extrasolar planets have (so far) been discovered by radialvelocity method. Why? Because they are mostly planets orbiting close totheir parent star, so moving fast, just what Doppler effect is sensitive to. More than 300 extrasolar planets have been discovered so far, over100 since the publication of the last edition of your textbook.Basic result: Much more variety than expected, mostly in orbital properties. Most have masses comparable to Jupiter’s (several hundred timesthe mass of the Earth). This is purely a selection effect because it is moredifficult to detect smaller-mass planets--less effect on parent star’s wobble (orfainter transit to detect--see later). But indications that there are manymore low-mass planets to come. Current record is ~ 5 Earth masses. Orbits are generally much smaller, and in some cases very muchsmaller, than the orbit of Jupiter. Almost completely unexpected--theory of migration of protoplanets had been forgotten. Orbits have high eccentricity compared to solar system planets.This is still not understood.Results so far (nearly all from radial velocity method)Orbits of extrasolar planetsThis plot shows the semimajor axis and eccentricity for some of the knownextrasolar planets, with Jupiter and Earth included for comparison.Knowing these are mostly Jupiter-class planets, notice how strange thisdiagram is compared to what we expected from our own solar system.Mercury is at 0.4AU.Why so many close-inextrasolar planets, and none in our system?Planets in our system havenearly circular orbits witheccentricity near zero--whyso many eccentricextrasolar planets?Another way to see the strange results: Orbits of 60 of the knownextrasolar planets. Note that some of them are very close to their star:Orbits of extrasolar planets (continued) Theories show that Jupiter-likeplanets can migrate inward,through friction with gas in theprotoplanetary disk of the parentstar. This process must take placeearly in the life of a planetarysystem, since we see no gas disksaround stars older than about 5-10 million years.Sinking planets: Migration in young planetary systems Could an Earth-mass planet survive the passage of a migrating Jupiter?Could such a passage actually assist the formation of Earth-like planets??Recently, more Jupiter-like orbits have been found; this one has twice the mass ofJupiter and an orbital period of 6 years. You can see why it has taken so long todiscover planets in Jupiter-like orbits: You have to observe for at least one orbitalperiod.It takes six years for this planet to orbit just once, and observers usually demandmore than one orbit for confirmation.The blue line is the radialvelocity curve for Jupiter.Notice how much more difficultits detection would be.Jupiter-like planetary orbits have been foundThis represents about ten years of observations! Planets orbiting within 0.1 AU of their starsare called “hot Jupiters”; they are notincluded in the previous figure but arenumerous. Some are vaporizing as they arecannibalized by their parent star. Seeillustration.More results on extrasolar planets: Stars with composition like our Sun (about1%
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