ASTR 330: The Solar System

ASTR 330: The Solar System • Homework assignment #5: results today. • Class average was: 38.5/50.0 (77 %) • Overall class course average is: 75%. • Extra credit term paper: due Dec 5th (next Tuesday). Announcements Dr Conor Nixon Fall 2006

ASTR 330: The Solar System Lecture 26:Extra-solar planets Dr Conor Nixon Fall 2006 Figure credit: Stephan Simon, SCIENCE-WORLD.

ASTR 330: The Solar System Amazingly, despite countless sci-fi films to the contrary, no-one knew for sure whether any other stars in the galaxy had planets at all until the 1990s. However, most of the astronomical community believed that many solar-type stars, would have planetary systems. (There is much less agreement as to whether these systems are inhabited or not!) It would be rather incredible if, out of 100 billion stars in the Milky Way galaxy (and 100 billion galaxies), our Sun had nine planets and no other stars had any at all! However, until such systems were found and counted, we could not ascertain whether such systems were common or rare. Why had no planets outside our solar system been found? Other planetary systems Dr Conor Nixon Fall 2006

ASTR 330: The Solar System Of course, we are most interested in planets around solar-type stars, which are the most likely to harbor life. In 1991, the first planets discovered since Pluto in 1930 were announced. But, rather than orbiting a Sun-like star, they were discovered orbiting a pulsar, a rapidly rotating neutron star. Neutron stars are the remains of giant stars, which explode at the end of their lives in supernovas. Pulsars send out two beams of radio energy as they rotate, like a lighthouse. Timing these pulses shows that pulsars are amongst the best time keepers in the universe. An unexpected start Dr Conor Nixon Fall 2006 Figure credit: A. Wolszczan, Penn State

ASTR 330: The Solar System Because pulsars are so regular, it was with surprise that Penn State professor Alexander Wolszczan noticed in 1991 that PSR B1257+12 was showing regular deviations in its pulsing. How could this arise? If another object were orbiting the pulsar, the gravitational pull would move the pulsar in a small circle. This wobble would cause the pulses to arrive first late and then early. In this case, the pulses showed two regular deviations, which showed the definite effect of two bodies in orbit about the pulsar. This was later shown to be three bodies: two Earth-sized and one Moon-sized. Pulsar Planets Dr Conor Nixon Fall 2006 Figure credit: A. Wolszczan, Penn State

ASTR 330: The Solar System Pulsars are the last place astronomers would think to look for planets. Why? During the supernova explosion, most of the mass of the star is blown off into space. Besides the intense heating which would kill off any life on nearby worlds, the massive reduction in gravity should allow the planets to immediately escape the system. Why were the pulsar planets the first detected? The answer is simple: technology. Accurate timing is relatively easy to achieve now on the Earth. If other types of stars behaved like pulsars, we could have found planets around them as well! In the case of solar-type stars, the problem was much harder however, and the eventual success had much less to do with luck than good planning. Why pulsars? Dr Conor Nixon Fall 2006

ASTR 330: The Solar System How would we go about finding planets around other stars? Can we see them orbiting other stars? Ideally we would like to see the point of light in a telescope. When Neptune was tracked down by mathematics in the 1800s, the finding was not considered definitive until the moving dot had been located. However, for extra-solar planets, this may be the last method we use. Why? Quite simply, the star is too bright (more than a million times brighter than any planet) and the planets too close to the star, for direct imaging in visible light to be successful with current technology. However, this is a distant goal that astronomers are already working towards. Can we see extrasolar planets? Dr Conor Nixon Fall 2006

ASTR 330: The Solar System • So, we can’t image a planet at the present time. How can we find planets then? • The answer is that we must use an indirect detection method, by looking at changes in the star caused by the planet. • Three methods are currently feasible for finding planets around MS stars: • Astrometric method. • Radial Velocity Method. • Transit Method • We’ll examine each in turn. How to find planets Dr Conor Nixon Fall 2006

ASTR 330: The Solar System The first of the indirect methods is to use the fact that the planet will pull the star into a small circle about the center of mutual mass, called the system barycenter. On the sky, the star will move from side to side. Astrometric Method Dr Conor Nixon Fall 2006 Figure credit: wikipedia/Zhatt

ASTR 330: The Solar System • Clearly, we are looking for a very small effect! The mass of the star far outweighs the mass of the planet, and the ‘wobble’ will be very small. • For example, the barycenter of the following systems: • Sun-Earth: barycenter is 450 km from the center of the Sun. • Sun-Jupiter: barycenter is 777,000 km from the Sun’s center. This is just outside the surface of the Sun. • This was the first method used to try to find planets. Over several decades of research, no positive results were found.However, it was clear from calculations that the effect would very likely be too small to measure. Astrometric Method Dr Conor Nixon Fall 2006

ASTR 330: The Solar System The Doppler Effect can also be used to find planets. How? In a familiar example, an ambulance racing down the street past us has a distinctly higher pitch as it approaches, and a lower pitch as it recedes. Why is this? The sound waves given off by the siren are affected by the forward motion of the vehicle. As it approaches, the waves get bunched up more closely together, and arrive closer together. We perceive that as a higher sound pitch. The opposite occurs as the vehicle moves away. The wave peaks get stretched further apart, and we hear a lower pitch. Thus the familiar pitch drop, from hightolow, as an ambulance passes. The Doppler Effect Dr Conor Nixon Fall 2006

ASTR 330: The Solar System The Doppler Effect also occurs with light. The light emitted from a star moving towards us is shifted to a higher frequency than we would see if the object was at rest. We see this as a shift to the blue end of the spectrum for visible light. Similarly, if a star or galaxy is moving away from us, the light is shifted to lower frequencies, which we call a red-shift. Incidentally, red-shifts of distant galaxies were our first evidence that the universe is expanding. Red Shift and Blue Shift Dr Conor Nixon Fall 2006 Figure credit: John Fix, McGraw-Hill

(or radial velocity method) ASTR 330: The Solar System We can use the Doppler Effect to find planets! How? We have just discussed how a planet orbiting a star will cause it to move in a small circle about the system’s center of mass. As the star moves first towards us, then away from us, its light is first blue-shifted, then red-shifted in a cyclical fashion. Of course, the period of this cycle may be days, months or years! Spectroscopic Method Dr Conor Nixon Fall 2006 Figure credit: physicsweb

ASTR 330: The Solar System Starting in the early 1990s, several groups of scientists began to look for such a shift. The size of the shift they were searching for was very small, but even so, this is an easier task than measuring the movement of the star on the sky. The key was in measuring very small shifts of dark stellar absorption lines, repeatably and accurately over months and years. A yardstick was needed: a pattern of fixed features, which the stellar lines would show movement against. For this, a gas cell was also inserted into the optical path, to add fixed lines which would not move over time. The positions of the stellar lines would be measured relative to the gas lines. Doppler Searches Dr Conor Nixon Fall 2006

(or transit, or eclipse method) ASTR 330: The Solar System The final of the three methods we will discuss is the photometric (transit or eclipse) method. This method is simple: it looks for the dimming of the starlight as the planet passes in front of the star. The main disadvantage of this method is that the eclipse takes just a few hours as seen from the Earth, and it may not occur again for years! So, out of all the stars in the sky, how do we know where and when to look? Photometric Method Dr Conor Nixon Fall 2006 Figure credit: Don Kalinski, UCAR STARE Project

ASTR 330: The Solar System In 1995, two teams were pursuing the radial velocity (Doppler) method to search for planets around main-sequence (solar-type) stars. The first success was announced by Michel Mayor and Didier Queloz of Geneva Observatory, Switzerland. They had found a roughly Jupiter-sized planet in orbit around the star 51 Pegasi. The figure (right) shows the sine-wave pattern of frequency shift as the star is pulled in a circle by the planet. First Success Dr Conor Nixon Fall 2006 Figure credit: Mayor and Queloz, Geneva Observatory.

ASTR 330: The Solar System The announcement by Mayor and Queloz took the astronomical community by surprise. After so many years of negative findings, no-one was paying attention to this corner of astronomy, which suddenly became front-page news. The 51 Pegasi finding was quickly confirmed by Geoff Marcy (UCB) and Paul Butler (Carnegie Institute) who had been observing 51 Pegasi and a bunch of other nearby stars, but had not yet examined their data. The team of Marcy and Butler went on to examine the rest of their observations, and quickly found the signature of planetary companions around two other stars. However, some scientists dissented, saying that the sinusoidal signal could occur naturally in a star, if it was pulsing in and out like a giant heart. Over time, this theory was shown to have flaws, and the planetary detections were accepted. Confirmation and Criticism Dr Conor Nixon Fall 2006

ASTR 330: The Solar System After the 51 Peg announcement, the number of planets found exploded from 1996. In just a few years, the number of planets known outside our solar system first equaled our total of nine, then rapidly overtook and is still climbing. As of November 2006, 197 planets around main sequence stars have been found using the Doppler technique, 121 by the Marcy and Butler team alone! Let’s now look at what information we can glean about these planets. A Multitude of Worlds Dr Conor Nixon Fall 2006

ASTR 330: The Solar System By studying the Doppler motion of the parent star, quite an impressive amount of information about the planet can be obtained. Firstly, of course we find the orbital period. Secondly, if we know the approximate mass of the star (which we can tell from its spectrum) , we can use Newton’s Law of Gravity to find the radial distance (semi-major axis) of the orbit. Thirdly, we can tell the eccentricity of the orbit from the shape of the oscillations. A sinusoidal oscillation shows a circular orbit, whereas a saw tooth shape on the other hand shows an elliptical orbit. The amount of ellipticity can be determined mathematically by fitting the shape. Information From Doppler Studies Dr Conor Nixon Fall 2006

ASTR 330: The Solar System This figure shows the distinctive signature of a planet in a very eccentric orbit about the parent star, in this case, the 16 Cygni system, with an eccentricity of 0.68. Eccentric Planetary Orbit Dr Conor Nixon Fall 2006 Figure credit: Cochran et al, Ap J 453.

ASTR 330: The Solar System The star Upsilon Andromeda was the first main-sequence star to show a multiple planetary system. The right figure below shows the oscillations due to two of the three planets detected. The left figure shows the orbits of the three planets. Upsilon Andromeda: Multiple System Dr Conor Nixon Fall 2006 Figure credit: Marcy and Butler/Berkeley

ASTR 330: The Solar System The fourth fact we can discern from the radial velocity graph is the minimum mass for the planet, from the amplitude of the star’s reflex motion. Why the minimum mass? Consider two extreme cases. If the planetary system was face-on as seen from Earth, the motion of the star would be side-to-side on the sky, not forwards and backwards. In this case, we would see no Doppler shift of the starlight at all. However, if the system was edge on, we would see the star moving forwards and backwards in the sky. The intermediate cases occur where the system is tilted partly towards us. In this case, we measure only the part of the stellar motion that is in the radial direction (forwards-backwards) not transverse (side-to-side). Planet Mass Dr Conor Nixon Fall 2006

ASTR 330: The Solar System By measuring the radial motion only, we can underestimate the mass of the star. In general, we can find only part of the effect that the planet has on the star, not all of the effect. So, when we measure the radial effect of the planet to determine its mass, we can tell only the minimum mass that the planet could have. Note that, if we were able to detect the transverse motion as well, using astrometry, we could then find the exact mass! Minimum Mass Dr Conor Nixon Fall 2006

ASTR 330: The Solar System When is a planet really a planet, and how big does it have to be to become a star? A true star is at least 70 Jupiter Masses (MJ). This is the minimum mass required to compress the core sufficiently to ignite true hydrogen fusion, the chain of adding protons together to make helium, which powers stars. However, between 13 and 70 MJ, an object can fuse deuterium (heavy hydrogen) to make helium, which occurs at lower temperatures and pressures. Such objects will never be able to fuse normal hydrogen like the Sun, and will glow very dimly: a ‘brown dwarf’. Below 13 MJ, there is no fusion at all, although of course Jupiter-sized planets can generate some energy by gravitational contraction and differentiation (‘helium rain’). Planet or Brown Dwarf? Dr Conor Nixon Fall 2006

ASTR 330: The Solar System If stars and planets were formed in the same way, then we would expect to see a continuous distribution of objects at all masses. However, most of our theories hold that stars form in the central condensation of the nebula, whereas planets form in the disk. The figure (right) shows the mass distribution of the first fifty or so exoplanets to be discovered. There is a distinct gap at around 13 MJ, which seems to indicate that different processes give rise to planets, or stars and brown dwarves. Mass Distribution Dr Conor Nixon Fall 2006 Figure credit: Geoff Marcy/Physics Today

ASTR 330: The Solar System Once the 51 Pegasi system was unravelled, the most startling thing about the planet turned out to be not the size, but the orbital period. At just 4.2 days (compare to Mercury’s88 days), this planet makes an orbit just 0.05 AU in radius about its parent star! Initially, there was some skepticism that a planet could possibly survive in a near-circular orbit so close to a star. However, calculations show that, although this planet would be very hot (1200 K) its minimum mass of 0.46 MJ (Jupiter Masses) was enough to hold its gases from totally evaporating. Many other extrasolar planets turned out to be ‘hot giants’ as well: large planets relatively close to the parent star. Why could this be? 51 Pegasi Dr Conor Nixon Fall 2006

ASTR 330: The Solar System • We must be careful when search methods not to delude ourselves that we are seeing all there is to see! • The Doppler search method has two biases: • Large planets: the bigger the planet, the further the stellar absorption lines will move, and the more obvious will be the effect. We could not detect Earth-sized planets with this method: the line movements are too small. • Close-in planets: the smaller the orbit about the parent, the less time we will have to wait to see the effect. For example, Saturn takes around 30 years to orbit the Sun. We would have to wait at least that long to see one full oscillation of the stellar lines. • So, it is not surprising that we tend to find large, close-in planets: hot giants! Selection Effects Dr Conor Nixon Fall 2006

ASTR 330: The Solar System This figure shows the dramatic concentration of exoplanets at small semi-major axis values. We can also see that several systems discovered so far are multiple systems. Distance Distributions Dr Conor Nixon Fall 2006 Figure credit: exoplanets.org

ASTR 330: The Solar System The detections we have made so far, even allowing for the biasing of the methods, show that our system may be far from typical. Our paradigm has been of terrestrial planets inside 5 AU from the star: small rocky worlds. Outside 5 AU are the gas giant planets. But in many exoplanet systems we see large giants close in, which we assume to be gas planets because off their size. This is not something our formation scenarios predict. Remember that theories were based on a core of icy planetesimals for the outer planets, followed by accretion of hydrogen and helium. These processes should have occurred at temperature of 30-100 K. How then can we account for the hot giants? What is a typical solar system? Dr Conor Nixon Fall 2006

ASTR 330: The Solar System If the hot giant planets turned out to be rocky, not gaseous, perhaps we don’t have a problem after all. We have no problem with rock and metal condensed inside of 5 AU. If only we could find the actual size of the planet, then we could calculate the density and find out which is right. But how to find the planet’s size? At this stage, a second of our indirect techniques came to the rescue: the occultation or photometry method. This method would show the existence of a planet, due to dimming of starlight as the planet passed in front. But, now that we know which systems have planets (from Doppler technique) we can see if any of them eclipse! Density Dr Conor Nixon Fall 2006

ASTR 330: The Solar System In 1999, two groups almost simultaneously discovered the first transit, in the system of star HD 209458. The odds that a given system will be in the correct orientation for an eclipse to occur (ever) are about 1 in 10. This figure shows the distinct dip in the light curve as the planet passed slowly in front of the stellar disk. The radius of the planet was 1.3 times that of Jupiter. First Transit Dr Conor Nixon Fall 2006 Figure credit: STARE project/UCAR

ASTR 330: The Solar System Since the initial detection using a small scope, the HST was trained on the system, showing the beautifully smooth lightcurve (right). The lower image shows an artists impression of the transit occurring. Using the derived radius, and the exact mass (which was known from the system orientation), the density was found to be 0.4 g/cm3, even less than that of Saturn. Although in a four-day orbit, the planet is indeed a gas giant world. HD 209458 Dr Conor Nixon Fall 2006 Figure credit: (i) HST/APL (ii) Lynette Cook

ASTR 330: The Solar System There are two plausible theories at the present time. The first theory is that the planets form very early, even before the star is burning at full strength. If the planets can accrete much of their mass by then, then they stand a good chance of being able to hang onto to the mass once the star really gets going. In this scenario, the planets can form inside the ‘snowline’: the 5 AU distance inside of which volatiles are not able to condense: after the star is shining properly. I.e. the planet condenses before the ‘snowline’ boundary is in place. How could hot giants form? Dr Conor Nixon Fall 2006

ASTR 330: The Solar System In the second scenario, the planet forms outside the snowline, as ‘normal’, but then migrates inwards. There is even some evidence that Jupiter too underwent this migration process, due to the enrichment in argon, which must have condensed at 30-40 K. How could migration occur? One theory involves the presence of the nebular disk, which could linger after the planet had formed. So long as disk material exists both inside and outside the planet’s orbit, the gravitational effect on the planet is neutral. But if the inner disk material is removed, by accreting onto the star for example, then the gravitational forces acting on the planet become unbalanced. The net effect is that the planet loses energy and falls into a smaller orbit. Planet Migration Dr Conor Nixon Fall 2006

ASTR 330: The Solar System Migration Schematic Dr Conor Nixon Fall 2006 Figure credit: physicsweb

ASTR 330: The Solar System The migration can end in one of two ways. In one scenario, the migration is stopped by tidal forces. When the planet comes close enough to the star to experience strong tides, it becomes synchronously rotating. Once its rotation period becomes equal to its orbital period (at about 0.1 AU or less), it can no longer drift further inwards. There are some systems which seem to defy this description however, having giant planets ‘stopped’ at 0.3 AU for example. A second possibility is that the planet simply continues to migrate inwards all the way until it is consumed by the star. Ths could possibly explain the high metalicity of many of the stars which have planets. In summary, our theories are still embryonic and require much further work. An End to Migration Dr Conor Nixon Fall 2006

ASTR 330: The Solar System • As a planet orbits a star, two eclipses occur: • When the planet goes in front of the star (primary eclipse). • When the planet goes behind the star (secondary eclipse). Secondary eclipses Astronomers were very keen to observe a secondary eclipse when the planet is invisible - much harder to see than the primary eclipse when the starlight is blocked. But why were they so interested in ‘not seeing’ the star? Dr Conor Nixon Fall 2006

Dr Conor Nixon Fall 2006 ASTR 330: The Solar System Resourceful astronomers had figured out a trick for detecting the faint glow of a planet (often compared to a firefly buzzing round a streetlamp). By observing the system first at elongation, when the planet is side by side with the star, and then during secondary eclipse, scientists could subtract the two signals and find the light due to the planet alone! However, this trick is much easier to perform in infrared light, where the star only outshines the planet by 400 times, rather than in visible light where the star is 10,000 times brighter! Vanishing planets Figure credit: NASA/JPL-Caltech

ASTR 330: The Solar System In late 2004 two teams used the Spitzer Infrared Space Telescope to search for the secondary eclipses of two known transiting exoplanets. The teams announced the findings in early 2005: a double success. Secondary eclipses were observed for both TrES-1 and HD 209458b. This infrared detection allowed the temperature of the planets to be directly measured: 1060 K for TrES-1 and 1130 K for HD 209458b - both much hotter than Venus! But, we must remember that these are both giant planets very close to their parent stars. Double Jackpot Dr Conor Nixon Fall 2006 Figure credit: NASA/JPL/Caltech

Dr Conor Nixon Fall 2006 ASTR 330: The Solar System By observing the secondary eclipse as well as the primary eclipse, the astronomers were also able to confirm that both planets are in perfectly circular orbits. This was expected: tidal forces from the parent star at such close distances can quickly circularize the planet’s orbit, removing any orbital eccentricity. However, this research has deepened another mystery: why HD 209458b is 35% larger than Jupiter, whereas TrES-1 is only 4% larger, yet both have near identical masses and heating from their suns. Why is HD 209458b so puffed up? No-one knows at present! HD 209458b - The mystery of the puffed-up planet

ASTR 330: The Solar System The field of extrasolar planets is still in its infancy, having just arisen less than 10 years ago. Many new telescopic searches and space missions have been proposed, and many will doubtless be approved. The Kepler mission is the brainchild of Bill Borucki at NASA AMES. This space-based telescope would simply stare at a patch of sky for 4 years, containing a rich field of solar-type stars, in the expectation of seeing many transiting systems. Kepler has the advantage of being sensitive enough to detect Earth-sized planets for the first time. About 100 are expected to be found, along with many more giant planets, starting in 2008. The Future Dr Conor Nixon Fall 2006

ASTR 330: The Solar System Although Kepler will enable us to detect the presence of planets as small as Mercury, we will not be able to tell much about them. To really find out whether they have atmospheres and the possibility of life, we need to be able to sense the planetary infrared radiation directly, and perform spectroscopy. This will require much better spatial resolution than any single-dish telescope can achieve: the telescope would need to be 100s of meters wide. The solution is to use interferometry: the technique of combining the light from an array of separate telescopes to achieve the spatial resoluion of a single large scope. To perform interferometry at optical wavelengths, we are required to deploy our array of telescopes in space! Space-based interferometry Dr Conor Nixon Fall 2006

ASTR 330: The Solar System A mission called ‘SIM’ - Space Interferometry Mission will lead the way, in around 20011 if the mission goes ahead. SIM would be able to resolve giant planets from starlight. SIM would lead the way for TPF - the Terrestrial Planet Finder mission, in 2020 or thereabouts. TPF would be able to give us direct information about Earth-like planets: if the daunting technical challenges can be overcome. Terrestrial Planet Finder Dr Conor Nixon Fall 2006 Figure credit: JPL/NASA

ASTR 330: The Solar System • What unexpected discovery was made in 1991? What was found? • Why were pulsars a very dubious place to hunt for planets? • Can we see planets around other stars using existing technology? • What indirect methods can be used to find planets? • How does the Doppler Effect help us find planets? • What information about extra-solar planets can we determine from radial velocity measurements? • What is a brown dwarf? Do planets and brown dwarves form in the same ways? Quiz-Summary Dr Conor Nixon Fall 2006

ASTR 330: The Solar System Why does the Doppler method give us a minimum mass for the planet? What biases do we expect to occur in our planet detections when we use the Doppler method to find them? How can we determine whether close-in massive planets are gas or rock? How could hot gas giants planets form? In what ways could the migration process end for the planet? What is the objective and scientific method of the Kepler mission? Why do we need space-based interferometry? Quiz-Summary Dr Conor Nixon Fall 2006

ASTR 330: The Solar System