As recently as five years ago, the search for planetary systems beyond our own was the subject of a few painstaking surveys by a small but dedicated band of planet-hunters. Their goal was to find extrasolar counterparts of our own giant Jupiter, which circles the Sun once every 11 years. To do this they relied on the fact that the star around which any planet orbits also moves: for instance, the Sun circles its common centre of gravity with Jupiter at the leisurely pace of 12 metres per second. The aim of these early programmes therefore, was to develop techniques that could detect the periodic changes in the Doppler shift of the light from a star as it wobbled back and forth in response to the gravitational tug of an unseen Jupiter-like companion (figure 1).
Several teams achieved the required precision by the early 1990s. These efforts included a regular monitoring programme of 120 nearby stars that was started at the Lick Observatory in California in the late 1980s by Geoff Marcy and Paul Butler of San Francisco State University. However, the computational overheads needed to analyse the results were so great that much of their data remained unstudied in an archive. This strategy made good sense: by the time any candidate Jupiters had completed enough of their orbits to be clearly identifiable, processor speeds would have increased to the point where the data analysis could be carried out far more quickly. Other groups at the University of Victoria in Canada and the University of Texas at Austin adopted similar philosophies.
When Michel Mayor and Didier Queloz of the Geneva Observatory announced the first discovery of stellar reflex motion due to a planetary body in 1995, its form was so unexpected that it demanded independent confirmation. Their data showed the solar-like star 51 Pegasi to be wobbling back and forth at 56 m s-1, completing one orbit every 4.2 days. The only plausible explanation for this wobble was the presence of an unseen body with at least half the mass of Jupiter in an orbit with a radius of 0.05 astronomical units (AU). An astronomical unit is defined as the mean distance between the Earth and the Sun, which is around 1.5 x 108 km.
Fortunately Marcy and Butler had been monitoring the same star for several years, and confirmed almost immediately that the wobble was present in their data as well. At the same time, they found similar signatures of close-orbiting giant planets for three other stars: tau Bootis, 55 Cancri and upsilon Andromedae. A spate of similar discoveries followed. New monitoring programmes were established, and the total number of stars that are currently under observation stands at about 1000. Among these, the tally of nearby stars known to possess at least one planet has risen to over 50. Several of these stars show extra, slower wobbles superimposed on the main signal, which betray the presence of one or more additional planets in larger orbits. Most notable among these is upsilon Andromedae with a family of three giant planets.
Last year saw the record for the lowest mass extrasolar planet being broken several times. Mayor currently holds the record for detecting a planet weighing just 0.16 times the mass of Jupiter that is orbiting around HD 83443, a bright star in the constellation Vela some 141 light-years from Earth.
Meanwhile in August 2000, a team led by Bill Cochran of the University of Texas at Austin announced that it had detected the closest extrasolar planet to Earth. At a distance of just 10.5 light-years away, the star epsilon Eridani shows evidence of a seven-year wobble, although this result still needs to be confirmed.
Reflex orbit
The period and size of a star's wobble encode important information about the planet's mass. Usually it is only possible to determine a lower limit on the mass, because for most systems astronomers cannot measure the tilt of the orbit relative to the line of sight. Assuming that the orbit is edge-on to the line of sight gives the smallest possible value of the planet's mass. This is not as serious a problem as it might appear. If the orbital axes of the planetary systems are oriented randomly in space, there is a natural statistical tendency for us to see many more orbits edge-on rather than face-on. (Try throwing a handful of coins in the air to convince yourself of this.) All the planets discovered so far are giants with masses ranging from slightly less than the mass of Saturn, which is 95 times heavier than Earth, to a dozen times the mass of Jupiter. Jupiter is the most massive planet in our solar system and is 320 times heavier than the Earth.
A planet in a circular orbit around its star produces a symmetric wobble that varies sinusoidally with time. The dozen or so known planets with periods of less than a week - the so-called hot Jupiters, deemed hot because they are substantially closer to their stars than our Jupiter is to the Sun - have orbits that are nearly circular, as expected (figure 2). The reason is that a close-orbiting planet in a highly elliptical orbit produces tides on the star, which move the star's centre of gravity in such a way that the orbit gradually evolves into its lowest energy state, a circular orbit. The greater the distance between the planet and the star, the smaller the tidal effects and the longer it takes for the orbit to become circular. For planets that have orbital distances larger than a small fraction of an astronomical unit, the orbit will remain elliptical for longer than the star's lifetime.
A planet in an elliptical orbit speeds up when it is close to the star and slows down when it is further away from it, giving the star a characteristic lopsided wobble. Astronomers have used this idea to deduce that the orbits of the longer-period planets are much more eccentric (i.e. more elliptical), in stark contrast to the near-circular orbits of the planets in our solar system. Indeed, nobody has yet found a well behaved Jupiter-like planet in a circular orbit with a radius of several astronomical units.
The high masses and short orbital periods of the planets discovered so far makes them easier to detect than conventional Jupiter-like planets with long periods. This is because the speed, v, of the parent star in its reflex orbit is given by the relation v = 12(MP/MJ)(5.2MSun/aMstar)1/2 m s-1 where MP, MJ, MSun, Mstar are the masses of the extrasolar planet, Jupiter, the Sun and the star, respectively, a is the orbital distance of the planet and 5.2 AU is Jupiter's distance from the Sun. This favours the discovery of massive planets in close short-period orbits, particularly as most of the monitoring programmes have only been running for a few years. However, it does not explain why so many of the orbits are so much more eccentric than those found in our solar system. To understand these differences, we need to look at the conditions under which planetary systems form.
How do Jupiters form?
Theories of giant-planet formation fall into two main categories. The "top-down" approach has planets forming from large-scale perturbations in the flattened, gaseous disc that surrounds a new-born star for the first few million years of its life. Meanwhile, the "bottom-up" approach requires dust grains with ice mantles to clump together to form bodies a few times the mass of the Earth. Once this critical mass is attained, the planet's gravitational pull becomes strong enough for it to accrete large amounts of gas from the disc and grow rapidly into a gas giant.
As the planet grows, however, it causes slow-moving material outside its own orbit to speed up and faster-moving material with smaller orbits to slow down. This tidal effect sweeps the planet's own orbit relatively free of material (figure 3). Computer simulations show that accretion can only occur along a pair of spiral shocks extending inward and outward from the planet. However, several things can go wrong. For example, the growth process itself may be self-limiting due to a lack of material in the region swept clear by the planet. And in many models, the angular momentum that is inevitably exchanged between the planet and the surrounding disc material causes the planet's orbit to decay, spiralling in to be swallowed by its sun before it can attain a high enough mass.
Other models have shown that top-down planet formation occurs in local clumps in the wake of a growing giant planet, producing a system of several giant planets in dynamically unstable orbits. In this case, interactions between the planets may lead to some bodies being ejected from the system altogether, while others are left behind in eccentric orbits.
Tantalizing new evidence emerged recently that top-down formation of planetary-mass bodies may even occur in interstellar space. Maria Zapatero Osorio and co-workers at the Instituto de Astrofisica de Canarias in Spain and the California Institute of Technology in the US recently announced the discovery of several faint objects in the star-forming region around the massive star sigma Orionis (see further reading). The spectra of these "freely floating" objects, which appear unbound to the star, are a good match with theoretical models of gaseous bodies that weigh a few Jupiter masses and are between 1 and 5 million years old. At these very young ages, the objects still shine brightly as they contract by radiating gravitational energy. However, it is not yet clear from the observations whether the free-floaters have formed in isolation or have been ejected from nearby protoplanetary systems.
These new ideas - inspired largely by the properties of the exoplanetary systems discovered so far - paint a much more violent picture of the planet-formation process than is needed to explain our solar system. While the same problems of spiral-in and self-limiting growth were encountered as long ago as the mid-1980s, much of the fine-tuning of the models was carried out under the assumption that the end-product should look like our system, with well behaved giant planets in circular orbits at more or less the distances where they formed.
The orbits of the giant exoplanets suggest that many of the things that can go wrong in building a tidy system like our own, do go wrong elsewhere. Neither top-down nor bottom-up scenarios can produce Jupiter-like planets with four-day orbits, such as the planets around 51 Pegasi, tau Bootis, upsilon Andromedae and the star HD 187123 in the constellation Cygnus. If the cores of these exoplanets formed from rock-ice planetesimals, they must have done so several astronomical units from their stars, accreted their atmospheres, spiralled in and had their migration halted near 0.05 AU by some as yet unknown mechanism. The giants in eccentric orbits between a few tenths and a few AU must have undergone a similar formation and migration history, combined with violent dynamical interactions with other newly formed giants.
Hints from other worlds
The exoplanets themselves may give some clues about their formation history. The radius of a Jupiter-like planet depends weakly on its total mass and internal composition, and also on its age. Even today, Jupiter is shrinking, radiating 65% more energy than it absorbs from the Sun. Recent calculations by Adam Burrows at the University of Arizona in the US and Tristan Guillot at the Observatoire de la Côte d'Azur in France show that the shrinkage rate for a giant planet with a given mass can be slowed considerably if it is prevented from radiating efficiently. The extreme irradiation experienced by the hot Jupiters due to their close proximity to their stars has precisely this effect, as the planet can only radiate efficiently from its dark side.
Much of the shrinkage occurs early in a planet's history when it is hot and has a large radiating surface area. The present-day radii of the hot Jupiters are therefore quite sensitive to the ages at which the planets reached their current orbits. For example, a planet that took a long time to form and spiral in will have plenty of opportunity to radiate and shrink. On the other hand, a planet that formed quickly and spiralled in rapidly would have arrived at its new orbit with a large radius, and would subsequently find it harder to cool.
Since the ages of the parent stars can generally be determined to within a billion years or so (from their luminosities, temperatures, heavy-element abundances and axial-spin rates), observational determinations of the radii of exoplanets with known masses can provide important insights into the interior compositions and histories of these planets.
Late last year, David Charbonneau, Tim Brown and others at Harvard University and the High-Altitude Observatory (HAO) in Boulder, Colorado discovered a planet in a 3.5-day orbit about the star HD 209458, which lies roughly 200 light-years from the Sun in the constellation of Pegasus. As the plane of the planetary orbit lies along our line of sight, the new planet passes across the face of its parent star once every orbit. The Harvard-HAO team, and others, quickly found that nearly 1.5% of the star's light was blocked each time the planet crossed in front of it. This allowed the researchers to measure for the first time the relative sizes of the planet and the star directly, and so obtain the first confirmation that the hot Jupiters are indeed gas giants. With a radius 1.35 times that of Jupiter, HD 209458's planet is over-sized for its age and mass, in agreement with recent models published by Burrows' team.
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