The Past and Future of Planet Hunting
May 2009: Steven Pravdo and Stuart Shaklan of JPL announce the detection of VB 10b -- the first planet discovered through astrometry.
Astrometry is the science (and art!) of precision measurement of stars' locations in the sky. When planet hunters use astrometry, they look for a minute but regular wobble in a star's position. If such a periodic shift is detected, it is almost certain that the star is being orbited by a companion planet.
Astrometry is the oldest method used to detect extrasolar planets. As early as 1943 astronomer Kaj Strand, working at the Sproul Observatory at Swarthmore College announced that his astrometric measurements reveal the presence of a planet orbiting the star 61 Cygni. Although the announcement was greeted with enthusiasm at the time, the claim has remained unproven and astronomers today are highly skeptical of Strand's results. The tradition of planet hunting through astrometry nevertheless remained strong at Sproul, where Strand's announcement was followed decades later by two other contentious claims. In 1960 Sproul astronomer Sarah Lippincott published a paper claiming that the star Lalande 21185 was orbited by a planet of roughly ten Jupiter masses, and in 1963 the observatory's director, Peter Van de Kamp, announced the discovery of a planet orbiting Barnard's Star.
The fact that all of these claims, based on decades of meticulous observations, were subsequently cast into serious doubt, testifies to the immense difficulties confronting an astrometric hunt for planets. Until recently, the level of precision required to detect the slight shifts in a star's position that indicate the presence of a planet was at the outer edge of technological feasibility. This, however, is fast changing. The Keck telescopes in Hawaii, the largest in the world, are currently being fitted for astrometrical measurements of the unprecendented accuracy of 20 micro arcseconds, when directed at single stars. This is equivalent to the diameter of a golf ball at the distance of the Moon
The future of astrometry, however, lies undoubtedly in space, because atmospheric interference limits the accuracy of ground-based measurements. Two space missions, now in their planning stages, will make possible astrometrical measurements of unprecedented accuracy within a few years. NASA's Space Interferometry Mission (SIM) has been delayed multiple times and is now scheduled tolaunch no earlier than 2015. Once in orbit around the Sun, SIM will be able to make angle measurements of single stars as accurate as 1 micro arcsecond -- the width of a human hair at a distance of 500 miles.
The European Space Agency's Gaia mission, scheduled to launch in 2011, will make wide-angle observations of over a billion stars in our galaxy at an accuracy of around 20 micro arcseconds. Whereas SIM's goal is to detect small Earth-like planets, Gaia will provide a broad survey pointing to the prevalence of larger planets throughout the galaxy.
Astrometry is one of the most sensitive methods for detection of extrasolar planets. The future SIM mission is so sensitive that it has the potential of detecting an Earth-mass planet orbiting within its star's habitable zone. Unlike transit photometry, astrometry does not depend on the distant planet being in near-perfect alignment with the line of site from the Earth, and it can therefore be a applied to a far greater number of stars. Furthermore, unlike the radial velocity method, astrometry provides an accurate estimate of a planet's mass, and not just a minimum figure.
In several of its key characteristics, astrometry is an excellent complement to the spectroscopic method, currently dominant among planet hunters. Whereas spectroscopy works best when a planet's orbital plane is "edge on" when observed from the Earth, astrometry is most effective when the orbital plane is "face on," or perpendicular to an observer's line of site. This is because astrometric observations cannot detect a star's displacement towards or away from the Earth, as this does not produce any change in the star's position in the sky. Astrometry can only detect that component of a star's wobble that moves it to a different location in the sky - i.e. perpendicular to the line of site of the Earth-bound observer. The closer a planet's orbital plane is to a "face-on" position when seen from the Earth, the larger the component of its movement that can be astrometrically measured.
Furthermore, whereas spectroscopy is at its best in detecting planets with short periods, orbiting very close to their stars, astrometry will excel in detecting stars of long periods, orbiting further away. This is because a planet with a long orbit causes a greater displacement of its star's location during the course of its orbit than a planet that remains in close proximity to its star.
The result of this is that, in contrast to spectroscopy, the sensitivity of astrometric detections actually grows with the increasing distance of a planet from its star. This means that astrometry can actually detect relatively small planets orbiting far from their stars -- a crucial advantage for scientists looking for Earthlike planets rather than the hot Jupiters favored by spectroscopy.
First of all, discovering extrasolar planets through astrometry is extremely hard to do. It requires a degree of precision that has seldom been achieved even with the largest and most advanced telescopes. This is made plain by the fact that although astrometry is the oldest method of searching for extrasolar planets, and despite numerous claims of discovery that have been made over the past 60 years, no extrasolar planets have as yet been found by this method. The space missions SIM and Gaia will likely address many of the difficulties and produce measurements of unprecedented accuracy. But the launch of both spacecraft is years away and, like all space missions, depends on many unforeseen contingencies.
Even improved accuracy cannot change some fundamental limitations of the astrometric approach. Astrometry, by its very nature, is highly sensitive to the distance of a celestial object from the Earth. This is because the same actual displacement in an object's position would appear as a greater change in position in the night sky if that object was close by than if it were further away. Therefore, while astronomers believe that astrometry will be very useful for detecting planets in the Solar neighborhood, the method will be far less effective when applied to more distant objects.
Then there is the fact that even accuracy in measurement can have its drawbacks. The new astrometric measurements could be so sensitive that they might be affected by star spots - the darker regions on the face of a star that appear to move as the star rotates. When observed with the new highly accurate astrometric systems, this could create a periodic shift in the star's "photometric center" - the exact location in the sky where the star's light appears to be generated. This effect can create the illusion that the star is wobbling to the tug of a planet, when in fact it remains in its place.
Finally, there is an inherent difficulty in observing planets with long periods, the very planets that astrometry should excel in. In order to detect a planet, it is necessary to observe the repeated periodic displacements of its parent star. This means that the star needs to be observed for longer than a single orbital period. When dealing with planets of long periods, comparable to those of our own Solar System, this can obviously be a problem. A star must be observed continuously for years or even decades before the presence or absence of a planet can be established. Such longterm projects are very hard to sustain given the realities of scientific funding, publishing pressures, and the length of scientific careers.
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