perpendicular to the line of sight of the Earth-bound observer. Astrometry can only detect that component of a star's wobble that moves it to a different location in the sky - i.e.
This is because astrometric observations cannot detect a star's displacement towards or away from Earth, as this does not produce any change in the star's position in the sky. Whereas spectroscopy works best when a planet's orbital plane is edge-on when observed from Earth, astrometry is most effective when the orbital plane is face-on, or perpendicular to an observer's line of sight. In several of its key characteristics, astrometry is an excellent complement to the spectroscopic method. Furthermore, unlike the radial velocity method, astrometry provides an accurate estimate of a planet's mass, and not just a minimum figure. Unlike transit photometry, astrometry does not depend on the distant planet being in near-perfect alignment with the line of sight from the Earth, and it can therefore be a applied to a far greater number of stars. The Gaia catalog has already helped to refine exoplanet size estimates made from transit photometry, but has not yet yielded any more potential exoplanets.Īstrometry is one of the most sensitive methods for detection of extrasolar planets. The European Space Agency's Gaia mission, which launched in 2013, is making wide-angle observations of more than a billion stars in our galaxy at an accuracy of around 20 micro arcseconds its first data release was in September 2016. However, followup radial-velocity observations of VB 10b did not detect the signal that would be expected based on the astrometric data, so most researchers consider it another false positive.
ASTROMETRY RATE ACROSS SKY TERM ARCHIVE
As of February 2020 there is only one confirmed planet on the NASA exoplanet archive which is listed as an astrometric discovery, DENIS-P J082303.1-491201b, also known as VB 10b. 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. 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.
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 tradition of planet hunting through astrometry nevertheless remained strong at Sproul, where Strand's announcement was followed decades later by two other contentious claims. 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. As early as 1943 astronomer Kaj Strand, working at the Sproul Observatory at Swarthmore College announced that his astrometric measurements revealed the presence of a planet orbiting the star 61 Cygni. Astrometry is the oldest method used to search for extrasolar planets.
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