Map: Research / Extrasolar planets / Imaging extrasolar planets	< Back		Next >

1.3 Imaging extrasolar planets - removing the bias towards detection of giant planets close to stars (Ghez, Hansen, Song, Zuckerman)

1.3.1 Development of the youthful star target database: identification of nearby, youthful stars offers the best nearterm opportunity for imaging Solar System-like planetary systems

The past few years have seen the discovery of numerous massive extrasolar planets (Marcy et al. 2000). All have been detected indirectly, by virtue of their gravitational tug on the star about which they orbit. Only when planets are imaged directly will it be possible to measure their spectra and thus their compositions.

	Figure 1.3.1. Plot of masses and star-planet distances for warm, giant planets that would be observable if they were to be in orbit around the 112 stars identified by Zuckerman and Song as very young and near to Earth. The plot shows the minimum masses that should be detectable using the near-infrared camera aboard the Hubble Space Telescope.

Advances in astronomy from the ground, specifically adaptive optics (AO; Beckers 1993), and the employment of an infrared camera on HST, now enable imaging detection of planets with masses comparable to that of Jupiter (Figure 1.3.1; Macintosh et al. 2001). But such detections must be of thermal emission from young, warm planets rather than of reflected starlight from old, cold planets, such as Jupiter. At wavelengths near a few mm, thermal emission from a giant planet not older than tens of millions of years can be hundreds of times brighter than reflected starlight; the latter is still much too faint to be detected with any existing imaging system.

The giant planets of our Solar System are 5 to 30 times more distant from the Sun than is Earth. Given the diffraction and instrumental scattered light properties of AO and of HST, imaging of Solar Systemanalogs requires finding stars within about 50 pc of Earth and not older than a few tens of millions of years. During the past few years, Zuckerman and his group have carried out a long term project to find the youngest, closest stars to Earth. Using all-sky survey data in X-rays (ROSAT) and major astrometric catalogs (Hipparcos, Tycho-2, and SuperCOSMOS), they have generated a list of a few 1000 very young star candidates within 60 pc of Earth. This project has been very successful. Zuckerman and Song have observed about one thousand stars from which they have identified about 200 young nearby stars including the nearest young stellar association, the Pictoris moving group (Zuckerman et al. 2001; Song et al. 2003).

1.3.2 Infrared searches for Jupiter-mass planets around low-mass stars

Zuckerman will continue the search for young nearby stars during the period of CAN-3 with greater emphasis on lower mass stars. This is of special interest for two reasons. First, for imaging faint planetary companions around stars, one can detect lower mass planets around low mass stars. For example, at 10 pc from Earth, with AO or HST, it is possible to detect a Saturn-mass planet at an orbital separation of 20 AU around a 10 million year old M-type star. On the other hand, around a 10 million year old A-type star, only planets with masses of about ten Jupiter masses or higher are detectable at the present state of the art. Second, there are many more less-massive stars than massive stars. In fact, regardless of age, there are fewer than 300 A-type stars within 50 pc of Earth while we expect 100,000 or more M-type stars in the same volume.

Various international teams of astronomers have been imaging young stars with AO and/or HST for evidence of cooling planets. But it is generally impossible to know at which nearby star to search without a preceding survey (like that of Zuckerman and Song) to identify "young stars" and to classify them according to mass and age. Thus, all competing planet hunting groups are dependent in various ways on the results of Zuckerman and Song's search. For example, stars identified in the beta Pictoris moving group will be observed early-on with SIRTF by a variety of teams. Indeed the young stars identified by Zuckerman and Song will be re-observed again and again in coming years, each time a more sensitive spaced-based telescope becomes available to astronomers. Young nearby stars from the UCLA survey will be obvious targets for upcoming NASA missions such as Space Interferometry Mission (SIM, lead team member Ghez is a Co-I), James Webb Space Telescope (formerly NGST), and SOFIA.

With compilation of a young nearby stars catalog, members of the UCLA lead team contribute to the astronomy/astrobiology community by providing a set of valuable targets while maintaining a leading position in an ongoing race for imaging detection of cooling extrasolar planets.

< Previous Section	Next Section >