A New Test for Extrasolar Planets

disk of star HD 141569
A near-infrared view of the disk surrounding HD 141569. Recorded in 1998 by the Hubble Space Telescope, the 120-billion-km-wide disk seems to have a dark band separating a bright inner region from a fainter outer region. The innermost parts of the disk, and the star itself, are hidden behind the camera's occulting mask.
A. Weinberger, E. Becklin (UCLA), G. Schneider (Univ. of Arizona), and NASA.
Although astronomers have found the gravitational signs of more than 100 planets circling other stars, they’ve never seen one directly. Nevertheless they’re convinced that planets comparable to Jupiter in mass will be, like Jupiter, gas giants. For example, the one exoplanet clearly seen transiting its star displays a silhouette 1.35 times Jupiter’s size, implying a heat-swollen gasball with 30 percent of Jupiter’s average density. On the other hand, the “hot Jupiters” known to lie close to several stars are probably unlike anything in our solar system, with atmospheres blackened by clouds of rock dust or perhaps mists of liquid iron.

Now a new observation, published yesterday in Nature, offers hope for a new probe of exoplanet atmospheres and may also provide a glimpse of a gas-giant planet under construction. Using NASA's Infrared Telescope Facility (IRTF) in Hawaii, Sean D. Brittain and Terrence W. Rettig (University of Notre Dame) probed HD 141569, a 7th-magnitude star about 320 light-years away in the constellation Libra. This very young star still retains a circumstellar disk that extends out to perhaps 400 astronomical units (60 billion kilometers). But Brittain and Rettig concentrated on a region closer in, near where the star's energetic winds have created a clearing in the disk. There they discovered distinct infrared signatures from carbon monoxide (CO) and H3+ (essentially a hydrogen molecule with an extra proton).

The presence of carbon monoxide was expected, since it's the dominant form of carbon in circumstellar nebulae. Brittain and Rettig calculate that the CO emission comes from a wide annulus whose inner edge, 17 astronomical units (2.5 billion km) from the star, marks the limit of the central clearing. The CO emission is not particularly strong, suggesting that most of the disk has already dissipated.

But the H3+ emission comes as a surprise — although this molecule has been identified in several astrophysical settings, it has been detected by emission only in the auroral glows of Jupiter, Saturn, and Uranus. So does HD 141569 also harbor a gas-giant planet? Brittain and Rettig cautiously suggest that it might be coming from one "under construction," a protoplanetary blob circling within the clearing about 7 a.u. from HD 141569. Perhaps 300 million km across, the object would have roughly five times the mass of Jupiter. Alternately, the H3+ may be concentrated near the disk's inner edge, where molecular hydrogen (H2) could be ionized by ultraviolet radiation from the young star.

HD 141569 schematic
Based on emission from CO molecules, the inner edge of HD 141569's vast disk probably lies 17 astronomical units from the young star. But spectra also reveal an unusual form of hydrogen closer in, suggesting that a large protoplanetary mass may orbit the star.
Sean Brittain and Terrence Rettig (University of Notre Dame).
Brittain and Rettig lean toward the protoplanet scenario, if only because CO in the disk would readily destroy any H3+. They'll know if that's the case in a couple years, when Doppler shifts should divulge the putative blob's orbital motion.

But until then they and other researchers will be wrestling with an enigma: the IRTF data reveal only two of the expected three lines from the tri-hydrogen ion. "That's given us some pause," Rettig admits. On Jupiter, the emission is essentially triggered by a cascade of magnetospheric electrons, but some unknown means of excitation is likely occurring around HD 141569.

Further observations may resolve the mystery of the missing line, but in the meantime the detection of this unusual molecule is "big news," notes Takeshi Oka (University of Chicago), who has studied it extensively. "Brittain and Rettig's findings will certainly stimulate interest in the formation of giant planets and H3+ spectroscopy," Oka writes in the same issue of Nature.

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Kelly Beatty

About Kelly Beatty

J. Kelly Beatty, S&T's Senior Editor, joined the staff of Sky Publishing in 1974 and specializes in planetary science and space exploration. Learn more about him here.
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