spacetoday.net: Spectra allow astronomers to measure mass of exoplanet

Posted: Thu, Jun 28, 2012, 6:05 AM ET (1005 GMT)
Tau Bootis b illustration (ESO)

Astronomers announced this week that they have used spectroscopic observations of an extrasolar planet to directly measure its mass. Astronomers at the European Southern Observatory used an infrared spectrometer to study Tau Bootis b, an exoplanet orbiting the star Tau Bootis that was originally discovered in the mid 1990s. At the time, and as is the case with most extrasolar planets discovered through the radial velocity technique, astronomers could only compute the minimum mass of the planet since they did not know the inclination of the solar system to the Earth. The new observations were able to directly detect carbon monoxide emissions from the planet's atmosphere at infrared wavelengths; the shifts of that spectra as the planet moved around the star allowed astronomers to calculate the orientation of the orbit and, from that, the planet's mass of 5.6 Jupiter masses. The observations also allowed astronomers to probe the exoplanet's atmosphere, showing that the atmosphere is cooler at higher altitudes, the opposite of other "hot Jupiter" exoplanets studied to date.