ScienceShot: Weighing an Alien Planet

Step on a tilted scale and it probably won't read your weight right. Planet hunters have a similar problem: When they detect a planet tugging on its star, they often don't know how tilted the planet's orbit is—and so they can't measure the world's exact mass. Now, as astronomers report online today in Nature, infrared spectra from the Very Large Telescope in Chile have unveiled the orbital tilt of one of the first planets ever found, allowing the researchers to derive its true mass. Located 51 light-years from Earth, this planet—named Tau Boötis A b—is a "hot Jupiter" that whirls around its sun every 3 days, 7 hours, and 30 minutes. The astronomers discovered carbon monoxide in the planet's air. Coupled with the orbital period, the poison gas's Doppler shift reveals that the planet's orbit is tilted 45° to our line of sight and that the world itself weighs six times more than Jupiter . Furthermore, the spectra of the carbon monoxide molecules indicate that unlike many other hot Jupiters, the planet's atmosphere has no temperature inversion; instead, temperature drops with increasing altitude. The reason? Tau Boötis A spins fast and emits ultraviolet radiation, which destroys compounds in the planet's upper air that would otherwise absorb starlight, heat up, and cause the temperature to rise with altitude.

*Update on 28 June: On 1 July in Astrophysical Journal Letters, another team of astronomers reports detecting carbon monoxide in the planet's atmosphere and derives an orbital tilt and planetary mass that agree with the results above.

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