Space

Rotation Of A Planet Orbiting Another Star Measured For The First Time

February 22, 2016 | by Stephen Luntz

Photo credit: Artist’s impression of 2M1207b and the barely visible brown dwarf it orbits. NASA, ESA, and G. Bacon/STScI

For the first time, the rotation of an exoplanet – a planet outside the Solar System – has been observed directly. Considering it's only been 12 years since the first direct observation of planets outside the Solar System, the achievement is remarkable.

The planet, 2M1207b, has around two to five times the mass of Jupiter. It has no parent star, instead orbiting 2M1207 – a brown dwarf five to seven times as massive as the planet. Graduate student Yifan Zhou of the University of Arizona used the Hubble Space Telescope to measure the changes in 2M1207b's brightness. In The Astrophysical Journal, Zhou argues that these changes indicate differences in the clouds that surround the giant planet.

The feat was only possible because the clouds are patchy. The planet's rotation creates a 10.7-hour-long cycle with a 1.36 percent variation in brightness in the near-infrared, and 0.78 percent variation at slightly longer wavelengths. This means a "day" on the planet, one rotation, is roughly equivalent to Jupiter at 10 hours.

2M1207b's heat is almost entirely internal. Not only does its parent put out very little heat, but the gap between the two is 8 billion kilometers (5 billion miles), more than 50 times the Earth-Sun distance. Despite this, 2M1207b is still very hot because it is only 10 million years old, and its contraction is still releasing a lot of energy. "The atmospheric temperatures are between about 1,200 and 1,400 degrees Celsius (2,200 and 2,600 degrees Fahrenheit)," Zhou said in a statement.

These temperatures vaporize rock, which cools in the upper levels of the atmosphere. "So at higher altitudes it rains glass, and at lower altitudes it rains iron," said Zhou.

Such high temperatures cause 2M1207b to emit a lot of infrared light, allowing its detection. Nevertheless, it is not as bright as expected for its size and age, and even before these observations, this led to speculation it might be shrouded by patchy clouds.

Two years ago, a weather map was created of a brown dwarf, and similar techniques have enabled the measurement of the rotation of several of these objects. However, the paper notes, “High contrast amplifies the challenges for directly imaged exoplanets and planetary-mass companions to acquire high-precision light curves compared to isolated brown dwarfs.” Moreover, the most detailed brown dwarf maps have been made of objects many times closer than 2M1207b.

Zhou noted that the similarity in mass between 2M1207 and 2M1207b, along with the distance between them, indicates they “formed differently than our own Solar System.” He suggested that the origins of the system lie in two small disks of gas that collapsed independently, rather than smaller objects forming from the outer reaches of a large disk.

The fact that Hubble was able to detect 2M1207b's rotation has increased excitement about the prospects for the James Webb Telescope (JWST), which will launch in 2018, and will be far more sensitive at these wavelengths. Potentially, the JWST will be able to provide significant detail about the upper clouds around 2M1207 and other young gas planets.

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