WASP-12b, discovered in 2008, is a real outlier among the 400 or so exoplanets discovered to date. Not that it’s particularly massive (it’s a gas giant, not unlike Jupiter), nor that its homesun (host star) is particularly unusual (it’s rather similar to our own Sun), but it orbits very close to its homesun, and is considerably larger than any other gas giant discovered to date.
Results from recent research explain why WASP-12b is so unusual; we’re watching it die a painful death at the hands of its homesun, which is snacking on it.
“This is the first time that astronomers are witnessing the ongoing disruption and death march of a planet,” says UC Santa Cruz professor Douglas N.C. Lin. Lin is a co-author of the new study and the founding director of the Kavli Institute for Astronomy and Astrophysics (KIAA) at Peking University, which was deeply involved with the research.
The research was led by Shu-lin Li of the National Astronomical Observatories of China. A graduate of KIAA, Li and a research team analyzed observational data on the planet to show how the gravity of its parent star is both inflating its size and spurring its rapid dissolution.
WASP-12b, like most known exoplanets discovered to date, is large and gaseous, resembling Jupiter and Saturn; however, unlike Jupiter, Saturn, or most other exoplanets, it orbits its homesun at extremely close range – 75 times closer than the Earth is to the Sun, or just over 1.5 million km. It is also larger than astrophysical models predict. Its mass is estimated to be almost 50% larger than Jupiter’s and it is 80% larger, giving it six times Jupiter’s volume. It is also unusually toasty, with a daytime temperature of more than 2500° C.
Some mechanism must be responsible for expanding this planet to such an unexpected size, say the researchers. They have focused their analysis on tidal forces, which they say are strong enough to produce the effects observed on WASP-12b.
On Earth, tidal forces between the Earth and the Moon cause local sea levels rise and fall, modestly, twice a day. WASP-12b, however, is so close to its homesun that the gravitational forces are enormous. The tremendous tidal forces acting on the planet completely change the shape of the planet into something similar to that of a rugby or American football.
These tides not only distort the shape of WASP-12b. By continuously deforming the planet, they also create friction in its interior. The friction produces heat, which causes the planet to expand. “This is the first time that there is direct evidence that internal heating (or ‘tidal heating’) is responsible for puffing up the planet to its current size,” says Lin.
Huge as it is, WASP-12b faces an early demise, say the researchers. In fact, its size is part of its problem. It has ballooned to such a point that it cannot retain its mass against the pull of its homesun’s gravity. As the study’s lead author Li explains, “WASP-12b is losing its mass to the host star at a tremendous rate of six billion metric tons each second. At this rate, the planet will be completely destroyed by its host star in about ten million years. This may sound like a long time, but for astronomers it’s nothing. This planet will live less than 500 times less than the current age of the Earth.”
About this image: The massive gas giant WASP-12b is shown in purple with the transparent region representing its atmosphere. The gas giant planet’s orbit is somewhat non-circular. This indicates that there is probably an unseen lower mass planet in the system, shown in brown, that is perturbing the larger planet’s orbit. Mass from the gas giant’s atmosphere is pulled off and forms a disk around the star, shown in red.
The material that is stripped off WASP-12b does not fall directly onto the parent star; instead it forms a disk around the star and slowly spirals inwards. A careful analysis of the orbital motion of WASP-12b suggests circumstantial evidence of the gravitational force of a second, lower-mass planet in the disk. This planet is most likely a massive version of the Earth – a so-called “super-Earth.”
The disk of planetary material and the embedded super-Earth should be detectable with currently available telescope facilities. Their properties can be used to further constrain the history and fate of the mysterious planet WASP-12b.
In addition to KIAA, support for the WASP-12b research came from NASA, the Jet Propulsion Laboratory, and the National Science Foundation. Along with Li and Lin, co-authors include UC Santa Cruz professor Jonathan Fortney and Neil Miller, a graduate student at the university.
Source: KIAA; the paper published in the February 25 issue of Nature is “WASP-12b as a prolate, inflated and disrupting planet from tidal dissipation” (arXiv:1002.4608 is the preprint).