The Ongoing Demise of a Planet

Our own Jupiter is a tranquil paradise compared to some exo-planets undergoing tidal disruptions.
Jet Propulsion Laboratory

The atmosphere of planet WASP-12b, more than 800 light-years from Earth, is being pulled away by the gravity of its sun at the staggering rate of six billion metric tons per second. This is the first time astronomers have witnessed the ongoing disruption and death march of a planet. The discovery was made by an international group of astrophysicists, primarily working at the recently established Kavli Institute for Astronomy and Astrophysics (KIAA), Peking University. The research findings have been published in the journal Nature.

Until 15 years ago, the only known planets were those in our own Solar system. To date, more than 400 planets have been detected around other stars. Most of the exo-planets discovered tend to be gas giants with a mass comparable to that of Jupiter, since these are easier to detect than smaller Earth-size planets. However, unlike Jupiter and Saturn, one in five of these exo-planets are on orbits very close to their host stars. In some of these systems, the planet regularly undergoes annular eclipses, passing between its host star and Earth. By accurately measuring how much of the star’s light is blocked out and how long the occultation lasts, astronomers can measure the planet’s size and mass. Astrophysicists then use this information to calculate the internal make up of these planets and construct scenarios on how they may have acquired their properties.

The planet WASP-12b has puzzled astronomers since it is one of the few planets that is significantly larger than astrophysicists had predicted. The planet’s mass is almost fifty percent larger than that of Jupiter, the most massive planet in our own Solar System. Although theoretical models had predicted that WASP-12b should have about the same size as Jupiter, it turned out to be 80% larger in size (or almost six times larger in volume). Some mechanism therefore has to be responsible for “puffing up” the planet.

WASP-12b revolves around its host star approximately once a day, at an average distance of 0.0207 AU. The small separation between WASP-12b and its host star makes it among the most intensely heated planets known. Its day-time temperature is more than 2500 degrees centigrade.

On Earth, tidal forces between the Earth and the Moon cause local sea levels to modestly rise and fall twice a day. WASP-12b, however, is so close to its host that the gravitational forces are enormous. The tremendous tidal forces acting on the planet change the shape of WASP-12b into that similar to an American football, oblong rather than spherical.

The continuous deformation of a planet causes friction in the planet’s interior, producing heat. Professor Doug Lin, director of the Kavli Institute for Astronomy and Astrophysics, comments, “This is the first time there is direct evidence that the internal heating (or “tidal heating”) is responsible for puffing up the planet to its current size.”

The research team at the Kavli Institute for Astronomy and Astrophysics at Peking University has shown that the combination of the strong tidal forces and the puffed-up size of WASP-12b is causing the planet to lose mass to the host star. The head author of the Nature article, Shulin Li, who carried out the bulk of the research work as part of her PhD in Beijing, commented: “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 WASP-12b will be completely destroyed by its host star in about ten million years.” She continues: “This may sound like a long time, but for astronomers it’s nothing. This planet will live less than 500 times shorter than the current age of the Earth.”

The material that is stripped off WASP-12b does not directly fall onto the host star. Instead, it forms a disk around the star, and slowly spirals inward. A careful analysis of the orbital motion of WASP-12b provides 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 are detectable with currently available telescopes. Their properties can be used to further constrain the history and fate of the mysterious planet WASP-12b.