Friday, April 3, 2015

Giant Planet near the Edge of Destruction

WTS-2b is a hot Jupiter in an unusually close-in 1.02-day orbit around a K-type star. The planet has 1.12 times the mass and 1.363 times the diameter of Jupiter. Like Jupiter, WTS-2b is a gas giant planet comprised mostly of hydrogen and helium. WTS-2b orbits so close to its host star that the separation between the planet and its host star is only 1.5 times the distance where the planet will start to get torn apart by strong tidal forces.

Figure 1: Artist’s impression of a hot Jupiter in a close-in orbit around its host star.

Figure 2: Transit light curve of WTS-2b. Birkby et al. (2014).

WTS-2b raises tides on its host star. The tides exert a strong torque that transfers energy from the planet’s orbit to the star’s spin. This causes the planet’s orbit to shrink and the star to spin up. WTS-2b is estimated to have another ~40 million years before its in-spiralling orbit brings it close enough to its host star to be tidally destroyed. Because WTS-2b transits in front of its host star every 1.02 days, the planet’s shrinking orbit can be directly measured. The in-spiralling is estimated to create a ~17 seconds decrease in the duration between consecutive transits over a period of 15 years.

Most hot Jupiters have orbital periods around 3 to 4 days. There appears to be a lack of hot Jupiters with orbital periods less than 2 days. This could mean that either it is very difficult to get hot Jupiters into very close-in orbits, or that they are quickly destroyed by tidal forces once they are in such orbits. Being so close to its host star, the amount of insolation WTS-2b gets is ~1,000 times more intense than what Earth gets from the Sun. As a result, the day side of WTS-2b is heated to an estimated ~2,000 K.

Birkby et al. (2014), “WTS-2 b: a hot Jupiter orbiting near its tidal destruction radius around a K-dwarf”, arXiv:1402.5416 [astro-ph.EP]