Rare “Hot Jupiter” Planet May Be Eaten by Its Star

The WASP-18 star is located roughly 300 light years away from Earth, but its close proximity to our planet is not what makes this star special. WASP-18 is being featured in this month’s issue of Nature because an unusual planet—a gas giant with a mass 10 times that of Jupiter—is orbiting dangerously close to the star.

“Hot Jupiters” are not uncommon in the universe. In fact, according to Ben Mathiesen of PhysOrg.com, the large gas planets account for most of the planets discovered to date. It is believed that these large planets form early in the life of a solar system from the huge cloud of debris that sometimes surrounds a star after formation. Because of their mass, it is thought that “hot Jupiters” get pulled inward toward their stars, sucking in debris and even baby planets as they get closer, until reaching a stable orbit around the sun.

What makes WASP-18b an unusual gas giant is its size and its closeness to its star. According to Seth Borenstein of the AP, WASP-18b is .02 AU away from its star (one AU is the distance from the Earth to the sun). Borenstein notes that both the planet and the star should experience tidal effects about 1000 times stronger than those felt between the earth and the moon.

WASP-18b orbits its star about once a day—compare that with Earth’s 365-day orbit. With all the tidal pulling between the two, Phil Berardelli of Science Magazine explains that eventually “it will smash into the star’s surface. Or … be shredded to form a Saturn-like ring system.”

Scientists calculated that the gas giant should have had a lifespan of less than a million years, but the star it orbits is much older than that, perhaps even 1 billion years old according to ScienceDaily. So why hasn’t the star eaten its planet?

Bernadelli reports that study co-author Andrew Collier Cameron looks to the possibility that astronomers might have made incorrect assumptions about how tidal and other forces work at such a close range; the planet may not actually be moving inward for some reason yet unknown. 

Scientists will continue to monitor WASP-18b, and if it doesn’t show signs of moving inward over the next few years they will have to rethink some longstanding theories.

Large space bodies have the potential to pull in smaller orbiters and eventually “eat” them, or break them apart. After years of wondering how Jupiter’s roughly 62 moons formed without being absorbed by the planet, astronomers now think that the giant planet “ate” its earliest moons and that the orbiting bodies we see now are the survivors or latecomers to the planet’s orbit.

Many of the extrasolar planets discovered until now have been found through indirect methods, meaning that they are not seen but rather that scientists assume they exist based on the gravitational effect they exert on their star, or through infrared observation. The planets are usually extremely hard to see because of the brightness of their star. In late 2008, two teams of scientists made history by “seeing” new planets around the Fomalhaut star and the HR 8799 star.

Often scientists will look to Sun-like stars for signs of planets (and hopes of finding rocky Earth-like planets), but newer evidence shows that binary (two) star systems may have equal potential for harboring planets as single star systems like our own.

Also, it was formerly thought that the presence of “hot Jupiters” like the one orbiting WASP-18 would destroy any rocky planets forming in the same system, but recent models of solar system formation suggest that rocky planets (and potentially even rocky planets in the “habitable” zone of a solar system) could still exist in systems with a gas giant near the star.