A newly discovered planet that whips around its star in less than a day may have been found mere cosmic moments before its demise.
The planet, WASP-18b, is one of the "hot Jupiter" class of planets that are huge in size (10 times the mass of Jupiter in this case), but orbit very close to their stars. Their very existence was surprising to astronomers when the first of them were found a few years back. Now they've become common discoveries.
But this scorched, gaseous world is only one of two known exoplanets that orbits its star in less than one Earth day (0.94 days to be exact). Coupled with its hefty mass, this leads to strong gravitational tugs between the planet and its star, WASP-18. (WASP stands for the Wide Angle Search for Planets, run by several universities in Britain.)
These gravitational interactions create tides thought to squeeze and stretch the planet, and even alter its orbit. If the planet orbits faster than the star spins, the planet should be pulled inward toward the star; if the star spins faster, the planet should be pushed outward. (The latter is the case with the moon's orbit around Earth; the moon is moving away from us as you read this.)
Given what astrophysicists know about the dynamics involved, astronomers think WASP-18b is moving toward its host star, but that would make observing the planet a 1-in-1,000 shot: While planets spend most of their lives sort of growing up, they perish in a cosmic blink of the eye. And so there is only a small time window where a planet would be in this position of impending demise. Statistically, it would be more likely that the star would be observed much earlier in the planet's lifetime, or after the planet's destruction (which means the planet wouldn't have been seen at all).
"Either the odds of finding it are really small, and we just got lucky," or there's something fundamental about the tidal interactions between stars and their planets that astronomers are missing, said Douglas Hamilton, an astronomer at the University of Maryland in College Park who was not involved with the finding.
Knowledge of how tidal interactions work between stars and planets is largely based on our own solar system, having to do with all the rotational (or tidal) energy that a system has when a cloud of gas and dust condenses to form a star. Material left over from the star's formation flattens into a rotating disk where planets form. Estimates of the rate at which a star dissipates all that tidal energy could be off-base, however. If the star isn't very good at dissipating this energy, the planet will survive a lot longer before spiraling into its host star.
Or there could be "more exotic possibilities," Hamilton told Space.com, such as some force holding the planet up against the inward drag of the tidal forces. Hamilton described the possibilities in an opinion essay in Thursday's issue of the journal Nature, where the discovery of the planet is also detailed.
Astronomers will be able to test which scenario (a planet near its death or some funky physics) is more likely — they just have to wait a few years. If the planet is, in fact, spiraling in toward its star, the demise might not occur for thousands or millions of years, but there should be noticeable changes in its orbital period in about a decade. Astronomers just have to keep their eye on the system.
"We just gotta wait and be patient," Hamilton said. Whatever the answer turns out to be, "this one is going to teach us something," he said.
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