The Demise of Planet WASP-18b

We’ve reported previously on WASP-12b and WASP-17b, two of the nearly twenty planets discovered by the UK SuperWASP team. (“WASP” stands for “wide-angle search for planets.”) But it’s planet WASP-18b that made headlines this week.

WASP-18b is so close to its star that it takes only 94 percent of an Earth day to complete an orbit.

The newly discovered WASP-18b belongs to a class of exoplanets known colloquially as “hot Jupiters”—extremely large planets (WASP-18b is ten times the mass of Jupiter) that orbit extremely close to their stars. WASP-18b is so close to its star that it takes only 94 percent of an Earth day to complete an orbit. (By contrast, Mercury’s relatively short orbital period is nearly 88 Earth days.)

Because of the gravitational pull the star exerts, astrophysicists believe WASP-18b is on a collision course with the star it orbits. But that raises a question: how is it that we found the planet so close to its demise? As LiveScience’s Andrea Thompson puts it, “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—it would be statistically more likely to have found it much earlier in its lifetime, or after its destruction (which means it wouldn’t have been seen at all).”

University of Maryland–College Park astronomer Douglas Hamilton said the same: “Either the odds of finding it are really small, and we just got lucky”—or astrophysicists are misunderstanding the full nature of the gravitational interactions between planets and stars. However, that understanding is partially based on evolutionary models of the origin of solar systems. Also, billions-of-years dogma dictates that a “cosmic blink of the eye” may actually be thousands or millions of years.

Regardless, astronomers are eager to keep a close eye on WASP-18b for changes in its orbit. It could be that astrophysicists are mistaken and that we don’t fully understand the true nature of physical forces in orbital contexts. Or it could be that WASP-18b is indeed on a collision course, perhaps heading toward impact even sooner than scientists guess. If that is the case, we will again have to revisit the issue of whether it was merely a one-in-a-2,000 chance (as The Independent reported) that we observed the planet so near its demise, or whether perhaps evolutionary timetables fail to predict the actual speed of events in the universe.

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