New "Impossible" Planet May Be on a Death Spiral

Brian Handwerk
for National Geographic News

August 26, 2009

A newfound planet orbiting a star about 325 light-years away has astronomers believing in the impossible.

Dubbed WASP-18b, the planet is ten times more massive than Jupiter but is so close to its star that it only takes one Earth day to make a full orbit, according to a new study.

Based on current theories of how planets and their stars interact, WASP-18b simply shouldn't exist—unless it's on death's door.

It's possible astronomers have gotten a lucky glimpse of a so-called hot Jupiter about to meet its fiery end, said study co-author Coel Hellier, of the U.K.'s Keele University.

But there's also a chance the planet is a mystery that could force scientists to rethink established ideas about planetary forces known as tidal interactions.

(Related: "Planet Found Orbiting Same-Size Star.")

Gambler's Luck?

Astronomers think that massive hot Jupiters are born far away from their stars and approach them gradually during eons of orbiting.

WASP-18b is now nearly next to its star, at a distance of only 1.4 million miles (2.2 million kilometers).

Current theories say that such a massive planet so close to its star should be pulling on the host star, creating a tidal effect similar to the moon's pull on Earth.

At that range the planet's pull would be so strong that it would drain energy from its orbit, causing the planet to rapidly fall into the star.

If this is the case for WASP-18b, that world would be much closer to the end of its life than any known planet.

But Hellier sets the odds of spotting a "dead planet orbiting" at about the same as drawing two consecutive red aces from a deck of cards.

"It could be a one in a thousand sort of thing," he said. "That is unlikely, but sometimes unlikely things happen."

What's more, the researchers calculate that a doomed planet as massive as WASP-18b would plunge into its star in less than a million years.

"That's a very short [timeframe] on a billion-year timescale of stellar evolution," Hellier said. That's why it's so unlikely that's what we are seeing."

Tidal Turmoil

In a paper describing the discovery in this week's issue of the journal Nature, Hellier and colleagues suggest another intriguing explanation.

If WASP-18b doesn't go into a death spiral soon, it might represent the first known example of a new way for stars and planets to counteract each other's tidal forces.

(Related: "Eccentric Exoplanet Gets Hot Flashes.")

The behaviors of stars and planets subjected to tides are measured by a complex, catch-all factor dubbed Q.

This factor is a bit like what happens when you ring a bell, said Doug Hamilton, a planetary scientist at the University of Maryland who was not involved in the new study.

"You hit a bell and it starts to ring. Q kind of measures how long [the sound] goes on."

In this case, striking the bell can be compared to an approaching planet, and the ring is like the energy the planet loses from its orbit, which then gets transferred to its star.

"The planet is stressing the star a little bit, altering its shape," said Hamilton, who wrote a commentary on the new paper that also appears in this week's Nature.

"If you took the planet away, how long would it take the star to regain its original shape? That's the measure of Q for how effectively tides are dissipated by the star."

If WASP-18b's Q factor is weak enough for it to maintain its cozy orbit, scientists will be challenged to reconcile that new reality with the far different tidal interactions they see elsewhere.

According to study co-author Hellier, "it may be peculiar to the type of star that this planet is orbiting around. Maybe those stars dissipate tidal energy a lot more weakly than other types of objects."

Stay Tuned

Luckily, we may not have to wait long for answers to the puzzling planet's fate.

If it is indeed at the very end of its life, astronomers should be able to see the telltale decay of WASP-18b's orbit in a decade or less.

"If the orbit changes in the way we expect, it tells us the planet is really going in. If not, something [perhaps unknown] is preventing it from going in, so we're going to learn something very soon," the University of Maryland's Hamilton added.

"No matter what happens, it's going to be fascinating. Of the 400 [known] extrasolar planets, I think this is the one to watch."