The Why Files The Why Files --

Fatal attraction: Young hottie nuzzles close to star!

The hot-blooded companion, named WASP-18b, has 10 times the mass of Jupiter, and is one of the heftiest planets to emerge from searches of nearby space.

So far, about 360 extra-solar planets have been detected, says Coel Hellier, a professor of astrophysics at Keele University in the United Kingdom. About 300 appeared because the planet regularly tugs its star, subtly changing the color of the starlight due to the Doppler effect.

A color graphic shows the two wavelengths emitted from a detector towards a wobbling star
Image: NASA
One way to detect a planet is to study the color of a star's light. Large planets cause stars to wobble. As the star moves toward us, its light shifts toward shorter, or bluer, wavelengths. As the star heads away, its light stretches into longer, or redder, wavelengths.

Hellier's group, in contrast, looks for the slight dimming that occurs when a planet passes ("transits") in front of its star.

A straight line with a U-shaped dip in the center represents a scattering of points on the graph
Image courtesy Coel Hellier.
More than 8,000 light measurements at WASP-South created this light curve for WASP-18. The dip at center marks where planet WASP-18b passed before the star, blocking some light.

Although the transit method only "sees" planets that happen to orbit directly between the star and Earth, transits reveal the planet's size and mass, and that allows a density calculation. "With the transit method, we get a lot more information than with any other technique," says Hellier, who is first author of the new report on 18b.

18b is about 7 times as dense as Jupiter, and is likewise composed mainly of hydrogen.

Many of the previous discoveries from WASP -- the Wide-Angle Search for Planets -- orbit in less than four Earth days, but 18b is the first confirmed hot-Jupiter planet that orbits in less than one day.

And that means its orbital distance is about 2 percent of Earth's distance to the sun, which makes it the first planet ever seen with such a speedy orbit.

WASP-18 and 18b are, in a word, quite an item.

Two rows of four telescopes are mounted on a metal swing arm, pointed towards the sky on an angle
Photo: NASA
Each of the two SuperWASP observatories carries eight cameras equipped with heavy-duty light detectors. One camera covers up to 2,000 times as much sky as a regular telescope, and can "see" up to 100,000 stars per image. Those monster numbers boost the odds of detecting a planet.
Artist depiction of a Jupiter-like planet silhouetted by large orange fiery star
Artist's rendering from NASA

British astronomers reveal sizzling details of cosmic tryst

Just as a Hollywood babe can get burned by close proximity to a Brad Pitt, 18b endangers itself by nuzzling so close to a stellar biggie. And this big ball of hydrogen is more scorching than a Weekly World News romance: Hellier and Co. estimate its surface heat at 2111 °C.

For reasons of size and body heat, 18b is called a "hot Jupiter."

More important, a big planet in such a close orbit would create a huge tidal interaction and distort the star. "It's a bit like Earth's ocean being influenced by the moon, causing a bulge in the ocean," says Hellier. "In the same way, the planet is causing a bulge in the star."

A white star with an orange halo to the left of a much smaller white planet also with an orange halo
2005 image of GC Lupi, NASA
Most extra-solar planets are too close to their stars to be photographed, but the European Southern Observatory snapped this hot Jupiter orbiting about 1,500 times further from its star than 18b.

Earth-bound paparazzis have snapped a dazzling new image of a hot young thing, a plus-size planet clinging perilously close to the star WASP-18, located about 320 light years from Earth in the southern sky.

The size of the bulge is difficult to estimate and impossible to see, but it could be tens of miles high, Hellier says.

And now we move beyond simple star-gazing. Creating that bulge steals energy from the planet's momentum, which is what causes it to orbit in the first place.

A brilliant item, but a brief one...

If the star is built like our closest stellar neighbor, then "one-millionth of the planet's tidal energy would be lost in each orbit," says Hellier, which would cause 18b to crash into its star within 1 million years.

Since we are less likely to detect brief events, it's highly unlikely that we would catch this system in this particular condition.

But if the star has less internal friction than our star, then the orbital decay could be much slower, which would raise the odds of viewing 18b.

Why Files dishes on how long 'til this twosome goes up in flames!

A blue ellipse that circumscribes a green circle representing earth bulges out towards the moon
Image: AndrewBuck
Just as the moon's gravity attracts Earth's ocean, the gravitational interaction between 18b and its star steals energy from the planet, which will eventually crash into its star.

18b offers a "heads I win, tails you lose" situation. If the internal friction in WASP-18 is like that in our sun, then 18b will slow noticeably within 10 years, allowing astronomers to watch tidal interaction in action.

Otherwise, we may need to revise the assumption that our sun's innards reflect the situation throughout the universe. "There is a lot of theory, but not a lot of data, regarding the dynamics inside a star," says Hellier. "WASP-18 is telling us that the value for Q [a measure of internal friction] is probably not correct for this situation."

Even though almost 400 planets have been discovered, "There are very few that have a decay period shorter than a billion years, so this is spectacular," says Douglas Hamilton, a professor of astronomy at the University of Maryland. "If the decay is fast, we can measure it, and get a measure of Q based on evidence rather than theoretical arguments. Or if we watch for 10 years, and it's not moving in, that will show that Q is off by a spectacularly large amount, or something else is going on. It has potential to teach us something in 5 to 10 years, and none of the other planets will do that."

So which is it: is Q wrong, or is the planet as fleeting as Tara Reid's latest hunk? "Most likely it's a bit of both," says Hellier. "The Q value for this system is probably much higher than it is in our solar system, but the planet is probably a rare, short-lived object anyway."

David J. Tenenbaum

Related Why Files

• Detecting Planets: Lost and Found in Space

• Scoping Out The New Telescopes

• Extra Planetary Perception


• An orbital period of 0.94 days for the hot-Jupiter planet WASP-18b, Coel Hellier et al, Nature, 27 Aug. 2009 (see also News and Views by Douglas Hamilton, same issue).

©2021, University of Wisconsin, Board of Regents.