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An artist's concept of an exoplanet orbiting a star.

An artist's rendering of a "hot Jupiter" in front of its star.

Image courtesy C. Carreau, ESA

Richard A. Lovett in Seattle, Washington

for National Geographic News

Published January 24, 2011

In the search for life on Earthlike planets, scientists have been particularly excited about finding worlds in the so-called Goldilocks zone, the region around a star that's just right for liquid water. (Related: "First Truly Habitable Planet Discovered, Experts Say.")

But new research suggests that—Goldilocks or not—many of the known planets outside our solar system are orbiting stars that may be too hazardous for life.

According to a new study, Jupiter-size planets in close orbits around their stars can make their middle-age stellar parents unexpectedly regain the violence of youth.

Such stars produce gigantic flares, which could shower otherwise habitable planets with dangerous radiation, searing blasts of heat, and ozone-destroying ultraviolet light.

Video: Solar Eruptions Captured in 3-D.

In a related study, astronomers also found that old, dim stars in tight-knit pairs appear to be experiencing a similar effect, producing mega-flares that quickly and suddenly brighten the stars by up to 10 percent.

"Imagine our sun brightening by 10 percent," said Geoffrey Marcy, an astronomer at the University of California, Berkeley, who was not part of either study. "Such powerful flares [would] bode ill for life close to that star."

"Nightmare" Flares Could Wreck Ozone Layer

Young stars are often spinning very rapidly shortly after birth, creating strong magnetic fields. Various forces on the star's surface cause magnetic field lines to get tied into knots, which produce flares and other types of radiation bursts.

An extreme example is the red dwarf star YZ CMi, a young star that's probably only a few hundred million years old. By contrast, our sun is considered middle aged, at 4.5 billion years old.

YZ CMi rotates on its axis once every 2.8 days, nearly ten times faster than the sun. This spin is fast enough to produce stunningly violent flares, said Adam Kowalski, a graduate student at the University of Washington.

In 2009, Kowalski watched as a particularly large flare jacked up YZ CMi's ultraviolet emissions by a factor of more than 200—enough to totally wreck the ozone layer of any rocky world orbiting within the star's habitable zone.

"I have nightmares about this flare," Kowalski said earlier this month during a meeting of the American Astronomical Society in Seattle, Washington.

Big, Hot Planets Are "Fountains of Youth"

As a star ages, its spin will slow, which should tame its violence. Our sun, for example, still goes through cycles of activity and produces flares—some of which can be powerful enough to disable satellites or knock out power grids—but nothing on the scale of YZ CMi.

(See "Giant 'Tornadoes' Seen Erupting From the Sun.")

Recently, however, Villanova University astronomer Edward Guinan saw giant x-ray flares, large star spots, and powerful coronal mass ejections coming from a star that should be about the same age as the sun.

HD 189733 is an orange dwarf star that's 80 percent the size of the sun but that spins twice as fast: once every 12 days. Based on its activity levels, the star would seem to be about 600 million years old.

The hyperactive star has a distant, more sedate companion star, which astronomers estimate is at least 4.5 billion years old. That's puzzling, since the two stars almost certainly formed at the same time and should be the same age, Guinan said.

What's keeping HD 189733 dangerously young, Guinan said, is a Jupiter-size planet that's slowly spiraling into its host. (Related: "Star 'Eating' Superhot Planet's Atmosphere.")

This "hot Jupiter" is so close to the star that it orbits once every 2.2 days. In the process, the planet's magnetic field is pushing against the star's, Guinan said, transferring angular momentum and making the star spin faster.

Astronomers don't know if the HD 189733 system hosts any rocky, Earthlike worlds. But thanks to the hot Jupiter, the star's youthful behavior would likely spell doom for life on any other potential planets.

(Related: "NASA Finds Smallest Earthlike Planet Outside Solar System.")

The finding could be a blow to the wider hunt for habitable worlds: Many of the more then 500 extrasolar planets found so far are hot Jupiters, and computer models suggest that more than a third of the star systems containing these giants may also harbor Earthlike planets.

"In our study of other hot Jupiter systems," Guinan said, "it looks like many of the host stars rotate fast and appear young."

Hopes Dim For Life Under Twin Suns

In addition to hot Jupiters, some older stars may be tapping into a "fountain of youth" in the form of close stellar siblings.

At the AAS meeting, the University of Washington's Kowalski presented Hubble Space Telescope data on 200,000 dim stars about twice the age of the sun. These types of stars, among the most plentiful in the galaxy, should have been among the most tranquil.

But in the course of a week, Kowalski counted a hundred stars that emitted superhigh-energy flares, each strong enough to increase the star's brightness by up to 10 percent.

"This is much larger than the largest solar flare we have observed" on our sun, he said.

Astronomers don't yet know whether these stars host planets of any size. What scientists do know is that all these stars have stellar companions in extremely close orbits

(Related: "Many Planets Could Circle Twin Suns, NASA Says.")

The stars are so close they are tidally locked, which means that one side of a star always faces its companion, just as one side of the moon always faces Earth. Some of the tightest pairs orbit each other—and therefore spin on their axes—once every three days, Kowalski said, "which is really fast."

And the intense activity coming from such rapidly spinning stars would put a serious damper on the chances of life existing on nearby planets.

According to UC Berkeley's Marcy, "it's interesting that the most numerous stars in our galaxies pose these risks."

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