National Geographic News
Two newfound planets are seen in an illustration.

An artist's impression of planets orbiting close to a hot subdwarf star.

Illustration courtesy S. Charpinet

Ker Than

for National Geographic News

Published December 21, 2011

Two newfound Earth-size planets are probably the charred survivors of a near-death encounter with their fading parent star, scientists say.

The planetary pair, discovered using NASA's Kepler space telescope, are about 0.76 and 0.87 times Earth's radius, making the alien worlds the smallest planets detected so far around an active star, other than our sun.

But the planets didn't start small—astronomers think the worlds were once gas giants, akin to Jupiter or Saturn, that were stripped down after being swallowed by their swollen, aging parent.

(Related: "How Planets Can Survive a Supernova.")

News of the "deep fried" duo comes hot on the heels of the announcement of the first Earth-size planets found by the Kepler team.

Just yesterday scientists announced the discovery of Kepler-20e and Kepler-20f, two planets that are 0.87 and 1.03 times Earth's radius, respectively.

There are some similarities between the Earth-size planets discovered so far, said Valérie Van Grootel, an astronomer at the University of Liège in Belgium and a co-author on the latest study.

Kepler-20e and -20f "orbit very close to their star, so they are very hot planets, and that is the case with our planets, too," Van Grootel told National Geographic News.

"But there are major differences, too. In our case, we have a post red-giant star, whereas the other team's planets orbit a star like our own sun. We think that our Earth-size planets are the rocky cores of former gas planets."

Giant Planets Swallowed Whole?

The new planets orbit a subdwarf B star called KIC 05807616, which lies about 4,000 light-years away in the direction of the constellation Cygnus.

Subdwarf B stars are hot, blue stars that fall between red giants and white dwarfs—the final stage in the life cycles of stars like our sun.

When a sunlike star has depleted most of its fuel, it will swell up to become a red giant many hundreds of times its original size.

As the red giant star balloons, any planets closest to the star are completely vaporized in the inferno.

Astronomers think our sun will enter this phase in about five billion years and engulf the solar system's inner planets: Mercury, Venus, Earth, and Mars.

(Related: "Red Giant Sun May Not Destroy Earth.")

Meanwhile, interactions with gases in a red giant star's expanded atmosphere will force some planets a bit farther from the star to migrate inward—which actually gives those worlds a shot at survival.

Scientists think the newly discovered Earth-size planets were once gas giants that got sucked in toward the star during its red giant phase.

As the planets plowed through their star's hot atmosphere, their gaseous and liquid layers were stripped off, leaving behind only the rocky cores.

(Related: "Stars Can Strip Gas Giants Naked.")

No Chance of Life on New Planets

Not only did some of KIC 05807616's planets survive, scientists think the friction generated by their passage helped strip the star of most if its fiery envelope, hastening its progress from a swollen red giant to a hot subdwarf.

"We think this is the first documented case of planets influencing a star's evolution," study leader Stephane Charpinet, an astronomer at France's University of Toulouse, said in a statement.

And despite their Earth-like sizes, "we are sure there is no life on these planets," the University of Liège's Van Grootel added.

Both planets whirl tightly around the subdwarf at distances roughly 40 times closer to their star than Mercury is to our sun. That means the planets have mean surface temperatures on their star-facing sides of 15,947°F (8,842°C) and 14,110°F (7,821°C).

(Related: "Earthlike Planet Found Orbiting at Right Distance for Life.")

"It's impossible [for life as we know it] to survive at these temperatures so close to a star" that is five times hotter than our sun, Van Grootel said.

The new Earth-size planets are described in this week's issue of the journal Nature.

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