Saturday science subject: Could Earth survive?

Our Sun is about half-way through its life now, about four and a half billion years. In another four or five billion years, our star will run out of hydrogen and swell up into a red giant, and some believe it will engulf and vaporize all nearby planets—including Earth.

However, a report by Nature says planets that orbit close to their host stars may not be doomed. Roberto Silvotti of the National Institute for Astrophysics in Naples, Italy has spotted a planet orbiting around V 391 Pegasi, a star that has already gone through the red giant phase. The star has now evolved into a hot B-type subdwarf, and its neighbor planet seems to have survived its host star turning into a red giant.

The planet that Silvotti found . . ., V 391 Peg b, is 1.7 astronomical units away from V 391 Pegasi. This is just within the 2 astronomical units thought to be too close for comfort for planets near red giants (one astronomical unit is the distance from Earth to the Sun). The planet orbits the star roughly once every 3.2 years and is also one of the oldest planets known, Silvetti says in his paper published today in Nature.

Its discovery is exciting, says Matt Burleigh, an astronomer from the University of Leicester, UK. "This is the first time a planet has been found around a star that has evolved so far," he says. Burleigh is looking for planets around white dwarfs — stars further on still in their evolution than red giants, and Silvotti's research gives Burleigh hope that he will be successful. "This work suggests that planets should be there around white dwarfs," he says.

That said, our planet may still not live through the Sun's changing stages. Nature says the situation of V 391 Peg b and that of Earth aren't directly comparable. According to Burleigh, "We lie right on the borderline between survival and being engulfed by the Sun's expanding atmosphere when it becomes a giant." And even if the Earth does survive, "it will probably be a barren rock, unfortunately," Burleigh says.

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