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Scientific Method / Science & Exploration

Even dwarfs can have planets

A planet has been discovered in orbit around a dwarf star, suggesting for the …

When it comes to the eventual fate of the solar system, we've all heard the standard line. The sun, currently enjoying a healthy middle age converting hydrogen to helium will eventually turn into a red giant, and that'll be bad news for the planets that surround it. Once the core of a star is exhausted of hydrogen, it expands in radius  up to a hundredfold, and that tends to have serious implications for things in nearby orbit.

Although it had been assumed that this enlargement would destroy orbiting planets, new findings in Nature suggest that there's life after giant-hood, at least as far as planets go. The star in question in V 391 Pegasi, currently a B-type subdwarf that's in that relaxed stage of a star's dotage when it wants nothing more than to convert helium into carbon. V 391 Pegasi also pulsates, apparently not a very common trick for B-type subdwarfs, but it is thanks to this pulsation that we were able to spot a planet in orbit around it. This was done by a periodic change in the timing of the pulses, due to the planet's gravitational effect on the star.

The planet surrounding V 391 Pegasi was originally in orbit around the star at about the same distance that we orbit the sun, 1 astronomical unit (AU). When the star became a red giant, it would have expanded to around 70 percent of this distance, and now the planet, a massive gas giant, is around 1.7 AU. V 391 Pegasi is a rather uncommon type of dwarf star, having lost most of its atmosphere of hydrogen as a red giant. It is speculated that perhaps the presence of this planet might have contributed to its eventual fate as a B-type dwarf, with the gravitational pull from the planet somehow accelerating the loss of hydrogen from the star.

 Nature, 2007. DOI: 10.1038/nature06143

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