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Planets survive a cosmic tug of war

Dr Amanda Doyle, Feature writer
Dec 23, 2011, 21:35 UTC

Sen—Two Earth-sized planets have been discovered orbiting a dying star and it seems that they have survived being engulfed during the cataclysmic red giant phase of the star’s evolution. 

The planets orbit the star KOI 55 and are subsequently named KOI 55.01 and KOI 55.02. They are both smaller than our own planet, with radii of 0.76 and 0.87 times that of the Earth respectively. This means that they are among the smallest planets detected around another star.

The star KOI 55 is a subdwarf B star that has recently passed through the red giant phase of stellar evolution, during which it lost most of its atmosphere. When a star becomes a red giant it expands to tremendous proportions, and any planets caught in its path will be vaporised. Our Sun will turn into a red giant in about 5 billion years, with Mercury and Venus succumbing to a fiery death. The Earth will also most likely be engulfed by the treacherous expanding Sun, and if this happens the Earth will not survive. The planets orbiting KOI 55 are so close to the star that they must have been smothered during the red giant stage, so how did these planets endure?

It seems that the planets were born as Jupiter-like planets rather than Earth-sized ones, and they thus had massive gaseous atmospheres. The large mass of the planets is what prevented them from being completely vaporised by the star. When the dying star expanded into a red giant, it engulfed the unsuspecting planets, tearing away the thick gaseous atmospheres and leaving scorched remnants in their place, which are the Earth-sized bodies that we see today. However, this cosmic tug of war went both ways. It seems that while the planets were inside the atmosphere of the red giant, the friction of the planets with the gas of the star managed to strip the star of much of its atmosphere.

Neither of these planets are somewhere you would want to visit. Their star-hugging orbits mean that their “years” are only 5.76 and 8.23 hours respectively. Planets orbiting this close to their parent star are most likely tidally locked, meaning one side of the planet always faces the star and the other is in constant darkness. The dayside of these planets will get extremely hot due to the proximity to the star, but also because of the type of the star. As KOI 55 is a subdwarf B star, it is nearly 5 times hotter then our own G dwarf Sun. This means that the surface temperature of these planetary remnants is likely to be a whopping 8000 degrees Celsius.

It is this temperature that allowed the planets to be discovered. The planets are so hot on the dayside that they produce some light of their own, which causes a fluctuation in the overall light detected by observers on Earth. While they were discovered by the Kepler spacecraft, it was not via the usual transiting method.

“It's similar to a transit in that we use tiny variations in the brightness of the star to infer the existence of the planets,” Elizabeth Green from the University of Arizona’s Steward Observatory tells SEN. “However, in a transit, the planet blocks part of the star's light, so the star's brightness gets a tiny bit fainter during the time that planet is crossing in front of it. In our case, we see very tiny increases in light at times when the planets are on the opposite side of the star from us. When they're on the far side of the star, their bright sides are facing the star and us, so we see a tiny increase in the brightness of the whole system.”

The discovery of KOI 55.01 and KOI 55.02 came as quite a surprise as these astronomers weren’t actually looking for planets. They were studying the pulsations of the star; a field known as asteroseismology. Stars expand and contract due a battle between pressure and gravity raging within them. The pulsation of stars can be similar to breathing in and out, or they can pulse in an irregular fashion causing the star to twist into an asymmetrical shape. Astronomers observe the variations in the stars light caused by these pulsations, which in turn allows them to gather information about stellar interiors as the pulses move through the star.

While studying the pulsations of KOI 55, Green and her colleagues noticed that there were two “pulsations” that were very unusual for this type of star, with frequencies that are most likely caused by planetary orbits. “Our models show that subdwarf B stars like KOI 55 are capable of pulsating on time scales of about 45 minutes up to 4 hours. We've never found a subdwarf B star that has longer pulsations than about 4 hours, and our best models, taking into account everything we know about stellar evolution, indicate that a star like KOI 55 just can't sustain pulsations that are longer than 4 hours. There are other slightly different subdwarf B pulsators that theoretically might have somewhat longer pulsational time scales, perhaps up to five hours, but even then, the two pulsation frequencies corresponding to 5.7 and 8.2 hours would still be too long to be produced by a hot subdwarf.”

Is it possible that more of these planets could be detected with Kepler? “Over the next few years we expect to be using asteroseismology to analyse Kepler data for another ten to twelve pulsating subdwarf B stars,” says Green. “If there are similar planets with orbital periods somewhat longer than 5 hours, we would expect to find them.”