Some exoplanets around red giant stars may be no more than an optical illusion. Variations in starlight that appear to be caused by orbiting worlds could come from ripples in the stars’ surfaces instead.
One method for finding stars relies on measuring changes in the star’s velocity as an orbiting planet makes it wobble back and forth. Because those movements are too small to detect directly, we find them by looking for how they affect the colour of the star’s light.
at the University of Texas at Austin and his colleagues examined years of data from four different observatories to characterise the “wobble” of a star called Gamma Draconis. It is a red giant star, similar to the behemoth the sun will eventually evolve into, 154 light years away.
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From 2003 to 2011, the star’s wobbly signal seemed to indicate that it had a single planet with a mass at least ten times that of Jupiter, orbiting once every 702 Earth days.
But from 2014 to 2016, the wobble disappeared. It then re-appeared in 2016, but wobbling out of sync with the previous signal. At the time, astronomers thought this could potentially be caused by Gamma Draconis having two planets with similar orbits.
Not a planet at all
Cochran and his team found that simulations of such a two-planet system were unstable – the planets would have to be too close together, and they would jostle one another out of their orbits. The probability of the worlds staying in their respective lanes was just 1 to 2 per cent.
“For years, we thought we had a planet here, but there was always this nagging worry,” says Cochran. “Going through with a fine-tooth comb, we found that there probably is no planet.”
The uncertainty was based on the fact that the planets that were supposedly orbiting Gamma Draconis appeared to have long orbits that took about as long as one rotation of the star. If that were the case, the planet would remain above one spot on the surface of the star for its entire orbit. So, it could be that a particular area on the star’s surface is giving off a false signal, masquerading as a planet high above the surface.
Cochran says that the changes in starlight were simply due to enormous sunspots or ripples within the stars’ plasma atmospheres. He says this could bring into question whether some of the exoplanets that have been recently discovered are actually there, though it would be a small percentage.
This trick of the light might not be unique to Gamma Draconis – similar stars that seem to have exoplanets may be illusions, too. “Here is a note of caution,” says Cochran. “We really need to go back and look at the other ones in a lot more detail now.”
Read more: Fake planets reveal distance to Earth’s nearest twin
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