RICHARD BIZLEY/SCIENCE PHOTO LIBRARY
The Kepler space telescope has seen thousands of exoplanets, and now we can perhaps add exocomets to the list. Potential evidence of comets has been found around two stars known as KIC 3542116 and KIC 11084727, both about 800 light years away.
Kepler finds planets by measuring the intensity of a star’s light over time. When a planet passes in front of its host star, an event called a transit, the light dims slightly. The “light curve” – a graph of the star’s brightness over several days – shows a symmetrical shape, with the light levels dropping and then rising again at equal rates.
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This symmetry arises because both planets and stars are spherical, but the transits found by Andrew Vanderburg at Harvard University and his colleagues were asymmetrical, meaning that whatever made them was not a sphere. The team turned to comets as a possible culprit, because they release gas and dust from one side, creating long tails that stretch out into space.
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“If you have a bunch of dust that you put in space, it will expand quickly outwards, not necessarily be symmetric,” says Vanderburg. You would see a steep dip in the star’s light, followed by an uneven rise.
Over the four years of the initial Kepler mission, the team saw six transits of KIC 3542116, indicating between two and six comet-like bodies in orbit, but only one transit of KIC 11084727, making it harder to draw conclusions from that evidence. Each lasted for around a day.
It is possible that these light curves are caused by something else, but Vanderburg says the team thinks the evidence is good. “We can’t prove yet that these transits are due to comets, but we’ve put together what we think is a pretty strong circumstantial case.”
Comet-spotting
This isn’t the first time astronomers might have found exocomets. In 2013, Barry Welsh at the University of California, Berkeley, presented findings of changes in a star’s spectra, the absorption lines that show when gas of specific elements is blocking light from a star. However, that wasn’t a direct detection like the one claimed by Vanderburg’s team. Welsh says a spectrographic study would clinch their case for comets, or show they found rocks and dust.
Another star, KIC 8462852, also has weird transit signals that were initially linked to comets, but that explanation was later dismissed. Its true nature remains a mystery, with suggestions ranging from an “alien megastructure” to a Saturn-like ringed exoplanet.
“I think it is good work. A comet explanation is possible, and they have demonstrated to my satisfaction that the signals are real and astrophysical,” says Joshua Pepper at Lehigh University in Bethlehem, Pennsylvania.
For him, the only odd thing is that the proposed comets have very short “periods”, meaning they complete a full orbit in around three months or less, based on how long the transits last and how many were observed. Comets in our own solar system typically have periods measured in years or decades.
Reference: arxiv.org/abs/1708.06069
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