Another Earth or data disturbance? we may never know
The exoplanet, called HD 137010 b, may be the closest thing to “Earth 2.0” astronomers have ever seen. The problem is that it’s only been seen once—and may never be seen again
Artist’s concept of exoplanet candidate HD 137010 b, which astronomers believe might share many of Earth’s characteristics if it actually existed.
NASA/JPL-Caltech/Keith Miller (Caltech/IPAC)
Astronomers believe they have found a celestial unicorn: a potentially habitable near-Earth twin about the same size as our own familiar planet and perhaps a year across, orbiting a star just like ours. The only problem is that they’re not sure it’s actually there.
The first—and so far the only—hint of a possible planet came in observations from NASA’s now-retired Kepler space telescope. In 2017 the telescope recorded a sudden 10-hour dimming of HD 137010, a star slightly less massive and luminous than our Sun located about 146 light-years away in the constellation Libra. Archival Kepler data were initially examined by volunteers as part of a crowdsourced Planet Hunters ProjectThe signal matches the profile of a small, rocky exoplanet passing or transiting in front of the star. After further analysis, a team of astronomers reported the possible discovery on Tuesday Astrophysical Journal Letters.
If confirmed, this world would officially be called HD 137010 b – but that has proven tricky, to say the least.
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Kepler’s discovery strategy was to stare at a rich field of stars for years. By observing the Guppy transit, it discovered thousands of exoplanets. However, stars can dim for many non-planetary reasons. So mission scientists sought a total of three observed transits for Kepler’s smallest planet candidates before researchers would declare them real worlds. This time-consuming process was cut short a few years into the mission when the telescope’s stabilization equipment failed, forcing NASA to alter Kepler’s field of view to new parts of the sky. This new phase of the mission was called K2, and each mission lasted only about 80 days. One of them brought the only known transit of HD 137010 b.
By the time someone noticed that single, fascinating signal in Kepler’s vast dataset, the telescope had been idled in a graveyard orbit after running out of fuel.
“The authors have tried to rule out everything they can from the data, but with only one transit, you can only do so much,” says Jesse Christiansen, an astrophysicist at the California Institute of Technology.
“There can always be something wrong with a single transit,” says Harvard University astronomer Andrew Vanderberg, a co-author of the study. “That being said, it’s a very strong signal – it looks like a planet.”
Vanderberg and his colleagues managed to roughly measure the candidate’s orbital motion – and with it, constrain its plausible orbital period and distance from its star. But still, the authors say that a year of HD 137010 b could last about 300 to 550 days. That huge range not only complicates the search for more transits; It also means that Earth’s supposed nearest twin could be so far from its slightly fainter star that it would resemble a frozen, supersized version of Mars.
At present its confirmation does not seem possible. No other exoplanet-hunting telescope currently plans to visit the star, let alone examine it long enough to obtain more transits. “Two transits is a possibility, but three transits is exactly what you want,” says Christiansen. “It’s a little early to fire the rocket and head towards this star.” But because this star is relatively brighter and closer in the galaxy, it may become a desirable target for planned future observatories that will be able to find smaller exoplanets by photographing them.
Yet even that might be a stretch. Why devote billions of dollars of space telescopes to looking for one potential planet when other, more certain worlds could be studied instead? Vanderberg also points out that some exoplanets that were previously confirmed through three transits have turned out to be potential mirages. However, it stands out because of its sadly lonely signal strength and clarity. “I feel better about that one transit than I do about multiple transits from some of these other systems,” he says.
Still, he wishes the astronomers who planned Kepler’s original mission had magically guessed what this otherwise ordinary star might be hiding. “If we had seen it for four years,” he says, “it would have been ‘Earth 2.0’ that no one could dispute.”
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