Kepler 52 and Kepler 968: Young exoplanet siblings

Two exoplanet systems – Kepler 52 and Kepler 968 – that have been drifting across the galaxy for hundreds of millions of years have proven to be parts of a 400-member star cluster.

The two systems were discovered several years ago by NASA’s Kepler space telescope, which spotted them when they passed between us and their stars, causing their stars’ light to dim briefly.

At the time, they were thought to be unrelated. But in 2019, astronomers using data from the European Space Agency’s Gaia space telescope realised they were part of a far-flung cluster called Theia 520, which spans a 20-degree swath across the northern sky.

This isn’t a cluster you could see on your own. “It’s really diffuse and sprawling,” says Jason Curtis of Columbia University, speaking last week at a virtual meeting of the American Astronomical Society.

It was only the precision of the Gaia space telescope that allowed it to be spotted at all, because Gaia’s hyper-precise star-tracking data revealed all the stars in it to be moving in a single, coherent group. This indicated that they had come from the same birth cluster, now dispersing.

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The next step, Curtis says, was to figure out how old the two planetary systems were. Prior estimates of the ages of their stars had been inconclusive, serving up answers that spanned pretty much the entire age of the universe.

But once he knew they were both members of a cluster, Curtis says, it was possible to use a different method to determine the age of the cluster, rather than the individual stars.

To do that, he and a team of high school students used data from Kepler, Gaia, and a 48-inch telescope on Mount Palomar in Southern California to calculate the rotation rates of 130 of Theia 520’s stars, graphing them against the stars’ masses.

All of this was done with publicly available date, easily available online.

“This underscores the importance of all-sky surveys and public archives,” says Marcel Agüeros, an astronomer at Columbia University and a co-author of the study.

The results proved that the Kepler 52 and Kepler 968 stars aren’t all that ancient. Instead, Curtis says, they appear to be about 350 million years old.

That’s because stars in a cluster are born spinning at a fairly wide range of rates, ranging from a few hours to a few days or tens of days. But as they age, they slow down, with faster-rotating stars slowing more quickly than slower-rotating ones, and bigger ones responding differently from smaller ones.

By graphing the distribution of spin rates against mass, Curtis says, it’s possible to estimate the age of a cluster. “At any age there’s a unique signature,” he says.

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Doing this for clusters with known exoplanet systems is important, he adds, because it helps astronomers understand how planetary systems evolve over time.

“Planets in clusters provide us with a snapshot in time,” says Elisabeth Newton, an astronomer at Dartmouth College who was not involved in the study. “When we know exactly how old planets are, we can use them to piece together the story of how planets and planetary systems evolve. Knowing that Kepler 52 and 968 are only a few hundred million years old is especially valuable because we haven’t yet found many planets that young.”

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