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SYFY WIRE Bad Astronomy

A (possibly) record-breaking exoplanet found with a ten-year orbit

By Phil Plait
Artwork depicting a system of planets around a red dwarf star. Credit: ESO/M. Kornmesser/N. Risinger (

Well now, this opens up interesting possibilities: Astronomers have reported that they have found a possible exoplanet orbiting a star with a ten-year period. That's far longer than any other transiting exoplanet ever seen, and may be the first of many found like it.

The problem is they can't confirm it, at least not for a while. It looks like the astronomers are likely right to me, but it'll be years before we know for sure.

The star is called EPIC248847494 (from the Ecliptic Plane Input Catalog of stars, and its entry number there). It's located a little over 1,800 light years away, and it's something of an unusual star: it's a subgiant, a star that was once like the Sun but has nearly used up the hydrogen in its core, and is expanding on its way to become a red giant. It has a lower mass than the Sun (about 90%), and those kinds of stars live longer than the Sun, so the star is likely pretty old, 11 billion years or more (the Sun will become a subgiant when it’s about 11 billion years old). It may be one of the first stars to form in the galaxy!

It was observed by the Kepler spacecraft as part of its K2 mission — after Kepler had observed the same 150,000 stars for years, it lost use of one of its reaction wheels used to keep it pointed. A solution found was to let the spacecraft observe different parts of the sky, stabilizing itself using the pressure of sunlight. It couldn't observe the same stars for long, but it could observe more: a fair trade-off.

Kepler looks for dips in starlight indicating a transiting exoplanet. If a planet orbits a star edge-on as seen from Earth, then once per orbit it blocks a bit of the star, causing the light to drop. The amount of the drop tells us the size of the planet, and the time it takes to transit the star gives an indication of the size of the orbit.

EPIC248847494 was observed for nearly 80 days. The star itself appears normal enough, but about three-quarters of the way through the observations, a large dip appears in the data. And by large, I mean long: the transit lasts for an astonishing 53 hours! No other transit ever seen was that long, implying heavily this was a planet in a long orbit.

The light curve (brightness vs. time) for the star EPIC248847494. The x-axis is in days, and the y-axis is arbitrary. The possible transit is centered at about day 3133, and represents a 0.2% drop in light. Credit: Giles et al.

Well, a candidate planet. There was only one transit (what they call a monotransit), and that means the exoplanet is hard to confirm. Generally, you need three transits for a confirmation. The first one that makes you notice it, a second one some time later, and then a third after the same interval so that you know it's something periodic and not just two random dips. So, with just one transit, this can't be confirmed using the Kepler data.

The astronomers followed up with ground-based observations to see if they could detect a Doppler shift in the star's light. As a planet orbits, it tugs on the star, and this means the star sometimes is moving toward us and sometimes away. That changes the wavelength of its light.

Looking at the data, I certainly can't say they found this motion; the shift of 0.19 meters per second per day would be fine, but it has an uncertainty of 0.16! So realistically it's not really distinguishable from 0. It might be there, but we can't say for sure.

But the spectra they took did allow for other important characteristics to be found. The temperature and color of the star indicate it's a subgiant, and that it's about 2.7 times wider than the Sun. By the depth of the transit, this means the planet (if it is a planet) is about 1.1 times Jupiter's, making it a gas giant. The orbital period from the length of the dip is 3650 (± 1280/1130) days, so about 10 years.

This is all really cool, but we're not even sure it's real! Still, the data are pretty provocative. And it's likely there will be more single transit planets found in the Kepler data, as well as the newly launched TESS, which will very soon be returning lots of scientific transiting exoplanet bounty. Looking at this object and figuring it out will help astronomers understand what to do when all that data start coming in, too.

Most planets we find have short period orbits; that's because those are easiest to discover. In the case of cooler stars that's fine, because a planet can be closer to the star and still be temperate. But when it comes to planets like Earth orbiting stars like the Sun, and also finding solar systems that look more like ours (with the giant planets on orbits of a decade or more), every one of these monotransit planets we find is like a gift. We don't see many systems like ours out there because they're hard to find, so it would be nice to know just how rare we are in the cosmos.

One final thought: EPIC248847494 is dying, on its way to becoming a red giant. It's warming up, so it was cooler in the past, meaning the planet was cooler too. That makes habitability a problem (assuming it has big moons). But I always get a thrill when we find planets around a star we know is much older than the Sun. If life got started there, it may have a several-billion-year head start on us.

The science fiction fan in me thrills to hear this. It'll be a long time before we can study these planets in more detail, but I always am left wondering: What will we find when we do?

[Update (July 9, 2018): Hugh Osborn, one of the authors on the paper about this planet, pointed out to me that this is the longest period transiting exoplanet found; many have been seen with longe rperions using direct imaging and other methods. Mea culpa! I should've been more clear. I updated the text above to reflect that.]