The most comprehensive and detailed catalog released yet marks the end of an era for the planet-hunting scope.

That’s a wrap for the Kepler Space Telescope — at least its primary mission. On June 19th, astronomers released the eighth and final mission catalog with data gathered from Kepler’s first 4 years of life. With 219 new planet candidates, 10 of which are near-Earth size and orbiting in their star’s habitable zone, the catalog will help astronomers answer a question central to the telescope’s mission: How many Earth-like planets orbit Sun-like stars?

Small planets come in two sizes
Researchers have discovered a gap in the distribution of planet sizes, indicating that most planets discovered by Kepler so far fall into two distinct size classes: the rocky Earth-size and super-Earth-size (similar to Kepler-452b), and the mini-Neptune-size (similar to Kepler-22b). This histogram shows the number of planets per 100 stars as a function of planet size relative to Earth. The orange line shows regions where counting data is complete, while the white line marks incomplete data. (Though there appears to be a gap between smaller, Earth-size planets and rocky super-Earths, there's not enough data there to tell if that second gap is real.)
NASA / Ames Research Center / CalTech / University of Hawai'i / B.J. Fulton

By the Numbers

In total, Kepler discovered 4,034 planet candidates, of which 2,335 are bona fide exoplanets. Although the community has watched those numbers creep up over the last four years as astronomers continued to analyze the data pouring in from the planet-hunting scope, those numbers have now reached their peak. This is the last catalog that will be released from Kepler’s primary mission — the one that scoured 160,000 stars in a patch of sky in the constellation Cygnus.

In addition, Kepler discovered 50 near-Earth size habitable zone candidates, more than 30 of which have been verified. “Kepler really and truly has opened up our eyes to the existence of these small terrestrial-sized worlds,” Susan Mullally (SETI Institute) said at Monday’s news conference at the NASA’s Ames Research Center in California.

This final catalog reprocessed the entire set of data from Kepler’s primary mission with a new trick: The team introduced their own simulated and false signals into the dataset to determine which types of planets were overcounted and which were undercounted. This will allow teams going forward to better characterize planets across the galaxy and answer one of astronomy’s most compelling questions: How many habitable worlds are there?

“The reason why I'm so excited about [Mullally’s] results, is that this catalog — because it was done in such a sophisticated, methodical way — really enables studies of habitable-zone-planet occurrence for sun-like stars in a way previous catalogs did not,” Courtney Dressing (Caltech) says. “It's laying fundamental ground-work.”

The Family Tree Divides

Exoplanet Family Tree
This sketch illustrates the family tree of exoplanets. Planets are born out of swirling disks of gas and dust called protoplanetary disks, which give rise to giant planets such as Jupiter, as well as smaller planets between the size of Earth and Neptune. Researchers have discovered that latter group can be cleanly divided into two smaller groups: rocky Earth- and super-Earth-like planets and gaseous mini-Neptunes.
NASA / Ames Research Center / JPL-Caltech / Tim Pyle

Already, Benjamin Fulton (University of Hawai‘i at Manoa) and his colleagues have taken advantage of Kepler’s latest dataset, posting their result on the arXiv preprint server. It helps astronomers better understand one of Kepler’s greatest surprises: that planets form with diameters between those of Earth and Neptune. Previously thought forbidden, astronomers classify these planets as super-Earths — rocky worlds a little larger than Earth with deep, crushing atmospheres — or mini-Neptunes — gaseous planets smaller than Neptune that don’t have a surface.

Although most of the exoplanets across the galaxy fall into this forbidden regime between Earth and Neptune, astronomers have struggled to find a dividing line between the two classes. Where does a super-Earth stop and a mini-Neptune begin? Fulton’s work might provide the answer.

He has found that between 1.75 and 2 times the size of the Earth, very few planets form, leaving a gap — and a dividing line — between the two classes. Any planet that is smaller than 1.75 times the size of Earth is likely a super-Earth and any planet that is greater than twice the size of Earth is likely a mini-Neptune.

“This is a major new division in the family tree of exoplanets, somewhat analogous to the discovery that mammals and lizards are separate branches on the tree of life,” said Fulton.

Although Fulton is unsure why nature prefers to keep these two planets clearly separated, he suspects that it all comes down to the lightest two elements: hydrogen and helium. “A very small amount of light hydrogen and helium gases goes a long way to inflate the size of planets,” he says. “Adding a tiny amount of hydrogen to one of these rocky planets, say about 2% by mass, would cause the planet to jump the gap and move into the group of larger planets.”

How to make a super-Earth / mini-Neptune
This diagram illustrates how planets are assembled and sorted into two distinct size classes. First, the rocky cores of planets are formed from smaller pieces. Then, the gravity of the planets attracts hydrogen and helium gas. Finally, the planets are "baked" by the starlight and lose some gas. At a certain mass threshold, planets retain the gas and become gaseous mini-Neptunes; below this threshold, the planets lose all their gas, becoming rocky super-Earths.
NASA / Ames Research Center / JPL-Caltech / R. Hurt

To boot, the results sharpen the dividing line between planets that are potentially habitable and those that would not make comfortable abodes. Anything above twice the size of the Earth would certainly not hold life as we know it and anything less than 1.75 times the size of the Earth just might, but it’s not a guarantee.

Although Monday’s news conference was hailed as the end of an era — it has, after all, been 23 years since William Borucki and David Koch first dreamed of a telescope that could catch the stellar dimmings required to spot planets — the aroma of the newsroom contained more excitement than sadness.

“It feels a bit like the end of an era — but actually I see it as a new beginning,” Mullally said. “I'm really excited to see what people are going to do with this catalog because it's the first time we have a population that is really well characterized and we can now do these statistical studies and really start to understand the Earth analogues out there.”




Image of Carl Kruse

Carl Kruse

June 20, 2017 at 7:10 am

What a marvelous trip it's been for Kepler and for all of us. I think Mullally is right, it's not the end of an era but the beginning of a new one. Could it be long before we conclusively answer the question, "are we alone?"

Carl Kruse

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June 24, 2017 at 3:30 am

We might as well be. They are out of reach and until we pick up a radio transmission let alone a warp drive signature it is time to recognize that this nightmare of a world is truly special. If you utilize the statistic that of the 10 billion forms of life that have inhabited this planet only one has become intelligent if you can call us that. That rapidly narrows the field.

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June 23, 2017 at 5:32 pm

So where does one find the publication of these planets?

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