Potentially Habitable Earth-Sized Exoplanet Spotted In The Kepler Reject Pile

Potentially Habitable Earth-Sized Exoplanet Spotted In The Kepler Reject Pile

For years, a dedicated team of experts has manually sifted through troves of rejected Kepler data in hopes of finding something the computers had missed—an exercise that has resulted in the discovery of what is now arguably the most Earth-like planet known to astronomers.

New research published yesterday in Astrophysical Journal Letters describes Kepler-1649c, a remarkably Earth-sized exoplanet located 300 light-years away. At 1.06 times the size of Earth—give or take a few decimal points—it’s practically the same size as our sparkling blue marble. This rocky planet also happens to reside within its star’s habitable zone, that sweet swatch of space within which liquid water can exist on a planet’s surface.

Incredibly, this terrestrial exoplanet might never have been discovered had it not been for a systematic visual inspection of all Kepler data rebuffed by a computer algorithm. More on that in just a sec—let’s take a closer look at this newly discovered world.

Side-by-side comparison of Earth and Kepler-1649c. (Illustration: NASA/Ames Research Centre/Daniel Rutter)

Kepler-1649c receives around 75 per cent of the starlight we get on Earth, and it features an equilibrium temperature around 234 degrees Kelvin (-39 degrees Celsius), according to the paper. Equilibrium temperature is based exclusively on incoming stellar radiation and excludes factors such as an object’s albedo (reflectivity) and atmospheric effects. In terms of Kepler-1649c’s actual surface temperature, that cannot yet be determined because the scientists have no clue about the composition of this planet’s atmosphere, if it even has one. By comparison, however, Earth has an equilibrium temperature of 278.5 Kelvin (5 degrees Celcius).

“In terms of size and likely temperature, this is the most similar planet to Earth that has ever been found with Kepler,” said Jeff Coughlin, a co-author of the study, in a SETI Institute press release.

Other notable exoplanets include TRAPPIST-1f, which is comparable to Earth in terms of size, while TRAPPIST-1d and TOI 700d are comparable to Earth in terms of temperature. But none of these hit both the size and temperature marks quite like Kepler-1649c.

This exoplanet requires just 19.5 days to make a complete orbit around its host star, a red dwarf known as Kepler-1649. With such a short year, it’s clear that Kepler-1649c lives in close proximity to its star. But at one-quarter the size of our Sun, this low mass star is not nearly as powerful as ours.

This is potentially bad news in terms of habitability. Red dwarfs are extremely common in the galaxy (around three of every four stars in the Milky Way is a red dwarf), but they’re prone to frequent temper tantrums in the form of powerful stellar bursts. This has led to concerns among astrobiologists that red dwarf systems are devoid of life.

Artist’s depiction of Kepler-1649c, its red dwarf, and a second planet previously known to exist in this system. (Illustration: NASA/Ames Research Centre/Daniel Rutter)

To date, NASA’s Kepler Space Telescope has contributed to the discovery of nearly 2,400 exoplanets, but this particular world almost slipped through the cracks.

Several years ago, a computer algorithm called Robovetter missed this planet while parsing Kepler data. These automated systems are critically important because Kepler, which was in operation from 2009 to 2018, produced hundreds of thousands of observations over the years, all of which need to be analysed for signs of possible exoplanets.

The first step in this automated process is for the system to use the transit method, in which it looks for dips in a star’s brightness—a possible sign of an orbiting exoplanet. The second step requires Robovetter to reject false positives caused by extraneous sources, such as variable stars (which exhibit fluctuating levels of luminosity), nearby objects passing through, and artifacts produced by Kepler’s electronics.

The system is not perfect, however.

“If we hadn’t looked over the algorithm’s work by hand, we would have missed it,” said Andrew Vanderburg, the lead author of the study and a NASA Sagan Postdoctoral Fellow at the University of Texas at Austin, in a NASA press release.

A dedicated team of experts called the Kepler False Positive Working Group looked through these rejects in hopes of finding something Robovetter might have missed. The new exoplanet was spotted in false-positive Kepler data released three years ago.

“This discovery highlights the value of human inspection of planet candidates even as automated techniques improve, and hints that terrestrial planets around… M-dwarfs may be more common than those around more massive stars,” wrote the authors in the new study.

It’s a fascinating new discovery, but the quest for a truly Earth-like planet continues. To date, nothing even comes close to our pale blue dot.