Artist concept of a scene from the surface of the extrasolar planet Kepler-1649c, which orbits a red dwarf star about 300 light years from our solar system. Almost exactly Earth's size, and located in its star's habitable zone, Kepler-1649c has the potential to possess liquid surface water, and a potentially life-friendly environment. (NASA/Ames Research Center/Daniel Rutter)
Scientists have made an exciting discovery in deep space — but not with an existing telescope or space probe.
Combing through a backlog of data collected several years ago by NASA's now defunct Kepler space telescope, they ran across a previously overlooked gem in the cosmos: an extrasolar planet, or "exoplanet," estimated to be almost exactly the size of Earth, in what's called the "habitable zone," at the right distance from its star to potentially harbor liquid water and a life-friendly environment.
The exoplanet, Kepler-1649c, orbits a small red dwarf star about 300 light years away in the constellation Cygnus — which means we won't be visiting it anytime soon. But with an estimated size of only 1.06 times that of Earth, and getting about 75% of the sunlight from its star that Earth receives from the sun, this exoplanet is the closest to Earth in size and solar heating of any discovered to date.
Whether Kepler-1649c possesses an atmosphere capable of supporting liquid water on its surface is not yet known, but follow-up investigations may give us a more complete picture of this tantalizing world.
Catching What a Computer Algorithm Overlooked
NASA's Kepler space telescope, the most productive exoplanet-finding spacecraft yet launched, was retired in 2018, after running out of the fuel needed to continue scientific observations. But over its nine years of service, Kepler amassed a huge amount of data — so much so, that scientists are still making new discoveries.
Here's how scientists look for evidence of exoplanets in the data. Kepler searches for the minor dimming of a star's light caused by an orbiting planet crossing in front of it, or "transiting." This "transit method" is responsible for most exoplanet detections made since the first discoveries nearly three decades ago.
Each measured dip in a star's brightness must be carefully analyzed to determine if it was caused by a transiting exoplanet or some other factor, like a fluctuation in a star's luminosity, or a random celestial object passing momentarily between us and the star.
With so much data to analyze, a first pass through it is done by computer programs, with algorithms designed to weed out all the non-transit events. Only about 12% of detections turn out to be transiting exoplanets, with the rest classified as "false positives." However, sometimes the algorithm gets it wrong, which is what happened with Kepler-1649c. Scientists in the Kepler False Positive Working Group discovered the mistake as they double-checked the computer's results.
Exoplanets that interest astronomers and astrobiologists most are the potentially Earth-like ones: planets close to Earth's size, and within their star's "habitable zone" — the right distance for liquid surface water to potentially exist.
Other exoplanets have been found that are closer to Earth's size than Kepler-1649c, like TRAPPIST-1f and Teegarden-c. Still others are known that receive more sunlight, that are closer to the warmth of the Earth. But none come as close as Kepler-1649c in both factors, making this once-overlooked exoplanet the nearest we've come to spotting another planet with Earth-like characteristicsin the cosmos.
But, as Earth-like as Kepler-1649c might appear, there are some significant differences between it and planet Earth. The exoplanet orbits close to a small, dim, red dwarf star — so close that it zips around it once in only 19.5 days, instead of 365. It also shares its system with at least one other planet, also close to Earth in size, but about half the distance from its star, and because of that, probably very hot. There is also some evidence for a possible third planet in the system.
Buried in the Data
Discoveries made from Kepler's hoard of backlogged data are not unique. Other completed space missions have piled up their own mountains of observations that scientists review and revisit to gain new understandings.
Examples include NASA's Galileo and Cassini spacecraft, whose missions were terminated in fiery burnups in the atmospheres of Jupiter and Saturn. But they gathered enough data on the gas giant planets and their systems of rings and moons that scientists are still studying it today.
NASA's Opportunity rover, which went silent two years ago following a major dust storm, collected enough images and other data along its 28 mile, 14-year trek across Mars that scientists are still analyzing it all.
How Many Exoplanets Have We Found?
As of June 30, 2020, a total of 4,183 exoplanets have been confirmed to exist in 3,092 planetary systems. The Kepler space telescope found 2,751 exoplanets.
Of the grand total, 160 are classified as "terrestrial" — rocky planets around Earth's size, with iron-rich cores, like Venus and Earth.
As more exoplanets are discovered by ground-based observatories and active spacecraft like NASA's Transiting Exoplanet Survey Satellite (TESS), more examples of Earth-sized planets within their stars' habitable zones are being found. An understanding is emerging that planets with potentially Earth-like conditions may be more commonplace in our galaxy than we previously thought.
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