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Astronomers have created a planet-by-planet guide to our galaxy—detailing a catalog of thousands of exoplanets detected by NASA's Kepler mission.
The Updated Catalog of Kepler Planet Candidates: Focus on Accuracy and Orbital Periods, published at the beginning of this month, contains details of 4,376 transiting planets, including 1,791 residing within 709 multi-planet systems.
Exoplanets are so-called because they orbit a star outside of our own solar system. The Kepler space telescope, launched in 2009, was NASA's first mission to hunt for these planets, and their orbiting stars. Nine years after its launch, Kepler showed that there were actually billions of these hidden exoplanets—and that they were promising areas to search for alien life.
Eight astronomers worked on the new catalog, which was posted on the open access archive Arxiv, operated by Cornell University.
"No planet other than Earth is known to have life—apart from the microbes that surely hitchhiked to Mars aboard spacecraft that have landed on that planet," Jack Lissauer, a research scientist at NASA's Ames Research Center, and corresponding author of the catalog, told Newsweek.
"Furthermore, there is no known exoplanet that is very similar to Earth in terms of size, star type and also orbital distance. That said, the exoplanets that have combinations of size, host star type and orbital distance most likely to be habitable by life such as that on Earth, which requires liquid water, were discovered by the Kepler spacecraft," Lissauer said. "Habitable planets were not the focus of this catalog, but we did compute better estimates of planetary size and the amount of energy from their stars impinging on the planets."
Although it will be a while before astronomers can estimate where life may lie among these 4,376 exoplanets, the catalog includes improved measurements of the Kepler planets and notes some new discoveries.
These planetary systems include "a lot more planets, very close to their host stars," than scientists expected, Daniel Jontof-Hutter, an Associate Professor of Physics at the University of the Pacific, in Stockton, California, who also worked on the catalog told Newsweek.
The catalog, with its near 4,400 exoplanets, is now large enough to study the demographics of planetary systems—this is like having a survey that represents "hundreds of billions of stars in our galaxy."
"One of our discoveries with the new catalog is a seven-planet system, Kepler-385, where all seven planets receive more than eight times as much heat from the star than the Earth received from the sun; all of them orbit closer to the star than Mercury's orbit around the sun," Jontof-Hutter said. "And yet the star itself is very similar to the sun, almost exactly the same mass but slightly hotter. Before discovering its bizarre compact array of planets, one might have imagined Kepler-385 to have a planetary system similar to our own. But nature has surprised us."
A fraction of planets in the guide are in what is known as the "Goldilocks Zone." This is the zone where the conditions for liquid water—and life—may be present. But deciphering which ones are in this zone remains a challenge.
Jontof-Hutter said astronomers are working hard to refine their estimates on this number.
"The reasons why this is so difficult are twofold. It is easier to detect planets that are closer to their star than Earth is to the sun, and very few of our discoveries are as far from their stars as the Goldilocks Zone," the professor said.
"The other reason is that it is easier to detect planets that are larger than Earth, and most of the planets that we have discovered are larger than Earth and smaller than Neptune, a class that is absent from the solar system," he said. "To estimate how many Earth-like planets are in the Goldilocks Zone, we have to extrapolate well-measured planet counts from orbits closer to the host stars, or planets that are larger than Earth, and this makes the estimate uncertain."
Despite the search for life remaining difficult, astronomers are not giving up. Catalogs like the one published at the beginning of the month will only bring scientists closer to finding what may be out there.
"Exoplanet scientists are working on the next steps in finding planets that may have life. It's exciting to look at our discoveries, try and understand the planetary population, and design the next mission that will bring us closer to finding life out there," Jontof-Hutter said.