Say Hello To EPIC 201497682.03, An 'Earth Twin' Planet In The 'Goldilocks Zone' - Forbes

The search for "Earth twins" is quickly advancing.

ESA - C. Carreau

Astronomers have found over 4,000 planets in other star systems, but almost all of these "exoplanets" are massive compared to Earth. In fact, 96% of them are larger than Earth. Mostly, astronomers have found gas giants like Neptune or Jupiter, and very few small planets. So the announcement today that 18 exoplanets have been discovered that fall into the category of an Earth-sized planet is big news.

The even bigger news is that one of them is slightly smaller than our planet. Have we just found Earth 2.0?

The eighteenth planet

This candidate for an "Earth twin" is called EPIC 201497682.03. It’s 69 percent of the size of the Earth, the closest fit in the 18 exoplanets found by scientists at the Max Planck Institute for Solar System Research (MPS), the Georg August University of Göttingen, and the Sonneberg Observatory, whose results have been published in Astronomy & Astrophysics. “The most remarkable object in terms of its small size is EPIC 201497682.03, which we found to have a radius of just 0.692,” says the paper. EPIC 201497682.03 likely orbits its red dwarf host star within the “habitable zone” (a “not too cold, not too hot” distance also called the “Goldilocks zone”) where liquid water could occur on its surface. That’s one of the basic prerequisites for life as we know it on Earth.

If an exoplanet travels in front of its star as seen from Earth, the planet darkens the star. How often that happens tells astronomers about the length of a year on the planet. However, the new algorithm by Heller, Rodenbeck and Hippke does not look for abrupt brightness drops like earlier standard algorithms, but for gradual darkening. This makes the new transit search algorithm significantly more sensitive to very small planets the size of the Earth.

NASA/SDO (Sonne), MPS/René Heller

Why are small exoplanets hard to find?

 Small planets are harder to track down than big ones. Exoplanet-hunters find their targets by detecting a drop in brightness as a planet transits in front of its host star. However, while large planets cause sudden drops in brightness, small planets cause such a tiny change in brightness that it's hard to separate it from the natural brightness fluctuations of the star. "Standard search algorithms attempt to identify sudden drops in brightness," said Dr. Rene Heller from MPS, first author of the paper. "In reality, however, a stellar disk appears slightly darker at the edge than in the center. When a planet moves in front of a star, it therefore initially blocks less starlight than at the mid-time of the transit. The maximum dimming of the star occurs in the center of the transit just before the star becomes gradually brighter again," he said.

How were these new exoplanets found?

As well as a sub-Earth, the scientists found 17 super-Earths, all of them found in data from the Kepler Space Telescope using a new search algorithm called the transit least-squares (TLS) algorithm. "Our new algorithm helps to draw a more realistic picture of the exoplanet population in space," summarises Michael Hippke of Sonneberg Observatory. The new algorithm does not look for abrupt brightness drops like earlier standard algorithms, but for gradual darkening. This makes the new transit search algorithm significantly more sensitive to very small planets the size of the Earth. "This method constitutes a significant step forward, especially in the search for Earth-like planets,” says Hippke.

This illustration depicts NASA's exoplanet hunter, the Kepler space telescope. The agency announced on Oct. 30, 2018, that Kepler has run out of fuel and is being retired within its current and safe orbit, away from Earth. Kepler leaves a legacy of more than 2,600 exoplanet discoveries.

NASA/Wendy Stenzel

What is the Kepler Space Telescope?

The researchers used data from NASA’s Kepler Space Telescope as a test bed for their new algorithm. In its first mission phase from 2009 to 2013, Kepler recorded the light curves of more than 100,000 stars, resulting in the discovery of over 2,300 planets. After a technical defect, the telescope had to be used in an alternative observing mode, called the K2 mission, but it nevertheless monitored more than another 100,000 stars by the end of the mission in 2018. The researchers decided to re-analyze all 517 stars from K2 that were already known to host at least one transiting planet.

Almost all known exoplanets are larger than Earth and typically as large as the gas planet Neptune. The 18 newly discovered planets (here in orange and green), for comparison, are much smaller than Neptune, three of them even smaller than Earth and two more as large as Earth. Planet EPIC 201238110.02 is the only one of the new planets cool enough to potentially host liquid water on its surface.

NASA/JPL (Neptune), NASA/NOAA/GSFC/Suomi NPP/VIIRS/Norman Kuring (Earth), MPS/René Heller

Could the other 17 exoplanets be habitable? 

 They’re probably too hot. Most of the new planets detected in the old data orbit their star closer than other known planets. The surfaces of these new planets therefore likely have temperatures well in excess of 100ºC/212ºF, while some have temperatures of up to 1,000ºC/1,832ºF.

Can the new algorithm be used to find another Earth 2.0?

Given their discovery rate of 3.5% new planets in old data, the researchers think that their method will help them to find more than 100 other Earth-sized worlds in the Kepler data. “This new method is also particularly useful to prepare for the upcoming PLATO mission to be launched in 2026 by the European Space Agency”, says Professor Dr. Laurent Gizon, Managing Director at the MPS.

PLATO, the PLAnetary Transits and Oscillations of stars mission, is a next-generation planet hunter with an emphasis on the properties of rocky planets up to the habitable zone around Sun-like stars – the location from a star where liquid water can exist on the planet’s surface. Importantly, it will also analyse the planet’s host star, including its age, and thus give insight into the evolutionary state of the entire extrasolar system.

ESA

What is the PLATO mission?

Planned for launch by 2026, the PLATO (PLAnetary Transits and Oscillations of stars) mission will assemble the first-ever catalog of confirmed planets, including planets in the habitable zone of their host stars. It will study hundreds of rocky, icy and giant planets and accurately measure their radii, masses, and ages. It’s expected to revolutionize astronomers’ understanding of planet formation and the evolution of planetary systems.

"Does a second Earth exist in the Universe? is one of the exciting questions in astrophysics today," says Johann-Dietrich Wörner, Director General of ESA. "With our PLATO satellite, we are focusing on Earth-like planets orbiting up to the habitable zone around other stars which are similar to our Sun. This will be a major step towards finding another Earth."

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