'Mirror image' of Earth and Sun discovered 3000 light-years away, say scientists
KOI-456.04 orbits a Sun-like star, and it does so with a star-planet distance that could permit planetary surface temperatures conducive to life
Scientists have discovered a potentially habitable planet outside our solar system, which along with its star are a "mirror image" of the Earth and our Sun. According to the research team, at a distance of just over 3000 light-years from the solar system, the star Kepler-160 and its companion KOI-456.04 are "more reminiscent of the Sun-Earth system than any previously known exoplanet-star pair".
KOI-456.04 orbits the Sun-like star, and it does so with a star-planet distance that could permit planetary surface temperatures conducive to life, says the research team, led by the Max Planck Institute for Solar System Research in Göttingen. Experts say KOI-456.04's host star is unlike the central stars of most other exoplanets, and it actually emits visible light. On the other hand, the central stars of almost all other exoplanets emit infrared radiation, are smaller and fainter than the Sun and therefore belong to the class of red dwarf stars.
"The star Kepler-160 was located in the field of view of the Kepler primary mission and was continuously observed from 2009 to 2013. Its radius of 1.1 solar radii, a surface temperature of 5200 degrees Celsius (300 degrees less than the Sun), and its very Sun-like stellar luminosity makes it an astrophysical portrayal of our own parent star," says the analysis published in Astronomy & Astrophysics.
The team says KOI-456.01 is less than twice the size of Earth, with a radius of 1.9 Earth radii. But it is relatively large compared to many other planets that are considered potentially habitable. It is the combination of this less-than-double the size of the Earth planet and its solar-type host star that makes it so "special and familiar," they add.
As a consequence, the surface conditions on KOI-456.04 could be similar to those known on Earth, provided its atmosphere is not too massive and non-Earth-like. The amount of light received from its host star is about 93% of the sunlight received on Earth. The scientists estimate that If KOI-456.04 has a "mostly inert atmosphere with a mild Earth-like greenhouse effect, then its surface temperature would be +5 degrees Celsius on average," which is about 10 degrees lower than the Earth's mean global temperature.
KOI-456.01 has an orbital period of 378 days. "Given its Sun-like host star, the very Earth-like orbital period results in a very Earth-like insolation from the star – both in terms of the amount of the light received and in terms of the light color. Light from Kepler-160 is visible light very much like sunlight. All things considered, KOI-456.04 sits in a region of the stellar habitable zone – the distance range around a star admitting liquid surface water on an Earth-like planet – that is comparable to the Earth's position around the Sun," explain researchers.
Space telescopes such as CoRoT, Kepler, and TESS have allowed scientists the discovery of about 4,000 exoplanets within the past 14 years. Most of these planets are the size of the gas giant planet Neptune, about four times the size of the Earth, and in relatively close orbits around their respective host stars. But scientists have also discovered some exoplanets as small as the Earth that could potentially be rocky. A handful of these small planets are also at the right distance to their host star to potentially have moderate surface temperatures for the presence of liquid surface water – the essential ingredient for life on Earth.
Kepler-160 has been known for about six years to be a host star of two exoplanets, called Kepler-160b and Kepler-160c. Both these planets are substantially bigger than Earth and in relatively close orbits around their star. Tiny variations in the orbital period of planet Kepler-160c gave scientists the hint of a third planet. The analysis suggests that Kepler-160 is orbited by a total of four planets.
One of the two planets that scientists found is Kepler-160d, the "previously suspected planet responsible for the distorted orbit" of Kepler-160c. The other planet is KOI-456.04, probably a transiting planet. "We conclude that Kepler-160 has at least three planets, one of which is the non-transiting planet Kepler-160 d. We also find the super-Earth-sized transiting planet candidate KOI-456.04 in the habitable zone of this system, which could be the fourth planet," says the study.
Experts, however, say that more data is needed to formally declare KOI-456.04 a planet. "It cannot currently be ruled out completely that KOI-456.04 is a statistical fluke or a systematic measurement error instead of a genuine planet," they say. The team estimates the chances of a planetary nature of KOI-456.04 to be about 85% "pro-planet" and getting a formal planetary status requires 99%. While some of the Earth's most powerful ground-based telescopes might be able to validate this candidate with observations of one of its upcoming transits, there is also a good chance that the PLATO space mission of ESA will be capable of confirmation.
PLATO is scheduled for launch in 2026 and one of its major science goals is the discovery of Earth-sized planets around Sun-like stars. "If PLATO will be oriented in such a way as to re-observe the field of view of the Kepler primary mission, then KOI-456.04 will have a chance of being confirmed and studied in even more detail," says the team.