A star like our sun just 12 light years away might host a rocky world in its habitable zone where conditions allow for liquid water.
Jeremy Dietrich and D谩niel Apai at the University of Arizona devised an algorithm called DYNAMITE to predict the existence of planets around a given star based on data from planets already known to orbit it.
They applied this algorithm to a star called Tau Ceti, which is the closest single sun-like star to our solar system. Like our sun, it is also a G-type star known as a yellow dwarf. From previous studies, Tau Ceti was known to have four planets orbiting outside its habitable zone.
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Using the known properties of those planets, Dietrich and Apai used the DYNAMITE algorithm to simulate potential configurations of the Tau Ceti system and predict where other planets could exist. They found that three more planets, labelled PXP-1 to PXP-3, could be orbiting close to the star, each with years of less than 100 Earth days and not in the habitable zone.
But a fourth hypothetical planet, PXP-4, was also predicted by the algorithm. Its year would be between 322 and 468 days, and it would orbit between 0.8 and 1.1 times the distance from Earth to the sun 鈥� within the habitable zone.
PXP-4 would have a mass of up to several times that of Earth. At the lower limit, the planet might be a rocky world capable of supporting liquid water on its surface. But even if it were larger, it could support potentially habitable moons. 鈥淭here are all sorts of possibilities that this opens up,鈥� says Dietrich.
Future telescopes, like the Giant Magellan Telescope being built in Chile, might be able to look for the planet later this decade. While finding such planets around sun-like stars is difficult 鈥� their small size makes it hard to detect the dimming of their star鈥檚 light when they pass in front of it or their gravitational effect on the star 鈥� their existence isn鈥檛 unexpected.
In 2020, Steve Bryson at the NASA Ames Research Center in California and his colleagues predicted that up to half of the sun-like stars in our galaxy could be home to rocky planets in their habitable zones, with considerable interest in finding such worlds. 鈥淲e have an example of life on such a planet around such a star,鈥� says Bryson, namely Earth.
Even if we can find such planets, studying them isn鈥檛 easy. A by Billy Edwards at University College London and his colleagues used the Hubble space telescope to study a nearby super-Earth called LHS 1140b, which is 40 light years away orbiting a smaller red dwarf star. It garnered a lot of excitement in the search for life as it sits in its star鈥檚 habitable zone.
Edwards鈥檚 team found tentative evidence for water in the atmosphere of the planet, but the uncertainty in the measurement 鈥渋s quite high,鈥� he says. 鈥淲e can鈥檛 really say too much about the atmosphere and what it鈥檚 going to be like on the surface.鈥�
Nonetheless, finding a planet in the habitable zone of Tau Ceti would be of huge excitement. The only closer G-type star, called Alpha Centauri A and set in a triple star system 4.4 light-years away, isn鈥檛 yet known to have any habitable zone planets, making PXP-4 perhaps one of the closest true Earth-like worlds yet predicted to exist.
鈥淚f there鈥檚 a potentially habitable planet around one of our biggest and closest stellar neighbours, right in our backyard, that would be incredible,鈥� says Dietrich, 鈥渁nd a great place to search for extraterrestrial life.鈥�
The Astronomical Journal
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