Finding a potentially habitable exoplanet is not as easy as you might think. Orbiting at a warm distance from the host star is only the first step. Size and composition also play a role, as does the level of flare activity in the star. And all of that doesn’t mean much if the system is so far away that we can’t take detailed observations to find out if it’s habitable.
It seems like a newly discovered system could check a good number of those boxes. And it’s incredibly close, just 10.7 light years away from the Solar System. This means that it could soon become one of the most studied systems in our local neighborhood.
“These planets will provide the best possibilities for more detailed studies, including searching for life outside our Solar System,” said astrophysicist Sandra Jeffers of the University of Göttingen in Germany.
The star is called Lacaille 9352, or GJ 887. In its orbit, scientists have found two exoplanets that could be terrestrial: rocky, like Earth and Mars. Surprisingly, there is also a hint of a third terrestrial exoplanet orbiting at a greater distance: a distance that could make it warm, neither too hot nor too cold to prohibit liquid water on the surface.
This hint of the third planet is considered inconclusive at this stage, but the discovery of the two close-orbiting planets (and the potential for the third planet) are enough to warrant a much closer look at the GJ 887 system.
The star itself, which is about half the mass of the Sun, is a red dwarf, a type of small, relatively cold, long-lived star that is the most common type of star in the Milky Way.
We have found many exoplanets orbiting red dwarfs; And because these stars are not as hot as stars like the Sun, the temperate habitable zone for orbiting planets is much closer than it is for Earth.
However, the problem with red dwarfs is that they are often quite noisy, spitting out intense stellar radiation and flashes that would render many of these nearby planets uninhabitable, stripping their atmospheres.
This is where GJ 887 stands out. For a red dwarf, it is actually incredibly calm: it has very low star activity and its brightness remains more or less uniform. This makes it of great interest to astronomers with the Red Dots Survey, a project to search for terrestrial worlds around nearby red dwarf stars.
As part of this survey, the star was studied for three months using the high-precision Planet Searcher instrument of the radial velocity Planet of the Southern European Observatory at the 3.6-meter La Silla telescope in Chile.
This sensitive instrument looks at the stars, looking for very slight changes in their light as they move a little bit, moved by the gravitational influence of planets in orbit around them. In GJ 887, these movements revealed two distinct periodic signals.
The amount the star moves can be used to calculate the mass of the objects they throw. This is how the researchers discovered the two exoplanets, GJ 887b and GJ 887c, confirmed by comparing them to 200 days of archival data obtained from 2002 to 2004.
GJ 887b has a minimum mass of about 4.2 times Earth’s mass, and orbits the star once every 9.3 days. GJ 887c has a minimum mass of about 7.6 times Earth’s mass, and orbits the star once every 21.8 days.
Those masses place exoplanets in the ‘super-Earth’ category, but without further study, it is impossible to know whether they are terrestrial or gaseous. In their respective proximity to the star, the two planets are unlikely to be habitable, but they are very close to the inner edge of the habitable zone.
However, the third signal, if it turns out to represent an exoplanet, would constitute a super-Earth with an earth mass of 8.3 in the middle of the star’s habitable zone, with an orbital period of 50.7 days. There is only one problem: the signal was only detected once in the HARPS data.
This suggests that there might not be an exoplanet there at all. “We consider the third signal at ~ 50 days to be dubious and likely related to stellar activity,” the researchers wrote in their article, but the possibility cannot be ruled out with the current data.
That means the researchers will return for another look, to see if that signal can be picked up again, and planetary scientists will also want to take a closer look at GJ 887b and GJ 887c anyway.
Due to a lack of star flare activity, the two exoplanets may have retained their atmospheres, and because the light from the star is so stable, those atmospheres might be detectable when the star’s light bounces off them.
Our current instruments are not yet capable of measuring this, but it is one of the tasks that the James Webb Space Telescope, scheduled for launch next year, has been designed to perform. It will be sensitive enough to get direct images of nearby exoplanets, which should revolutionize the field of planetary science.
“These kinds of observations could tell us about the atmospheric composition of these planets,” explains astronomer Melvyn Davies of the Lund Observatory in Sweden, who was not involved in the research, explains an article accompanying the article.
“If other observations confirm the presence of the third planet in the habitable zone, then GJ 887 could become one of the most studied planetary systems in the Solar neighborhood.”
The research has been published in Science.
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