Scientists using NASA’s space telescope have discovered a tantalizing world. It is about the size of Earth, is remarkably close to our solar system, and could be suitable for life as we know it.
The extrasolar planet, or “exoplanet,” called Gliese 12 b orbits a small and cool red dwarf star located only about 40 light-years from Earth in the constellation Pisces. The exoplanet, which the team discovered with NASA’s Transiting Exoplanet Survey Satellite (TESS), is believed to be about 1.1 times the width of Earth, making it similar to our planet as well as Venus. which is often called our world’s “twin” solar system.
Gliese 12 b orbits its star, Gliese 12, so close that its year lasts only 12.8 Earth days. But because the red dwarf Gliese 12 is only about a quarter the size of the sun, it is also much cooler than our star. This means that even though Gliese 12 b is at a distance from its red dwarf parent equivalent to only 7% of the distance between the sun and Earth, it is still in the habitable zone of its planetary system. Also known as the “Goldilocks Zone,” the habitable zone is the region around a star that is neither too hot nor too cold for planets to host liquid water, a vital ingredient for life as we know it. Although, importantly, the two teams behind the discovery of Gliese 12 b still can’t say for sure whether it has an atmosphere. Therefore, it remains unclear whether the world could be habitable, but researchers have some cautious optimism.
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“The main interesting thing is that this is a planet that’s really close; it’s actually one of the closest transiting planets to Earth,” scientist Larissa Palethorpe of University College London, who led the study along with astrophysicist Shishir of the University of Southern Queensland Dholakia, told Space.com. “Either it’s in the habitable zone of its star, or it’s right on the edge of it — so it could be habitable.”
If Earth and Venus had a child
Scientists spotted Gliese 12 b as it crossed, or “crossed,” the face of its red dwarf parent star. These transits cause small dips in light that TESS is adept at spotting. Palethorpe added that when the team embarked on this project, they didn’t know for sure what the orbital period or size of the planet would be.
“Finding it was so similar to Earth was a wonderful surprise,” she continued. “So that was a really nice thing to be able to nail it down, but I think especially knowing that habitability-wise it could be between Earth and Venus is really exciting.”
Gliese 12 b receives about 85% of the radiation that Venus receives from the sun, but is thought to have a much lower surface temperature of 107 degrees Fahrenheit (42 degrees Celsius) compared to Venus’ surface temperature of 867 degrees Celsius. Fahrenheit (464 degrees Celsius).
Although Earth and Venus are in the sun’s habitable zone, one can support life and has a favorable atmosphere, while the other is an inhospitable hellscape with temperatures hot enough to melt lead. A study of Gliese 12 b can help us understand why this is so.
“Gliese 12 b can teach us a lot about how our own solar system evolved as well,” Palethorpe added.
The team will now investigate whether Gliese has an atmosphere – but early indications are that if it does, that atmosphere will be relatively thin. Perhaps surprisingly, however, the lack of a dense atmosphere is good news for the planet’s habitability prospects.
“We know that some planets have a very dense atmosphere of hydrogen that covers the entire planet. This very thick layer of gas is actually bad news for habitability,” Palethorpe’s fellow UCL researcher Vincent Van Eulen told Space.com. “Typically these planets are two or three times the size of Earth. Gliese 12 b is the actual size of Earth, so it probably doesn’t have that thick an atmosphere.
“It could either have no atmosphere, which wouldn’t be great for habitability, or it would have this kind of thin atmosphere, kind of like Earth.”
Still, even if Gliese 12 b does not have an atmosphere, it could still be an important test object for advancing our search for life elsewhere in the Milky Way. That’s because, as a red dwarf, the star it orbits happens to be the most common form of star in our galaxy — but one we know relatively little about when it comes to red dwarf planetary systems.
Life around red dwarfs
In the Milky Way, red dwarfs make up the largest family of stars still fusing hydrogen with helium in their cores, a process that determines the so-called “main sequence” life of a star. It is estimated that 60% to 70% of the stars in our galaxy are red dwarfs like Gliese 12, and of the 30 closest stars to Earth, at least 20 are red dwarfs.
“It’s interesting to know about planets around small stars, what they might be like, and whether such planets could have life,” Van Eulen added.
Formally known as K-type or M-type stars, red dwarfs have between 7.5% and 50% of the mass of the Sun. This low mass relative to the sun means that such stars burn at a lower temperature, reaching only 6,380 degrees Fahrenheit (3,500 degrees Celsius) compared to our star’s 9,900 degrees Fahrenheit (5,500 degrees Celsius). For example, Gliese 12 has a surface temperature about 60% that of the Sun.
This lower temperature means that faint red dwarfs can exist as main-sequence stars much longer than moderately massive stars like the Sun. Although the sun is expected to live for about 10 billion years, red dwarfs are predicted to have lives tens or even hundreds of times longer than that. Sometimes this figure can reach trillions of years. This means that life would have more time to develop on planets orbiting red dwarfs than on planets around larger main-sequence stars.
But not all the news is good about the prospects for life on exoplanets orbiting red dwarfs.
Although they are cooler than the sun in their stellar age, red dwarfs are thought to be much wilder than our star. This class of stars is believed to be highly magnetically active, firing frequent and powerful bursts of high-energy light in the form of X-rays. These X-rays can violently strip the atmosphere of a planet close to a red dwarf.
Furthermore, recent research has shown that even red dwarfs that remain quiescent for many years can suddenly erupt in superflares 100 to 1000 times more powerful than solar flares on the Sun. These eruptions are more common in young stars of this class and are also capable of destroying atmospheres and boiling liquid water, even in habitable zones.
Currently, however, both teams involved in the discovery of Gliese 12 b believe the red dwarf is relatively calm in orbit, which could be good news for the exoplanet’s chance of having an atmosphere.
Red dwarf exoplanets are good targets for TESS
The fact that red dwarfs are cooler than stars like the sun, and therefore their habitable zones are closer to the stars, actually makes finding exoplanets around them a bit easier for TESS and its planet-hunting transit method.
“We have a bias towards finding planets that are close to their host stars essentially, simply because they transit more often. When we find planets orbiting red dwarfs, because they are smaller stars, the transit dimming is greater,” Palethorpe said. “Because red dwarfs are slightly cooler, the habitable zone is located closer to the star than it would be for our kind of sun, meaning we are more likely to find planets in the habitable zone with TESS.”
The team will have to turn to instruments other than TESS to study this planet further. They will also switch to a different exoplanet detection method to better define the characteristics of Gliese 12 b. One is called the “radial velocity method,” which uses the tiny wobbles caused by planets in the motion of their stars as they gravitationally pull on those stars.
“I think the next thing really is to determine the mass of the planet. We are already actively doing this as part of the High Precision Radial Velocity Planet Search Team for the Northern Hemisphere (Harps North), which is a radial velocity telescope,” Pailthorpe said. “Then we have another proposal accepted by the European Southern Hemisphere Astronomical Research Organization (ESPRESSO), which is another radial velocity telescope. And so hopefully, from the look of the radial velocity observations, we’ll do that. “
Palethorpe and Van Eylen also hope to get time with the James Webb Space Telescope (JWST) to further study the planet’s atmosphere. This is possible because as Gliese 12 b passes across the face of its star, the light passing through its atmosphere will carry the characteristic fingerprints of elements in the atmosphere.
This process is called “transmission spectroscopy,” and Gliese 12 b is just one of a handful of temperate Earth worlds that are close enough to be studied this way.
JWST is currently conducting a similar investigation for the seven Earth-like planets of the TRAPPIST-1 system, located about 40 light-years away. These planets are similar to Gliese 12 b in that not only are many of them in the habitable zone of their star, but this star is also a small and cool red dwarf.
“I think with JWST we’re going to get at least some clues about the atmosphere of this planet, which would be, I think, the most, the next most exciting thing to do after it’s already been discovered,” Van Eulen said.
As for the possibility that Gliese 12 b contains life, the two scientists are extremely cautious. After all, it is still early days for both our understanding of this world and the methods that could detect signs of life in the atmosphere of an exoplanet, even one as relatively close as Gliese 12 b.
“I think Gliese 12 b will teach us a lot about life, but we can’t say anything for sure. I think it’s very exciting and we should definitely look forward to more research on Gliese 12 b,” Palethorpe concluded. “Not a bad place to start looking for life.”
The two teams’ research was published Thursday (May 23) in The Monthly Notices of the Royal Astronomical Society and The Astrophysical Journal Letters.
Originally published on Space.com.
