There was a time not too many decades ago when an astronomer suggesting the existence of life-bearing planets beyond our solar system would be considered heresy, albeit without the punishment of house arrest suffered by Galileo until his death in 1642.
Fast forward to 2019, Michel Mayor and Didier Queloz, using HARPS (High Accuracy Radial velocity Planet Searcher) on the ESO 3.6-metre telescope at the La Silla Observatory in Chile, were awarded the Nobel Prize in Physics for the discovery of the first exoplanet around a Sun-like star, 51 Pegasi b, later formally named Dimidium. They would share the prize “for contributions to our understanding of the evolution of the universe and Earth’s place in the cosmos” with James Peebles, Albert Einstein Professor Emeritus of Science at Princeton University.
Super Earth Gliese 486b
This week, a newly discovered planet, Gliese 486b, that could be our best chance yet of studying rocky planet atmospheres outside the solar system, was announced by an international team of astronomers in Science. The exoplanet is a ‘super-Earth’, a rocky object bigger than Earth but smaller than ice giants like Neptune and Uranus., orbiting a red dwarf star only 26 light-years away. With scalding surface temperature of 430 degrees Celsius, Gliese 486b (image above) is too hot to support human life but still within the tolerance of Earth-bound extremophiles.
“This is the kind of planet we’ve been dreaming about for decades,” says Dr Ben Montet, an astronomer and Scientia Lecturer at the University of New South Wales and co-author of the study. “We’ve known for a long time that rocky super-Earths must exist around the nearby stars, but we haven’t had the technology to search for them until recently. This finding has the potential to transform our understanding of planetary atmospheres.”
“Not a Future Vacation Destination”
Although Gliese 486b may serve as a template in our search for rocky, Earth-like planets, with an equilibrium surface temperature of 700 kelvin, it’s most likely a planet only astronomers “dream of” –it likely will not prove to be a vacation destination for future generations of Homo sapiens. The planet is 30 per cent bigger and almost three times heavier than Earth. It’s possible that its surface – which is hot enough to melt lead – may even be scattered with glowing lava rivers, according to Science.
A Transiting Planet With an Atmosphere
Super-Earths themselves aren’t rare, but Gliese 486b special for two key reasons, report the authors led by Trifon Trifonov with the Max Planck Institute for Astronomy: “firstly, its heat ‘puffs up’ the atmosphere, helping astronomers take atmospheric measurements; and secondly, it’s a transiting planet, which means it crosses over its star from Earth’s perspective – making it possible for scientists to conduct in-depth analysis of its atmosphere.”
“Understanding super-Earths is challenging because we don’t have any examples in our backyard,” says Montet. “Gliese 486b is the type of planet we’ll be studying for the next 20 years.”
A planet’s atmosphere can reveal a lot about its ability to support life. “We think Gliese 486b could have kept a part of its original atmosphere, despite being so close to its red dwarf star,” says Montet. “Whatever we learn about the atmosphere will help us better understand how rocky planets form.”
As a transiting planet, Gliese 486b gives scientists two unique opportunities to study its atmosphere: first when the planet passes in front of its star and a fraction of starlight shines through its atmospheric layer (‘transmission spectroscopy’); and then when starlight illuminates the surface of the planet as it orbits around and behind the star (‘emission spectroscopy’).
Best Bet for Studying Emission and Transiting Spectroscopy
“This is the single best planet for studying emission spectroscopy of all the rocky planets we know,” says Montet, when starlight illuminates the surface of the planet as it orbits around and behind the star or ’emission spectroscopy’. “It’s also the second-best planet to study transmission spectroscopy,” a tool that splits light according to its wavelengths – to decode the chemical makeup of the atmosphere.
“The gravity is also 70 per cent stronger than on Earth, making it harder to walk and jump. Someone who weighed 50 kilograms on Earth would feel like they weighed 85 kilograms on Gliese 486b.” On the plus side, the quick transition of the planet around its star means that interstellar visitors would have a birthday every 36 hours.
“The planet is really close to its star, which means you’d really have to watch out for stellar storms,” says Montet. “The impacts could be as innocuous as beautiful aurorae covering the sky, or they could completely wipe out electromagnetic systems.”
“If humans are able to travel to other star systems in the future, this is one of the planets that would be on our list,” he says. “It’s so nearby and so different than the planets in our own solar system.”
The study was part of the CARMENES project, a consortium of eleven Spanish and German research institutions that look for signs of low-mass planets around red dwarf stars,
Downside for Life on a Red Dwarf Planet
Red dwarfs, or M dwarfs (M-type main sequence stars), are the most common type of star, making up around 70 per cent of all stars in the universe. They are also much more likely to have rocky planets than Sun-like stars. Based on these numbers, the best chance for finding life in the universe may be looking around red dwarfs, says Montet – but this comes with a catch. “Red dwarfs are known to have a lot of stellar activity, like flares and coronal mass ejections. This kind of activity threatens to destroy a planet’s atmosphere. “Measuring Gliese 486b’s atmosphere will go a long way towards deciding if we should consider looking for signs of life around red dwarfs.”
The findings were made possible using data from NASA’s all-sky survey called the Transiting Exoplanet Survey Satellite (TESS) mission and telescopes in Spain, USA, Chile and Hawaii.
T. Trifonov et al. 2021. A nearby transiting rocky exoplanet that is suitable for atmospheric investigation. Science 371 (6533): 1038-1041; doi: 10.1126/science.abd7645
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