What if you put an Earth-sized planet in a close orbit around an M dwarf star? This is more than an academic question since M dwarfs are the most numerous stars we know. A group of astronomers studying the planet GJ 1252b have found an answer and it’s not pretty.
Since this planet is so close to its star, it receives a lot of heat. And that closeness is deadly in another way. “The radiation pressure from the star is immense, enough to blow away a planet’s atmosphere,” said Michelle Hill, an astrophysicist at the University of California, Riverside and co-author of a recent paper focused on GJ 1252b. The planet is about 65 light-years from Earth and orbits its star twice every 24 Earth hours. The heat of the star makes this world inhospitable.
It’s not much different from Mercury in our solar system. There is no atmosphere, and the planet is alternately heated and frozen in orbit around the Sun. In fact, the Earth also loses some atmosphere due to solar activity. However, volcanism and other processes release gases into our atmosphere. Earth is lucky; planets like Mercury and GJ 1252b are not. And this has profound implications in the search for life-sustaining worlds.
What about the M Dwarf Stars?
There are millions and millions of M dwarf stars in our galaxy alone. Their size varies from about one tenth to two thirds of the mass of the Sun. These can be active, sending flares and explosions through their systems. Most have at least one planet in their habitable zones and others at various distances.
Not a good combo if you want to find life on their planets. The stellar activity that blows up planetary atmospheres obviously also destroys any chance of life on those worlds. And, since M dwarfs are so numerous, their ubiquity may reduce the number of planets in the galaxy that actually support life. This is not good news for planets like GJ 1252b.
“It’s possible that the state of this planet is a bad sign for planets even further away from this type of star,” Hill said. “That’s something we’ll learn from the James Webb Space Telescope, which will observe planets like these.”
Even though M dwarves can be atmosphere killers, it’s not all gloomy. For example, many of the 5,000 stars in Earth’s solar neighborhood are M dwarfs. could foster conditions conducive to life on their worlds. “If a planet is far enough away from an M dwarf, it could potentially retain an atmosphere. We cannot yet conclude that all of the rocky planets around these stars are down to Mercury’s fate,” Hill said. “I remain optimistic.”
Searching for an atmosphere on GJ 1252b
The science behind the situation at GJ 1252b is intriguing. Astronomers used data from the Spitzer Space Telescope to assess the planet’s infrared radiation as a secondary eclipse blocked its light. Measurements showed that the star is exploding the planet. Daytime surface temperatures range around 1227 C (2242 F). It’s hot enough to melt gold, silver and copper.
The heat, coupled with a supposedly low surface pressure, led researchers to believe there was no atmosphere there. But, let’s assume for a moment that there was an atmosphere of carbon dioxide. This would trap heat on the surface and possibly allow this blanket to exist for a while. However, it turns out that GJ 1252b is not so lucky. “The planet could have 700 times more carbon than Earth, and it still wouldn’t have an atmosphere. It would initially accumulate, then decrease and erode,” said Stephen Kane, UCR astrophysicist and co-author of the study.
In the long term, if this study holds true on a substantial population of M dwarf stars, it will shift the search for habitable planets to other candidates around less volatile stars.
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