According to two separate teams of astrophysicists from the Harvard-Smithsonian Center for Astrophysics (CfA) and the Lowell Center for Space Science and Technology, behavior of the parent star in the TRAPPIST-1 planetary system makes it much less likely than generally thought, that planets there could support life.
This artist’s impression shows the view from the surface of one of the exoplanets in the TRAPPIST-1 planetary system. Image credit: ESO / M. Kornmesser / Spaceengine.org.
In February 2017, astronomers announced that the star TRAPPIST-1 hosts at least seven planets — TRAPPIST-1b, c, d, e, f, g and h.
All these planets are similar in size to Earth and Venus, or slightly smaller, and have very short orbital periods: 1.51, 2.42, 4.04, 6.06, 9.21, 12.35 and 20 days, respectively.
Three of these planets lay in the star’s habitable zone, meaning they may harbor suitable conditions for life.
The host star, TRAPPIST-1, is a red dwarf star in the constellation Aquarius, 38.8 light-years away.
The star is barely larger than Jupiter and has just 8% of our Sun’s mass. It is rapidly spinning and generates energetic flares of UV radiation.
In the first study, CfA theorists Manasvi Lingam and Avi Loeb considered many factors that could affect conditions on the surfaces of planets orbiting red dwarfs.
For the TRAPPIST-1 system they looked at how temperature could have an impact on ecology and evolution, plus whether UV radiation from the central star might erode atmospheres around the seven planets surrounding it.
These planets are all much closer to the star than the Earth is to the Sun, and three of them are located well within the habitable zone.
“The concept of a habitable zone is based on planets being in orbits where liquid water could exist,” Dr. Lingam said.
“This is only one factor, however, in determining whether a planet is hospitable for life.”
The team found that the TRAPPIST-1 planets would be barraged by UV radiation with an intensity far greater than experienced by Earth.
“Because of the onslaught by the star’s radiation, our results suggest the atmosphere on planets in the TRAPPIST-1 system would largely be destroyed,” Professor Loeb said.
“This would hurt the chances of life forming or persisting.”
The researchers estimate that the chance of complex life existing on any of the three TRAPPIST-1 planets in the habitable zone is less than 1% of that for life existing on Earth.
The paper by Professor Lingam and Dr. Loeb is published in the International Journal of Astrobiology (arXiv.org preprint).
This artist’s impression displays TRAPPIST-1 and its planets reflected in a surface. Image credit: NASA / R. Hurt / T. Pyle.
In a separate study, CfA astrophysicist Cecilia Garraffo and co-authors found that the TRAPPIST-1 star poses another threat to life on planets surrounding it.
Like the Sun, the star is sending a stream of particles outwards into space.
However, the pressure applied by the wind from the star on its planets is 1,000 to 100,000 times greater than what the solar wind exerts on the Earth.
The researchers argue that the star’s magnetic field will connect to the magnetic fields of any planets in orbit around it, allowing particles from the star’s wind to directly flow onto the planet’s atmosphere.
If this flow of particles is strong enough, it could strip the planet’s atmosphere and perhaps evaporate it entirely.
“The Earth’s magnetic field acts like a shield against the potentially damaging effects of the solar wind,” Dr. Garraffo said.
“If Earth were much closer to the Sun and subjected to the onslaught of particles like the TRAPPIST-1 star delivers, our planetary shield would fail pretty quickly.”
The paper by Dr. Garraffo and colleagues will be published in the Astrophysical Journal Letters (arXiv.org preprint).
This diagram compares the sizes of TRAPPIST-1 planets with Solar System bodies. Image credit: ESO / O. Furtak.
While these two studies suggest that the likelihood of life may be lower than previously thought, it does not mean the TRAPPIST-1 system or others with red dwarf stars are devoid of life.
“We’re definitely not saying people should give up searching for life around red dwarf stars,” said CfA astrophysicist Dr. Jeremy Drake.
“But our work and the work of our colleagues shows we should also target as many stars as possible that are more like the Sun.”
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Manasvi Lingam & Abraham Loeb. Physical constraints on the likelihood of life on exoplanets. International Journal of Astrobiology, published online July 6, 2017; doi: 10.1017/S1473550417000179
Cecilia Garraffo et al. 2017. The Threatening Environment of the TRAPPIST-1 Planets. ApJL, in press; arXiv: 1706.04617
