More to life than the habitable zone

Two separate teams of scientists from the CfA have
identified major challenges for the development of life in
TRAPPIST-1. The TRAPPIST-1 system, depicted here in an artist’s
conception, contains seven roughly Earth-sized planets orbiting
a red dwarf, which is a faint, low-mass star. This star spins
rapidly and generates energetic flares of ultraviolet radiation
and a strong wind of particles. The research teams say the
behavior of this red dwarf makes it much less likely than
generally thought that the three planets orbiting well within
the habitable zone could support life. Credit:
NASA/JPL-Caltech/R. Hurt

Two separate teams of scientists have identified major
challenges for the development of life in what has recently
become one of the most famous exoplanet systems, TRAPPIST-1.

The teams, both led by researchers at the Harvard-Smithsonian
Center for Astrophysics (CfA) in Cambridge, Mass., say the
behavior of the star in the TRAPPIST-1 system makes it much
less likely than generally thought, that there could support life.

The TRAPPIST-1 star, a , is much fainter and less massive than
the Sun. It is rapidly spinning and generates energetic flares
of ultraviolet (UV) radiation.

The first team, a pair of CfA theorists, 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 ultraviolet 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
.

“The concept of a habitable zone is based on planets being in
orbits where liquid water could exist,” said Manasvi Lingam, a
Harvard researcher who led the study. “This is only one factor,
however, in determining whether a planet is hospitable for
life.”

Lingam and his co-author, Harvard professor Avi Loeb, found
that planets in the TRAPPIST-1 system 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,” said Loeb. “This would hurt the
chances of life forming or persisting.”

Lingam and Loeb 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.

In a separate study, another research team from the CfA and the
University of Massachusetts in Lowell found that the star in
TRAPPIST-1 poses another threat to life on planets surrounding
it. Like the Sun, the red dwarf in TRAPPIST-1 is sending a
stream of particles outwards into space. However, the pressure
applied by the wind from TRAPPIST-1’s star on its planets is
1,000 to 100,000 times greater than what the solar wind exerts
on the Earth.

The authors 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 acts like a shield
against the potentially damaging effects of the ,” said Cecilia Garraffo of the CfA,
who led the new study. “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.”

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 are devoid of life.

“We’re definitely not saying people should give up searching
for around red dwarf stars,” said Garraffo’s
co-author Jeremy Drake, also from CfA. “But our work and the
work of our colleagues shows we should also target as many
as possible that are more like the Sun.”

The paper by Lingam and Loeb was published in the
International Journal of Astrobiology and is available

online
. The paper by Garraffo et al, also available
online, has
been published by The Astrophysical Journal Letters.

Explore further:

Astrophysicists identify composition of earth-size planets in
TRAPPIST-1 system

More information: Manasvi Lingam et al. Physical
constraints on the likelihood of life on exoplanets,
International Journal of Astrobiology (2017).
DOI: 10.1017/S1473550417000179

The Threatening Environment of the TRAPPIST-1 Planets,
arXiv:1706.04617 [astro-ph.SR] lanl.arxiv.org/abs/1706.04617

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