Mars-like worlds orbiting red dwarf stars may not be habitable

Atmospheric loss caused by solar radiation would shorten the time such worlds are habitable.
By Laurel Kornfeld | Dec 17, 2017
A new study suggests red dwarf stars, which are long-lived and both cooler and dimmer than our Sun, may not be ideal places to look for habitable worlds.

Using data collected by the Mars Atmosphere and Volatile Evolution Mission (MAVEN), designed to study the Martian atmosphere, a team of researchers determined that Mars-like worlds orbiting red dwarf stars would be habitable for only short periods of time before losing their atmospheres and surface water.

Stellar activity on red dwarfs such as Proxima Centauri could strip the atmospheres of orbiting planets and reduce the time periods during which they are habitable by a factor ranging from five to 20.

The most active stars could shorten the duration in which orbiting planets are habitable by a factor of 1,000 or more.

In orbit around Mars since September 2014, MAVEN has been studying the rate at which Mars' atmosphere is being lost to space.

Solar activity levels have varied over the three years, ranging from quiet to flares, storms, and coronal mass ejections. The latter involve charged particles being ejected from the Sun's surface.

Because Mars has no magnetic field, solar radiation can remove the thin Martian atmosphere's lighter molecules.

MAVEN co-investigator David Brain used information collected by the probe on solar and Martian activity and inputted it into a computer model to create a hypothetical Mars-sized world orbiting a red dwarf star at the edge of the star's habitable zone.

For its temperatures to support surface liquid water, a planet orbiting a red dwarf has to be in a close orbit around the star. At that location, it would receive between five and ten times more stellar radiation than Mars receives from the Sun.

Higher levels of stellar radiation would significantly accelerate atmospheric escape. Energized charged particles would disturb atmospheric molecules, ejecting them into space.

Once the atmosphere becomes very thin, it can no longer support liquid water, causing the planet to dry up, much like scientists think happened on the real Mars several billion years ago.

Several factors could work against atmospheric loss on a Mars-like world orbiting a red dwarf, such as active geology that can replenish the atmosphere or the presence of a magnetic field.

Bigger planets have stronger gravity, which can help them keep their atmospheres.

The James Webb Space Telescope (JWST), scheduled to launch in 2019, will be capable of observing the atmospheres of large planets.

"Habitability is one of the biggest topics in astronomy, and these estimates demonstrate one way to leverage what we know about Mars and the Sun to help determine the factors that control whether planets in other systems might be suitable for life," noted MAVEN principal investigator Bruce Jakosky.

Findings of the study were presented on Wednesday, December 13 at a New Orleans meeting of the American Geophysical Union (AGU).

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