Dust may be a guide to exoplanet habitability

Atmospheric mineral dust may increase the potential habitability of exoplanets, British scientists suggest.

Writing in the journal Nature Communications, they say studies show that carbon-silicate material from the planet’s surface can have an effect on its climate system.

Planets with significant airborne dust may be habitable over a greater range of distances from their parent star, thus increasing the window for planets capable of sustaining life.

But dust also can obscure key biomarkers indicative of life, such as the presence of methane.

The study was carried out by a team from the University of Exeter, the University of East Anglia and the British Met Office, which performed a series of simulations using climate models.

Planets orbiting close to stars smaller and cooler than the Sun – so-called M-dwarfs – are likely to exist in synchronised rotation-orbit states, resulting in permanent day and night sides, they write.

Dust cools down the hotter dayside but also warms the night side, effectively widening the planet’s habitable zone.

They also found that that for planets in general, cooling by airborne dust could play a significant role at the inner edge of this habitable zone, where it gets so hot that planets might lose their surface water and become inhabitable.

As water is lost from the planet and its oceans shrink, the amount of dust in the atmosphere can increase and, as a result, cool the planet down. This process is a so-called negative climate feedback, postponing the planet’s loss of its water.

“On Earth and Mars, dust storms have both cooling and warming effects on the surface, with the cooling effect typically winning out, but these ‘synchronised orbit’ planets are very different,” says lead author Ian Boutle.

“Here, the dark sides of these planets are in perpetual night, and the warming effect wins out, whereas on the dayside, the cooling effect wins out. The effect is to moderate the temperature extremes, thus making the planet more habitable.”

Boutle and colleagues suggest their results also have implications for studies of the history of Earth before terrestrial vegetation covered large areas, with “a particular example being the faint young Sun problem of Archaean Earth”.

“The land masses that are believed to have emerged during this period will have been unvegetated, and therefore a significant source of dust uplift into the atmosphere if dry and not covered in ice,” they write in their paper.

“As we have shown, this dust would have a cooling effect on the planetary climate, potentially making the faint young Sun problem harder to resolve.

“However, it is also possible that microbial mats might have covered large areas of the land surface before vegetation evolved. The exact nature of such cover, and how much it would hinder dust lifting into the atmosphere, is yet to be quantified.”