Red Dwarfs Could Host 60 billion Habitable Planets in the Milky Way

A new study has effectively doubled the estimated number of life-friendly planets in orbit around red dwarfs. And remarkably, the astronomers attribute the revised figure to the presence of clouds.

Top illustration by Lynette Cook.

We know there’s a crap-ton of terrestrial planets in the Milky Way, many of them spinning around red dwarfs — those dim, main sequence stars that are the most populous in the galaxy.

'Tens of Billions'

Astronomers theorize that red dwarfs, which make up 75% of all main sequence stars in our galaxy, feature circumstellar habitable zones (HZ) that are considerably more interior than those of G stars (of which our sun is one). And in fact, owing to the low energy output of these stars, their HZs are about as close as Mercury is to our sun. But it’s within these sweet spots that water can remain in its liquid state — an important precursor to life.

Previous estimates have suggested that there may be as many as 4.5 billion potentially habitable planets in orbit around red dwarfs in our galaxy. But earlier this year, data from the ESO’s HARPS planet finder indicated that upwards of 40% of all red dwarf stars have a super-Earth orbiting in the HZ. This upped the number to tens of billions of such planets. What’s more, astronomers believe that virtually every red dwarf is host to at least one terrestrial planet.

And now, owing to a new calculation of the influence of cloud behavior on climate, the number has been extended even further, reaching 60 billion habitable zone planets. And the reason for the latest revision has to do with a newfound sense of the size of red dwarf habitable zones — a zone that’s now much larger, and more interior, than previously thought.

Thick, Reflective Clouds

As noted, red dwarf HZs are very interior. These close orbits cause planets to be tidally locked with their sun (i.e., the same side of the planet always faces the parent star, much like how our moon features a side that always faces Earth).

According to the new study, which was conducted by Dorian Abbot and Jun Yang, the star-facing side of red dwarf planets experience vigorous convection and exhibit highly reflective clouds at the substellar region — the location where the sun always sits directly overhead (call it “the place of perpetual noon”).

Red Dwarfs Could Host 60 billion Habitable Planets in the Milky Way

Simulated cloud coverage (white) on a tidally locked planet (blue) in orbit around a red dwarf star.

And indeed, Abbot and Yang’s 3D computer models showed that, if any surface water exists, clouds will result. What’s more, these clouds would be quite thick and have a significant cooling effect on any planet sitting inside the inner portion of the HZ — thus allowing these planets to sustain water on their surfaces at much closer distances to the sun than previously assumed.

The reason for the cooling effect is that a planet’s albedo is greatly increased by the presence of reflective clouds. The high insulation produces stronger substellar convection, resulting in a phenomenon in which climate is stabilized.

Additionally, the highly reflective clouds can prevent a lot of infrared radiation from reaching the surface.

“[We] use global climate models with sophisticated cloud schemes to show that due to a stabilizing cloud feedback, tidally locked planets can be habitable at twice the stellar flux found by previous studies,” the authors note in the study. “This dramatically expands the HZ and roughly doubles the frequency of habitable planets orbiting red dwarf stars.”

Don’t Believe the Red Dwarf Hype?

But not all astronomers are convinced that red dwarfs are the life-friendly stars they’re often made out to be. A separate study from St. Andrews University suggests that red dwarfs exhibit magnetic fields that are way too strong.

John Millis from Red Orbit reports:

Such strong magnetic fields can have a dramatic effect on nearby planetary objects. For a world close enough to maintain liquid water, the planetary magnetic field could be compressed by that of the nearby red dwarf; so much so that the field could be practically extinguished all together.

This is important, because an atmosphere is another element critical for life, and without a magnetic field the atmosphere would have considerably more exposure to cosmic radiation, especially high-energy charged particles. These charged particles can strip away the atmosphere all together. The chance of this is also increased by the fact the planet is so close to the host red dwarf, as the greatest source of charged particle radiation can come from the stellar wind itself.

Hope still exists, however. The team also revealed that the effect is driven by the strength of the red dwarf’s magnetic field, which varies depending on the rotation period of the star, which itself can slow over time.

At the same time, however, and as the Abbot and Yang study has shown, the clouds might be able to offset the dangerous levels of incoming particle radiation.

What’s clear, though, is that these red dwarfs feature complex environments far removed from what we’re used to here in our solar system. It’s an area that’s ripe for more research.

Read the entire study at The Astrophysical Journal, "Stabilizing cloud feedback dramatically expands the habitable zone of tidally locked planets."