

A team of astronomers at the New South Wales have announced the discovery of an exoplanet sitting solidly in the habitable zone of its parent star. The new planets — there are three altogether — orbit the red dwarf star Wolf 1061, which is just 14 light years from Earth.

The particular planet we’re interested in is Wolf 1061c. It’s four times the size of Earth and orbits its star every 18 days. Red dwarf stars are much smaller and dimmer than our own sun, which means Wolf 1061c is much closer to its star than Earth is to our own sun. All three planets could be rocky, but only Wolf 1061c sits within the habitable “Goldilocks” zone — and it’s solidly within that zone, to boot.

Many websites are reporting that this could make the planet habitable — and to be clear, it could — but based on what we know of red dwarfs, the odds don’t favor it. As we previously noted, red dwarf stars are much smaller and dimmer than our own sun. The so-called Goldilocks zone for a red dwarf star is much closer than it is for Earth, as shown in the chart below:


Gliese 581 and 667C are both red dwarfs.

Planets that orbit as close to their parent stars as these do are thought to become tidally locked, meaning one side of the planet always faces the star. This has a number of negative effects on a planet. If you’ve ever been camping on a cold night and huddled near a fire for warmth, you’ve experienced the sensation of being warm on one side, cold on the other. Standing closer to the fire doesn’t do much to improve your personal cold side — it just makes the part of you that’s already warm uncomfortably hot.

Now imagine that instead of a campfire, you’ve got a stellar furnace pouring out enormous amounts of energy. The cold side of the planet experiences a deep freeze, with temperatures low enough to make the atmosphere condense around that half of the planet. On the hot side, meanwhile, atmospheric gases expand. Some scientists think there might be a temperate band around the terminator of such planets where the land would be wreathed in a perpetual sunset / sunrise — but even if the temperature is right, the massive thermal differential between the hot and cold sides would create atmospheric storms of incredible power, as hot air rushed over the cold side of the planet. Scientists do not know if this would result in the gradual loss of atmosphere or if the planet would retain a thick storm cover.

A planet’s position relative to its host star is also likely to lead to substantial amounts of internal heating as well, which could leave a planet theoretically within the “Goldilocks” zone looking more like Venus than Earth, with a thick, tumultuous atmosphere made up of greenhouse gases and a volcanic, molten crust.


This image shows the Goldilocks zone in more detail for our own star. Venus orbits at 0.71 AU (compared to our own 1 AU, while Mars is at 1.6 AU). Mars and Earth are both considered to exist within the Goldilocks zone, but clearly something happened to Mars to prevent the planet from maintaining its own atmosphere. One theory is that Jupiter swung into the solar system and then back out to its current position, sweeping the solar system free from much of the material that would have otherwise accreted to Mars, Earth, or formed the asteroid belt (this is known as the Grand Tack hypothesis).

Jupiter’s theoretical migration could do much to explain why our solar system looks the way it does — and, by extension, some of the characteristics on Earth that made it amenable to life, while denying Mars the necessary mass it needed to achieve a similar state.

Even if Wolf 1061c isn’t habitable, it could still tell us a great deal about the behavior of tidally locked planets and the types of atmospheres that exist under these conditions. With enough data on similar planets, we may discover whether the concept of planets with a narrowly inhabitable terminator between two raging extremes is actually plausible, or if it belongs in the world of science fiction.