Habitable Exoplanet

Habitable Exoplanet

Climate modeling has identified the first exoplanet that really could support life.


A best bet for alien life. I'm Bob Hirshon and this is Science Update.

Scientists in France have identified the first planet beyond Earth that could support life as we know it. Many candidates have been found in the past. But according to exoplanet researcher Robin Wordsworth, of the Laboratoire de Météorologie Dynamique in Paris, this planet is the first to stand up to a three-dimensional climate model.

At the time of this discovery, it was thought to be too cold to have any chance of supporting life. And it turns out, in fact, that if it has a dense atmosphere of carbon dioxide, which is a very plausible scenario for a large planet like this, then it will be warm enough to have oceans—and hence life.

The planet, called Gliese 581d, is very different from Earth. For starters, one side is permanently lit, and the other is always dark. Yet the model shows that the dense atmosphere would redistribute heat to the dark side. Wordsworth says we'll have to develop more powerful telescopes to actually look for life there. I'm Bob Hirshon, for AAAS, the science society.

Making Sense of the Research

Since telescopes became powerful enough to look beyond our solar system, planetary scientists have been searching for "exoplanets" (planets orbiting stars beyond our sun) that could potentially support life.  However, we can't view these distant planets in great detail, as we can with nearby planets like Venus or Mars.  The closest known exoplanet orbits a star called Epsilon Eridani, which is 10.5 light-years away (63 trillion miles)—about 14,000 times further than Neptune, the most distant planet in our own solar system.

At that distance, we can't take photos of exoplanets like the ones you've seen of planets within our solar system. In fact, exoplanets are almost never observed directly, since the reflected light on an exoplanet's surface is too faint to detect from such a long distance. Instead, scientists infer the presence of an exoplanet from a set of secondhand clues. These include regular shifts in the wavelength of light coming from a star, which may be caused by the gravitational pull of an orbiting planet, or slight, regular dips in the star's brightness, which can be attributed to a planet passing in front of it.

Using methods like these, scientists have identified hundreds of exoplanets since the first confirmed detection in 1995. Once an exoplanet has been detected, researchers can begin evaluating it for the chance it might harbor life. Further analysis of the light spectrum radiating from the star can reveal properties of the planet's surface. Most extrasolar planets turn out to be gas giants (like Jupiter), poor candidates for supporting life as we know it. (This is probably because gas giants are easier to detect than smaller, rocky planets.) And most potentially rocky planets have been either too close or too far away from their star to have a hospitable climate. 

Exoplanets that fall within a "habitable zone," neither too close nor too far from their sun, remain on the short list for life. The next stage is to model the climate on these planets. So far, every planet that's been tested with a climate model, including other planets in the Gliese system, has been ruled out. 

Surprisingly, scientists originally thought Gliese 581d would be too cold to support life, because of its distance from the red dwarf star (Gliese) it orbits. It's also "tidally locked" with its sun, meaning the same side of the planet always faces it. That means there is no day and night cycle, just a permanent day on one side and permanent darkness on the other. It seemed that such conditions couldn't support a stable atmosphere because of the dramatic temperature differences on the two sides. 

However, Wordsworth's team's three-dimensional climate model projects that if Gliese 581d has a dense carbon dioxide atmosphere, which is likely for a planet of its size (seven times that of Earth), that atmosphere should redistribute heat from the lit side to the dark side. What's more, although the planet receives just a third of the energy from its red dwarf sun than Earth gets from our sun, it retains that energy more efficiently. Because of factors like these, Wordsworth says the planet should be warm enough for oceans, clouds, and rainfall. 

For the time being, that's the best evidence we're going to get that life could exist there. It will take a few leaps forward in telescope technology to observe exoplanets like these directly. In the meantime, planetary scientists will be applying this 3-D climate model to other candidate exoplanets, so they'll have a list of prime candidates to study when the technology catches up.

Now try and answer these questions:

  1. What is an exoplanet?
  2. What's unique about Gliese 581d?
  3. Why did the planet appear to be a poor candidate for life?
  4. What changed that perspective?
  5. Why look for signs of life on other planets?

Going Further

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