The “Goldilocks” Planet

Everyone is rightfully getting excited about the discovery of Gliese 581g, a new planet found orbiting a nearby star. Known as extrasolar planets or exoplanets, planets orbiting stars other than our Sun provide the best clues we have about the formation and evolution of planetary systems in general. However, Gliese 581g is even more exciting than usual because of where it lies relative to its host star. (The star’s name is Gliese 581, and the letter “g” indicates this planet is the sixth one discovered orbiting.)

Stars obviously produce light, and part of that light is in the infrared part of the spectrum, which we perceive as warmth. The amount of light a planet receives depends on the distance from the host star, so planets close in are warm, while planets far out tend to be cold. There’s a certain range of distances where the temperature is between 0° and 100° Celsius—water can exist in the liquid state. This region is known as the habitable zone, since liquid water is a necessary ingredient for life as we know it. In our Solar System, Venus and Earth are both in the habitable zone; Mars lies just on the far side of it.

Of course, the habitable zone by itself isn’t enough. Venus is very Earth-like in size and composition, but its thick atmosphere is mostly carbon dioxide, with other nasty ingredients like sulfuric acid mixed in. Due to the extreme greenhouse effect on Venus, the surface temperature is hot enough to melt lead, so even though it lies in the habitable zone, it wouldn’t qualify as habitable by our standards at all. On the other hand, Mars is a little better off: its carbon dioxide atmosphere keeps the planet barely warm enough in the equatorial region that liquid water could exist for some of the time. In fact, we know that Mars had substantial oceans in the distant past (though that’s a subject for another day).

Gliese 581g lies in the habitable zone of its host star, and it’s small enough to be rocky in composition (as opposed to gaseous, like Jupiter, Saturn, Uranus, and Neptune). But how Earth-like is it? First strike against it: it’s at least three times more massive than Earth, so if it’s of rocky composition, it’s likely to have much higher surface gravity than Earth. (We can only put a lower bound on mass estimates because of the way in which the planet was discovered.) Second strike: the host star Gliese 581 is a type known as an M dwarf, which (as the name suggests) is smaller than our Sun, but also much more red in color. Red stars are less hot than yellow stars, so the combination of temperature and size means that the habitable zone of Gliese 581 is very close in to the star. As a result, Gliese 581g orbits its host star at about half the distance Mercury lies from our Sun, which means it probably presents the same face to the star in the same way the Moon always presents the same face to Earth. In other words, it is always daytime on one side of Gliese 581g, and always night on the other side.

So does this rule out the possibility of liquid water and life on Gliese 581g? It’s too soon to tell. One problem with guessing either way is that we don’t know yet what the true conditions of life are. We have one planet—Earth—where we know life exists, and life exists in a wide variety of environments, from the hottest spots to the coldest, and extending deep below the surface. However, we don’t have any evidence for life on other worlds in our Solar System yet, so we don’t know what the probability is that life can develop. My guess is that Earth is far from unique in terms of having life of some kind, but that’s as far as I’m personally willing to go. Until we know for sure that life exists somewhere else, it’s hard to make guesses about other planets.

So I’m excited about Gliese 581g, but if you ask me whether it can harbor life, I’m going to stay noncommittal. A difference between science and pseudoscience is how willing we are to make definite pronouncements. True science always recognizes that our models are contingent: upon data, upon new discoveries, upon our ever-changing view of the universe, and so I look forward to the day when we can say a lot more about this Goldilocks planet and the possibility of life on worlds other than Earth.

6 Responses to “The “Goldilocks” Planet”

  1. 1 Rubbo October 13, 2010 at 19:40

    This is an interesting follow up article on the discovery of Gliese 581g

    I particularly like this article not because I have doubts about the discovery, but because it demonstrates how science really works.

  2. 2 Matthew R. Francis October 13, 2010 at 20:44

    Yes, I was thinking I should do a follow-up on this (unless you want to!).

  1. 1 Gliese 581g: Are you really out there? « Science in the Sky Trackback on October 26, 2010 at 02:23
  2. 2 Is There Anybody Out There? « Galileo's Pendulum Trackback on May 23, 2011 at 17:39
  3. 3 What Does “Earthlike” Mean? « Galileo's Pendulum Trackback on September 1, 2011 at 10:46
  4. 4 What Does “Habitable” Mean? « Galileo's Pendulum Trackback on December 6, 2011 at 10:15
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