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

  3. […] other small planets have been found before. But what makes Gliese 581g, as it’s referred to, special is that it’s just the right distance from its parent star that the planet’s surface […]

  4. […] Like many people, I’ve been watching the Kepler telescope’s exoplanet search with a great deal of interest. An exoplanet is a planet orbiting a star other than our Sun; the vast majority of known planets are now exoplanets, a big change over the last 10 years! One of the major goals of the Kepler mission is to identify terrestrial planets — planets that are roughly the same size and composition as Earth. Specifically, the ideal discovery would locate exoplanets in orbits that are within the host star’s habitable zone: the region where heat from the host star is sufficient for liquid water to exist. One potentially exciting discovery, Gliese 581g, was made last fall, which I wrote about here. […]

  5. […] to the hydrogen-based Jovian type — there’s no guarantee it will look like Earth at all. As I wrote in an earlier post about another possibly Earthlike planet, merely falling into the same general category is no guarantee greater of further similarity. To […]

  6. […] Hype aside (which is not supported by the original press release), Kepler-22b is a very interesting discovery, largely because the star it orbits is a slightly smaller version of our Sun: a yellow star. Even more significantly, it orbits at the right distance to be in its star’s “habitable zone”; more about what that means momentarily. Astronomers have found several exoplanets in the habitable zones of their stars, but those stars were all smaller and redder than our Sun, putting the planets very close in compared to Earth. […]

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