Goldilocks Globe—Part 2

[News flash! It was just announced by NASA on 2 February that the Kepler Space Telescope has discovered 1235 candidate exo-planets as of that date! Of those, some 68 are Earth size! NASA will be further examining these candidates over the next few months, to see which may really be planets. How timely! On to Goldilocks.]

A few months ago a so-called “Goldilocks” planet was found. It got tagged with the name Goldilocks, because its star had two previously-discovered planets that were either too hot or too cold to support life. But a little more analysis of the data revealed a smaller in-between planet, dubbed Goldilocks, and officially given the moniker “Gliese 581 g.” It’s far enough away from its star to be able to have water, rather than either ice (it’s too cold) or steam (it’s too hot). Furthermore, it’s not much larger than Earth, so even its gravity and atmosphere could be like ours. Gliese 581 g is the first planet that we have located that just might support life as we know it! It’s caused quite a buzz in the astronomical community.

Planet Gliese 581 g is orbiting a star some 20 light years (that’s about 120 trillion miles) away. The star itself is named Gliese 581 a—which is a naming system that gives stars and their planets a numerical moniker. This star is quite different from our sun—it’s a red dwarf, it’s about one-third the size of the sun, it’s much older, and it’s a little over half the temperature. That may seem to describe an unusual star, but red dwarfs (which are too dim to see with the human naked eye) constitute as many as 90% of all the many billions of stars in our Milky Way galaxy. So they’re very common, and if life really does exist out there (astronomers get more confident every day that we’re not alone), it likely is orbiting a red dwarf star.

So what more do we know about this Goldilocks Gliese 581 g? It’s probably a rocky planet (like Earth) with enough mass to hold onto an atmosphere, if it has one. It is about 1.5-2 times the size of Earth, so its gravity wouldn’t be so large as to crush life forms similar to Earth’s. Its orbital period (its year) is very short—only about 37 Earth days—because it’s very close to its little, cool star. That brings about the fascinating result that the planet would be tidally locked to its star (like our Moon is to Earth), so it presents just one face to the star. That means that its perpetually daylight face stays boiling hot, while its permanent night face is freezing cold.

So how could it foster life or have liquid water? Because the zone between night and day (Goldilocks’s “twilight zone”) could be just the right temperature for life. In fact, if the planet has an atmosphere, that twilight zone could be quite wide, due to winds blowing around and spreading out the habitable zone.

It would be a wild experience to stand on Gliese 581 g and gaze at its sun. Rather than appear yellow-white, it’d be orange-red. It would appear about twice the size of our sun (since it’s closer), but it would never roam across the sky. Just as if you were to stand on the Moon and watch Earth constantly hovering in the same spot, day after day, year after year, the star Gliese 581 a would be stuck in the same place, and it would remain near the horizon.

Goldilocks has astronomers pretty excited, but mostly its discovery is leading them to expect to locate more habitable planets soon. The race is on! Kepler is continuing to find more planets around neighboring stars (1235 and counting!). We are beginning to document the fact that planetary systems are quite common in our galaxy. Couple this fact with the robustness of life that we’re also discovering right here on Earth, and the probability of life existing on planets other than Earth grows ever higher. Just a couple of decades ago the question “Are we alone?” had to be answered with a high degree of speculation and doubt. It’s beginning to appear, however, that the answer might well be “Not at all.”