Lunar Life

To date, NASA's Kepler mission has discovered over 1000 planets circling other stars. This is humanity's first confirmation that planets do exist “out there,” and are thus very common in our galaxy. Therefore, we are gaining confidence that life may well be found on planets other than Mother Earth, if even only a tiny fraction of those planets may be inhabitable. (Since we now know there are so many of them, the odds of habitability proportionately increase.)

The discovery of these planets has required the ability to detect incredibly faint changes in the light levels of these stars, such as when a planet passes in front of its star (between us and the star) and ever so slightly (ever so slightly) dims the light that reaches us.

Even so, the incredible-but-still-modest sensitivity of our current instruments has permitted us thus far overwhelmingly to detect giant planets. The instrumentation techniques are getting constantly more refined, so smaller planets—more Earth-sized orbs—are being discovered and more will soon be found; but so far it's primarily large, gaseous planets (like Jupiter and Saturn) we've been able to observe. Life, as we know it, however, could not exist on these giant planets, both because they have no solid surface and because their strong gravity would crush any forms of life.

OK. So we've very recently found over a thousand planets, but for most of them, life is very unlikely to be able to exist. Does that possibly diminish the chances of our finding extraterrestrial life, even though we now have demonstrated that these uninhabitable planets are common? Not necessarily, because these giant planets may well have moons circling them (just as Jupiter and Saturn have dozens of moons), and some of those moons are likely to be Earth-sized, and thus possibly able to harbor life.

Imagine what it might be like living on a moon that is the satellite of a giant planet in some distant solar system. Our Moon offers Earth only one face, because Earth's strong gravity field has forced it to rotate in lock step around us. The Apollo astronauts who walked on the Moon saw the Earth suspended above them, in the same place, Moon day after Moon day.

The same thing would happen if we were to stand on a moon orbiting a giant planet orbiting a star somewhere. One face of this moon would permanently point toward the planet. The planet would be this huge orb filling much of our sky—never changing its position. It would be frozen in our sky. When our star is behind us, it fully lights up the face of the planet (we might call it a “full planet”) and it would bask us in its reflected light. If the planet was purple, we'd be bathed in a purple haze.

When positions change and our star is now behind the giant planet above us, we'd be watching its dark side. In fact, it would appear as this huge black disc in our sky (blocking out the star's light), surrounded by a starry background.

This is just one of many bizarre scenarios that could be found on moons around giant planets elsewhere. Some day humans may arrive at some of these worlds and be able to watch some fascinating celestial sights—quite alien to anything we're used to... whether or not life has greeted us there.

Seeing Double—Part 2

NASA’s Kepler mission has recently scored another success in its planet-finding work. (Kepler is a special telescope that orbits the sun as our Earth does, while keeping itself constantly pointed at a small, single patch of sky containing nearly 150,000 stars in a nearby part of our Milky Way galaxy.) This telescope has recently discovered several so-called “circumbinary” planets—worlds that orbit double stars. (Kepler has already found about a thousand planets orbiting single stars.) So now Kepler has also proven that planets orbiting double stars can and do exist. In fact, astronomers now estimate (from Kepler’s findings) that there may be “tens of millions” of circumbinary planets in our galaxy… in addition to the estimated billions of planets around single stars.

Yet it’s a whole other issue whether these circumbinary planets have any chance of harboring life. Our precious Earth orbits a single star in a nearly circular orbit—keeping the amount of solar heat falling upon us relatively constant. (We get seasonal temperature differences only because planet Earth’s axis tilts about 24° to its orbital plane, which points us towards the sun in summer and away from it in winter.) A planet that orbits two stars could experience wide and wild temperature swings, which would prevent life from either forming or surviving.

The Star Wars movie had a fascinating scene, in which the fictional planet Tatooine experienced a double sunset. Of course, the views of suns on real circumbinary planets could be bizarre and quite different from Tatooine. For example, it could be a case of one of two suns always shining down, bathing the planet in nearly constant daylight; or the lengths of days could vary wildly, as the double stars dance around each other; or the planet’s seasons could be random; etc.

Now we know that circumbinary planets do exist—thanks to Kepler. It’s one more fascinating piece of knowledge brought to us by our space programs. What’s next in this extra-terrestrial planet hunt? Astronomers are hoping to find an Earth-sized planet (around a solitary star) with an atmosphere like ours. If so, that would be a strong hint of the possibility of life “out there.” There’s no telling what the next discovery will be, but stay tuned—we are bound to learn more captivating details about our fantastic universe soon.

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.”

Goldilocks Globe—Part 1

The search is on for planets around nearby stars. In only the last decade or so have astronomers begun to find so-called extra-solar planets around other stars. Prior to that, they felt pretty confident that planets must be out there, but the difficulties of verifying their presence were beyond the tools at hand. More powerful telescopes and more powerful computers are now allowing these discoveries to be made.

How do astronomers detect planets circling other stars? There is as yet no telescope capable of adequately zeroing in on a planet in another solar system—a star is just too bright and the planets too small and dim to see them. But if the planets are there, they will exert the tiniest gravitational pull on their star, causing it to wobble a minute amount, as they circle. Today’s big telescopes, backed up by super computers, have the sensitivity to discern this miniscule quavering of a star and even infer what kinds of planets are causing the tremble. In fact, the planetary motion laws that Newton gave us can be used to determine how many planets are there, their masses, and the size of their orbits. It’s amazing what the laws of physics will allow us to discover indirectly!

So the search for extra-solar planets has kicked into high gear in the last several years. Different teams of astronomers using different facilities feed off of and compete with each other and keep the enthusiastic juices flowing. New telescopes and techniques are coming on line all the time and are racking up impressive findings.

Until a few years ago astronomers strongly suspected, but had no direct evidence of, planets beyond our solar system. The discoveries are now piling up quickly—on the order of 400 hundred planets or more have been found to date. (I give a rough number, because the tally mounts almost daily.) Last year NASA sent up the Kepler Space Telescope—especially designed to ferret out extra-solar planets. The Kepler team has already announced locating several more worlds out there. A formal NASA report is imminently due, that will likely add dramatically to the total.

So what has been found so far? The vast majority of recently discovered planets are huge—the size of Jupiter and more. The tools available thus far are not yet sensitive enough to find smaller worlds, like Earth, but Kepler will undoubtedly change that. Super Jupiters are nice to find, but they hold little chance of harboring life of a type anything like us—their gravity is too great, their atmosphere is too thick, and their temperatures are usually too high.

On to the Goldilocks planet next time…