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.