Earthlings have never been terribly sophisticated cosmic tourists. Ask us which worlds we’d like most to visit and our answers are always the same: Mars! Jupiter! Saturn! Neptune! But there’s a shortsightedness in that — a little like going on a world tour and limiting your stops to London, Paris, Tokyo and New York. They’re fantastic places all, but as any global traveler will tell you, if you really want to learn a culture and grasp its richness, you’ve got to visit the provinces.
The moons are our solar system’s provinces, and there are loads of them — up to 176 (depending on formal classification) circling six planets. Some are little more than giant bits of cosmic rubble, but many of them are much more — dynamic, even violent, and in one case at least, maybe capable of supporting life. What makes many of these satellites special is what’s known as tidal heating — a gravitational squeezing that takes place as the moon orbits its parent planet and is periodically tugged on by its sister moons in nearby orbits. On Jupiter’s Io, this produces explosive volcanoes; on Io’s neighbor Europa, it means a water-ice crust with what may be a global ocean underneath. On Saturn’s Enceladus it means sparkling jets of frosty exhaust, trailing behind the moon like the smoke in a wake of a steamship.
Now, scientists are coming to believe that what happens in our solar system may be happening all over the galaxy. A new study just submitted for publication by a pair of astrophysicists at Princeton University suggests that as investigators discover more and more exoplanets — worlds orbiting other stars — they might also discover tidally heated exomoons. This not only dramatically expands the scope of exoplanet research, it also represents a big step forward toward the field’s ultimate goal: finding worlds like our own — mirror Earths — that just might be home to life.
“Mirror Earth” is not just a term of scientific art, it’s also the title of a new book by my colleague Michael D. Lemonick, who has been chronicling the exoplanet hunt for years, as his Time.com columns here, here and here suggest. Lemonick begins his book with an improbable scene at the death bed of his father. A professor of physics at Princeton University, the older man had been drifting in and out of consciousness for a week, entirely unable to speak. Yet when the son began a visit with the pro forma question “Dad, what’s going on?” the old professor suddenly answered: “The Earth goes around the sun! That‘s what’s going on!” Lemonick goes on to write:
When my father first told me about the cosmos, astronomers didn’t know about quasars, or black holes, or pulsars. They didn’t know that most of the matter in the universe is not the atoms that make up stars, planets and people, but rather a mysterious, invisible substance known as dark matter. They knew that the universe is expanding but had no idea that the expansion is accelerating, driven by an equally mysterious force known as dark energy. And they didn’t know the answer to perhaps the oldest question of all: Do planets orbit distant stars? Do any of them harbor life? Is the human species alone in the universe?
(PHOTOS: Window on Infinity: Pictures from Space)
That puzzle finally pushed astronomers to act in a big way, and in 2009, NASA launched the Kepler Space Telescope with the singular purpose of looking for exoplanets. In the three years the spacecraft has been aloft, it has succeeded at that job brilliantly, identifying 77 previously unknown worlds and 2,321 candidates still to be confirmed. But searching for exoplanets is not easy. They’re almost always too small and too remote to detect visually. Instead, astronomers must infer their presence either by measuring the wobble in distant stars — a sign that something massive is orbiting them — or, as Kepler does, by looking at the slight dimming in light as a planet passes in front of a star’s facing side.
In the newly released paper, the investigators — Mary Anne Peters and Edwin Turner — wonder if tidally heated exomoons might serve as a sort of beacon to locate the planets they orbit. A pulsing moon may emit heat exceeding 1,000° K (1,303° F, or 727° C). That’s cold by the standards of the parent star — about 0.1% of stellar heat — but hot by comparison to the parent planet. And that may be just enough for the moon to be detected by infrared telescopes — essentially opening a whole new eye on the cosmos.
In some cases, the authors believe, once you’re able to take the infrared temperature of an exoplanet, you may find out it’s not an exoplanet at all. In 2008, the Hubble Space telescope discovered one such world, Fomalhaut B, orbiting a star 25 light years away. Preliminary infrared studies suggested it was a hot object, perhaps a young planet that was only then beginning to settle and cool. But the body’s temperature also falls into the range of what would be expected from a tidally heated moon. “If such exomoons exist,” the authors wrote, “it may well be far easier to image an exomoon with surface conditions that allow the existence of liquid water, than it will be to resolve an Earth-like planet.”
Exomoons may not even have to be tidally heated to harbor life. Rather, they could absorb plenty of solar heat if they happened to be orbiting a planet that lies in what’s called the Goldilocks Zone — the not-too-close, not-too-distant region around its parent sun in which liquid water could exist. Most of the exoplanets discovered so far are on a par with Jupiter in terms of mass, which would make a moon the size of Earth a plausible possibility. “If a Jupiter happened to orbit in its star’s Goldilocks Zone,” wrote Lemonick in an earlier Time.com post “and if that Jupiter happened to have a moon about the size of Earth — not impossible, surely — then that hypothetical moon might have a chance of harboring life. That’s a lot of ifs, which made talk of so-called exomoons seem like more of a marketing gimmick designed to gin up public interest in exoplanet science than a serious area of research. Not any more, though.”
Certainly, exoplanet research is still in its comparative infancy, with Kepler — the most prodigious of the planet-hunting telescopes — in service for not yet four years. But it’s not too early to say that the discovery of a mirror Earth is far closer than it’s ever been — or that when we do finally find it, it may turn out to be a moon.