Exploring the Possibilities of Life on Jupiter's Moons: A Million-Year Picnic
Step outside after dark this month and look up. That conspicuously bright white-gold star you see is no star at all but the planet Jupiter. Jupiter got some press last month when it reached opposition on January 10th. That’s the point when a planet is exactly opposite the sun from our point of view and therefore in the sky all night and at its very brightest. I heard from people who belatedly saw the news about opposition and thought they had missed their one chance to see Jupiter in 2026. But unlike meteor showers or comets, planets are very forgiving of weather and personal schedules. A planet remains bright and visible most of the night in the weeks and even months before and after opposition — a detail that sometimes gets lost in popular news coverage. So even right now and well into spring, you’ll be able to see Jupiter.
Jupiter is the largest planet in our solar system; all the others could fit inside it with room to spare. And it is its own solar system in miniature: 97 moons orbit Jupiter, four of them larger than the dwarf planet Pluto. Those four are the Galilean moons, so called after their discoverer, Galileo Galilei: Io, Europa, Ganymede, and Calisto. Ganymede, the largest of the four, is bigger than the planet Mercury. Astronomers are confident that three of the moons (Europa, Ganymede, and Calisto) have liquid oceans under thick crusts of ice and rock, warmed by tidal friction caused by their proximity to Jupiter. Oceans make these moons prime targets for exploration. A leading theory for the origin of life on Earth proposes that it began around underwater hydrothermal vents, where hot water interacts with minerals in rocks to create complex organic molecules. This activity continues today deep under Earth’s oceans. Similar environments may exist on those three Galilean moons.
NASA’s Europa Clipper spacecraft, launched in 2024 and arriving at the Jupiter system in 2030, will look for evidence of an ocean beneath Europa’s ice crust and determine its properties. It’s not specifically intended to look for signs of life but is capable of detecting molecules, or biosignatures, associated with life as we understand it. These would not be conclusive evidence of critters swimming in Europa’s lightless seas but would be strong evidence to support a future mission dedicated to discovering life.
You can see Jupiter’s Galilean moons for yourself with binoculars. Steady them on a railing, fence, or tree branch and aim them at Jupiter. The planet will be a bright disk, and you’ll see at least one right next to it; it will look like a tiny star. More likely you’ll see two or three or all four moons if you’re lucky. When a moon is behind Jupiter in its orbit, we can’t see it. If there’s more than one, they’ll form a line, which is the plane of their orbit around Jupiter. Observe them for an hour or more and you’ll notice them change position slightly. If your timing happens to be right, you’ll see one slip behind Jupiter and vanish, or one appear from the other side as its orbit brings it back in front of Jupiter.
But wait. Have you found Jupiter? It’s not the only bright star in the sky this winter. Your first clue is its color. Some of the other bright winter stars are blue-white, orange, or red. Jupiter shines with a distinctive white color with a hint of gold. The next clue to finding Jupiter is its position. On these February nights, it rises in the east just after sunset. As the night progresses, it soars to nearly directly overhead, then falls into the west, setting just before dawn. If you’re still in doubt, aim your binoculars at it. If the star in question resolves into a disk, it’s Jupiter. Actual stars will remain point sources of light in binoculars or telescopes.
While gazing at Jupiter, you can wonder what kind of life might evolve on its moons in an ocean of perpetual night and under crushing pressure. A resident of Europa would see no stars through the moon’s ceiling of ice; nor would a being there see Jupiter looming far larger than our full moon in the airless sky above, though such a being might deduce Jupiter’s presence from its life-giving effect of heat on its world. Do Europa dwellers dream of cutting through the ice to see what lies beyond? It’s a fanciful idea, but it’s the kind of imagining that inspires me to haul my telescope out in on a cold winter night. Clear skies!
About Justin Samaha
Amateur astronomer Justin Samaha sets up his telescope from time to time on various Outer Cape beaches and piers.
