The other night I stepped outside, just to the edge of the patio, away from the faint light cast from the window, and through the trees I could see a bright object. It was Jupiter, hanging high in the southeast in the constellation Sagittarius, with Saturn below and slightly to the east of it. Checking an astronomy app on my phone I could see that Pluto was there too. It’s too small to be seen without a telescope, but I thought it made an interesting addition: two planets and one ex-planet.
The boundaries of our solar system, and probably every other solar system, are fuzzy, and constantly changing. Even after Galileo proved Earth moved around the Sun and not the other way around Saturn was believed to be the most distant planet. And then came the discovery of Uranus in 1781, followed by Neptune in 1846, and Pluto was added in 1930, before being demoted to “dwarf planet” in 2006 because it’s one of hundreds, maybe thousands, of Kuiper belt objects hanging out past the orbit of Neptune but still within the Sun’s grasp.
I went outside to check out the sky because I was looking for Mars. Like the Sun it was below the horizon, but still out there, out of sight but not out of mind. On July 30th, if all goes according to plan, NASA will launch the Perseverance rover, bound for Mars, due to arrive in February 2021, where it will search for life. That makes the name Perseverance especially fitting; scientists have been looking for and hoping to find life on Mars since, well, probably as long as it’s been understood that Mars was a planet like Earth. Scientists hoped and even expected to find at least vegetation on Mars until the probe Viking 1 sent back pictures in 1976 that showed what a cold, hostile place Mars is. And yet there’s also a persistent belief that, having landed humans on the Moon, Mars is our next big leap.
Mars also seems like an end. The distance from Earth to Mars is roughly less than seventy million miles. It’s about another three hundred and forty million from Mars to Jupiter, and what we’d face out there makes just crossing the distance look easy. Jupiter has no solid surface. Maybe we could land on one of its bigger moons, one of the four first spotted by Galileo, but Io is one giant volcano, Europa holds the possibility of life and we’d have to ask whether we could risk contaminating it, and Callisto and Ganymede are bathed in Jupiter’s intense magnetic field. The other gas giants aren’t much better as far as human exploration is concerned.
In fact, moving outward, Pluto, former planet, could be our best next step. Terrifyingly cold and more than four and a half billion miles from the Sun it’s a solid, rocky island rich with water, at what we used to think of as the edge of the solar system. It’s even less like Earth than Mars is, and yet Pluto is the New York of space exploration: if we can make it there we can make it almost anywhere.
Before I went back in I turned to look at the rest of the sky, as much of it as I could see, anyway, considering boundaries and limits, and how they constantly shift in a universe in motion.