Is Mars really the only planet solely inhabited by robots? Yes, but no. The truth behind this meme is an excellent opportunity to investigate just how adventurous our robotic explorers are in visiting all sorts of places we squishy humans haven't.
Starting from the basics: at this exact moment in time, Mars is the only planet we know of with on-the-ground robotic explorers and no other confirmed life. If we use that as our definition, the statement is entirely true. Although the Spirit Rover went silent at Troy Hill and is presumed dead, both Opportunity and Curiosity continue to rove around the red planet. As long as the Mars 2020 rover arrives to start exploring before both the current inhabitants die, Mars will continue to be a planet inhabited entirely by robots until the Next Giant Leap, Mars One, or some other deep space mission successfully delivers the first (live) human explorers.
Would the declaration still be true if the rovers died? After all, Mars has quite a few residents within its gravitational influence, from the newly-arrived MAVEN and Mangalyaan to the longtime workhorses Mars Reconnaissance Orbiter, Mars Odyssey, and Mars Express. But, Mars isn't the only planet with robotic visitors loitering in orbit!
The many satellites of Mars will be hiding behind the planet when comet Siding Spring dashes past. Image credit: NASA
Working from inside-out, Mercury received its first visitor in 2011, with MESSENGER settling into orbit. Venus currently has the Venus Express, and with a bit of luck, may be joined by Akatsuki in November 2015. Skipping the Earth, as we're inhabited by more than just robots, and the moon, which isn't a planet, the next stop is the very-busy Mars with its host of robots. Beyond the asteroid belt, Saturn still has Cassini working hard. At the moment, Jupiter is a robot-graveyard with shrapnel from Galileo lost within the clouds, but Juno is due to arrive in July 2016. Uranus and Neptune are most certainly unoccupied by robots, the subject of cursory flybys but no long-term visitors and no upcoming missions. If we generously include Pluto as a planet, although it's currently unoccupied and hasn't even had a real flyby, New Horizons is rapidly creeping up on the dwarf planet with a scheduled arrival in 2015. Alas, the probe will not be making an orbital insertion. Instead, it will be just a fleeting flyby before New Horizons continues on to a To Be Decided target even farther away.
If we declare that any planet with long-term robotic visitors within its gravitational influence is inhabited by robots, Mars is joined by Mercury, Venus, and Saturn. Already, half the planets in our solar system are inhabited by robots. Now I'm hoping the existing probes will all hold on and keep functioning until Jupiter can be added to the list!
The first colour photograph from the surface of Venus, taken by the Venera 13 probe. Image credit: Roscosmos/NASA
If we exclude all these satellites and declare that "inhabited" requires a ground-presence, Mars is once again the only known planet currently inhabited exclusively by robots. Yet, even by this strict standard, it isn't the only planet in our solar system to ever have robotic inhabitants. The gas giants will be perpetually excluded due to their lack clearly identifiable "ground," and Mercury never had a ground lander. But Venus, our sister planet and object lesson in feedback mechanisms gone horribly awry, holds the title for being downright murderous towards robotic invaders.
The first several Venera probes launched by the Soviet Union were crushed by the planet's intense atmosphere before their robotic corpses hit the surface, but the 7th, attempt successfully survived entry to tattle about surface temperatures for a full 23 minutes before succumbing to the hostile environment. Now they'd mastered the knack of preparing their robotic explorers for a planet that clearly hated company, the Soviet space program sent more and more landers, photographing the surface and probing the soil (or a lens cap). While Venus was never occupied for more than an hour at a time, it was, at least briefly, inhabited exclusively by robots. Not only that, but its first inhabitant, Venera 7, arrived two years earlier and lasted 95 times longer than the first inhabitant of Mars, the Mars 3 lander.
Concept art of Philae on the surface comet 67P/Churyumov–Gerasimenko. The probe will attempt landing on November 12th. Image credit: ESA
The declaration also hinges on our definition of a planet: if we expand out to any objects within our solar system, quite a few places have robotic inhabitants. The Sun is by any definition very distinctly a star and not a planet, yet has an entire horde of orbiting satellites. After lingering at asteroid Vesta, Dawn is en-route to dwarf planet Ceres. Meanwhile, Rosetta is not just in orbit around a comet, but soon its Philae lander will attempt to actually land on and drill into the dirty mass of rock and ice. While nothing is on them at the moment, Saturn's moon Titan was briefly inhabited by the Huygens probe, and Jupiter's moon Europa may soon be getting its own explorer.
While we've done alright with our puny, fleshy bodies by reaching the moon and keeping a space station continually inhabited for years, we've done an excellent job of spreading robots throughout the solar system. Some of those places still have robots, while others are robot ghost planets with only the memories of explorers from long ago. Yes, Mars is at this very moment the only known planet occupied entirely by robots, but that won't always be true.
And, of course, we could always stumble upon a planet inhabited by the robotic representatives of yet-to-be-discovered alien lifeforms.
Hat tip to my brother who asked me to fact-check the meme.