For all the efforts to find another inhabitable planet orbiting a distant star, it might surprise you to learn that a very real Earth 2.0 exists in this solar systemjust one planet over. Not Mars (which actually isn't much like Earth at all), but rather, our other neighbor: Venus. Mental Floss spoke to geophysicist Bob Grimm, a program director at the Southwest Research Institute and chair of NASA's Venus Exploration Analysis Group. Here are a few things we learned about Earth's twin sister.

Venus has a radius of 3760 miles. Earth's is 3963. Its mass and gravity are 82 percent and 91 percent of Earth's, respectivelypretty similar as planets go. Venus is composed of a mostly basalt crust, silicate mantle, and iron core. Earth is the same. The two planets likely share common origins somewhere around 4.5 billion years ago.

In fact, by all accounts, we should be able to land our flying saucers on Venus, saddle up a dinosaur, and start building tract housing. It's perfect for colonization, but for a few minor differences. Its year is shorter, at 224.7 days. (And its days are much longer, at 243 Earth days per Venus day.) The Sun would rise in the west and set in the east because of the planet's retrograde orbit (which, by the way, is the most circular of any planet in the solar system). And then there's another small problem

Venus is hotter than Mercury, despite being 30 million miles farther from the Sun. How hot? Hot enough, on average, to melt a block of lead the way a block of ice would melt on Earth. Venus suffers from a runaway greenhouse effect. Sunlight penetrates the dense clouds surrounding Venus, heating the landscape. The ground in turn blasts out heat, which rises and tries to escape the atmosphere. But carbon dioxide, which makes up 96 percent of its atmosphere, traps the heat, keeping things nice and toasty, around 900F. And those clouds aren't the white, fluffy variety. They're made of droplets of sulfuric acid, which makes its lightning storms especially harrowing.

"'Does Earth-size mean Earth-like?' is a basic problem of planetology," says Grimm. "Understanding how Earth and Venus diverged is essential to understanding comparative planetology, and potentially exoplanetsthese worlds orbiting distant stars that are being discovered telescopically."

Knowing more about Venus would help scientists better distinguish potentially habitable worlds out there, and better understand how a good world can go bad, from a sustaining-life perspective. "Geology and meteorology are intimately related to the evolution of the Earth and the evolution of life on Earth," Grimm notes. "Even though we may not be looking for life on Venus, it's important to understanding Earth's place in the solar system and in the universe."

You might have run across old illustrations of Venus with conditions similar to the Carboniferous Period on Earth. Astronomers have known for just under a hundred years that Venus's atmosphere is devoid of oxygen, without which you can't have water. But even a modest backyard telescope can see the clouds enveloping our neighbor, and as Carl Sagan explained, from there you're only a couple of erroneous jumps from assuming a brontosaurus. (Thick clouds mean more water than land. More water than land means swamps. Dinosaurs lived in swamps. Dinosaurs live on Venus. QED.) Said Sagan: "Observation: There was absolutely nothing to see on Venus. Conclusion: It must be covered with life."

But seeing is believing, and the Mariner and Venera series of probes disabused us of the romantic notion of a swampy neighbor to the left. Still, we should probably send robots there to check. Just to be sure.

Venus was the first planet we visited, with Mariner 2 achieving the first successful planetary encounter in 1962. Four years later, Venera 3 on Venus became the first spacecraft to touch the surface of another planet. (Communications were lost long before impact, but unless a dinosaur ate it, the spacecraft probably touched the ground.) Our first graceful landing on another planet? Venera 7 on Venus. Our efforts to reach its surface go back much further than that, though. The transit of Venus in 1761 practically invented the notion of an international science community. But we abandoned the surface of Venus in 1984, and NASA hasn't launched an orbiter to Venus since Magellan in 1989.

Since then, the Venus-science community has been trying to get another mission to the launch pad. Presently, U.S. planetary scientists have submitted proposals to NASA for a sub-$1 billion New Frontiersclass mission. They are also working with their colleagues in Russia to launch a joint mission called Venera-D. "We need better radar views of the surface," says Grimm, "and that has to happen at some point to understand the geology. We need deep probes into the atmosphere to understand it better, and we need a new generation of landers."

"There is evidence in the deuterium-to-hydrogen ratio that Venus once had water, maybe hundreds of meters deep, more like a global sea than an ocean," says Grimm. A theoretical paper published last year posed a climate model for Venus suggesting that water could have existed on its surface as recently as 1 billion years ago. Clouds could form in a certain way, shielding the surface from the Sun and allowing stable water at the surface. Furthermore, near-infrared observations support the argument for a watery Venusian past. ESA's Venus Express orbiter in 2012 found evidence of granite-like rocks on some parts of the planet. Granite requires a multiple melting process in the presence of water. A mission to Venus could confirm this.

Meanwhile, one of the most significant revelations from Magellan is that there are only around 1000 craters on the surface with no differences in density, and it is hard to find craters that are obviously in a state of being wiped out by lava, or being faulted. Venus does not have plate tectonics, one of the central mechanisms that organizes all geology on the Earth. So what happened to the surface of Venus? Where is the evidence of the Late Heavy Bombardment seen on other terrestrial planets and moons? One hypothesis is that all of Venus was resurfaced at once. There may have been a global catastrophe on Venus, perhaps as recently 750 million years ago, that quickly "reset" its surface. Other models suggest a subtler resurfacing at work in which craters might be erased over billions of years.

"So this whole idea of the surface age of Venus is a pivotal question for how planets evolve geologically," says Grimm. "But what was Venus like before that? Was there a single catastrophe, or have there been many? Was there just one catastrophe and Venus was watery before that, or has Venus operated in a steady state going back to the first billion years? There is more consensus that in the first several hundred million years to billion years, there could have been water." Further landings on Venus could help us solve the mystery of when Venus's surface was formed, if there was ever water there, and why, if it existed, it went away.

If Matt Damon were to get stranded on Venus in a sequel to The Martian, he would need to be resourceful indeed to survive the heat and the corrosive air. But what he would find wouldn't be wholly alien. The winds at the surface of Venus are very gentle, around a meter or so per second. The vistas would consist of hills and ridges, with dark lava rocks of various types, mostly basalt. The atmospheric pressure is 90 times greater than Earth at sea level, so walking there would feel a lot like swimming here.

"I don't think [Venus] would look wavy and hot-hazy, because the atmosphere is pretty stable and uniform right at the surface," says Grimm. "It would be harder to walk through the dense atmosphere, but not as hard as walking through water. We know from landings that it's kind of yellow because of the sulfur in the atmosphere. So with the abundance of lavas in many places on Venus, it sort of looks like a yellowish Hawaii."

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