MICHEL MARTIN, HOST:
Earth may seem like a one-of-a-kind planet, but it actually has a twin. Today, Venus is the hottest planet in our solar system. But a long time ago, it looked a lot like Earth. For our series on the science of siblings, NPR's Regina Barber reports on how Venus grew up to be so different.
REGINA BARBER, BYLINE: When the sun sets in the sky, the first star you see is actually not a star at all. It's Venus. Venus is the second planet from the sun, basically right next to Earth. And it's roughly the same size. Some scientists call it our twin, especially the ones that study it.
MARTHA GILMORE: We, without a doubt, as a Venus community call ourselves Venusians.
BARBER: That's Martha Gilmore, a planetary geologist that studies Venus.
GILMORE: We write that in our emails to each other - dear Venusians. Venus, Venus, Venus.
BARBER: Venusians love Venus, even though it's an intense place. Its atmosphere is so hot and so crushing that it would be unbearable for humans. Even the spacecraft that have landed there didn't last long. Compared to that, Earth is way less harsh. So why are Earth and Venus called twins? It's because, Gilmore says, Venus, Earth and, even to some extent, Mars all started relatively the same.
GILMORE: If you were an alien visiting our solar system 4 billion years ago, you would see three rocky planets, each of which have oceans.
BARBER: Their locations and sizes affected all three planets. Let's talk about size first. Size determines if that planet stays hot from its initial creation. When Gilmore is talking to her class, this is how she puts it.
GILMORE: It's like Thanksgiving. You have a hot potato, you know, a baked potato, and you've got peas. And you want to eat that potato, but it's too hot. But the peas - they're ready to go, because they have radiated out their heat because they're small.
BARBER: Mars isn't exactly a pea, but it is smaller than Earth and Venus. It cooled off pretty fast, but...
GILMORE: Venus and Earth are the same size potatoes.
BARBER: They both were about the same temperature to start. They both were made of pretty much the same stuff. They both had active volcanoes, and these were releasing greenhouse gases like carbon dioxide to create an atmosphere. And they both had oceans that could store any excess carbon dioxide.
GILMORE: By all of our understanding, Venus and Earth should have been the same.
BARBER: ...But they're not. And that's because of one big difference. Venus is a bit closer to the sun. Think of twins separated at birth, growing up in very different environments. Venus got too hot being too close to the sun. Its oceans dried up.
GILMORE: And once you get rid of an ocean, you turn off the major mechanism to store carbon dioxide in rock. And therefore, it just stays in the atmosphere, and the greenhouse effect takes over.
BARBER: You may have heard of the greenhouse effect. Scientists think about it a lot because of its role in global warming on Earth. So studying Venus can show us what happens when this effect gets out of control. Gilmore has so many other questions when it comes to Venus. Can Venus show us how our planet operated billions of years ago, before plate tectonics altered volcanoes, mountains, seas and climate? Can it tell us what to look for as we search for habitable planets across the galaxy? She likens Venus to dozens of exoplanets like Earth that scientists have found outside of our solar system.
GILMORE: I mean, those are, like, our long-lost cousins that you find when you go on, like, AncestryDNA or whatever, you know, 23andMe. And you're like, what? Who's that? (Laughter) That's those exoplanets.
BARBER: We won't be able to visit those distant planets in the near future, but we can actually visit Venus. With three upcoming missions to Venus from NASA and the European Space Agency, we might soon be able to answer these questions.
Regina Barber, NPR News.
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