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 is a series exploring the ways our siblings can influence us, from our money and our mental health all the way down to our very molecules. We'll be 

Ask which planet in the solar system is Earth's closest sibling, and many people might point to Mars. It orbits nearby, just a little farther from the Sun. It was born at the same time and with the same stuff as Earth. And it is thought to have once had rivers and lakes, even oceans. NASA has sent rovers to its surface to help us learn whether the 'red planet' could have once hosted life.

But there are planetary scientists who would tell you to look in the other direction, to a planet that's far less explored but is actually closer to Earth in size, looks, composition and actual distance ... that is, toward Venus.

Scientists who study Venus affectionately call themselves Venusians. They like to refer to Venus as Earth's twin.

Martha Gilmore is a proud Venusian and a professor of Earth and Environmental Sciences at Wesleyan University. She says that in the past, our planet would not have looked so different from its two neighbors.

"If you were an alien visiting our solar system 4 billion years ago, you would see three rocky planets, each of which had oceans," Gilmore says.

Those planets — Earth, Mars, and Venus — look very different from each other today. Earth is a temperate, blue-green marble transformed by living things. Meanwhile, its siblings have migrated to two extremes: Mars is a dry, cold, dusty planet with a paper-thin atmosphere, and Venus is the hottest planet in the solar system, covered in a thick atmosphere that quickly destroys even nonliving visitors from Earth.

That's not an exaggeration: Ten probes that have made it to the Venusian surface; none of them have lasted more than two hours. Venus experiences temperatures over 800 degrees Fahrenheit and pressures that are more than 75 times that of what we experience on Earth.

So what happened to those ancient oceans on our two closest planets — and why is the surface of Venus such a harsh environment today?

Temperature and pressure are what set Venus apart from Earth. Gilmore says these differences stem from a couple of factors: distance from the sun, and the internal heat of the planet itself.

All planets are born with a certain amount of heat from when they were created, says Gilmore, who explains this phenomenon to her students using a holiday dinner.

"It's like Thanksgiving. You have a hot potato, you know, 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."

Mars is a smaller potato, so it lost its heat faster. Venus and Earth were similarly sized spuds, so they should have cooled at the same rate.

But other than internal heat, there was something else keeping Venus warm: the sun. Because Venus sits much closer to the sun, it receives more of its energy. And that extra bit of energy, delivered over billions of years, is a big reason that Venus's atmosphere became far more intense than what we experience on Earth.

Atmospheres act like "cozy blankets," Gilmore says. On Earth, for example, the atmosphere helps keep the planet habitable by shielding life from radiation and also keeps the surface at temperatures that we humans can live in. And to have a stable atmosphere, a planet needs a few things: volcanism, sufficient mass, and oceans.

"You have to be hot enough, big enough to have volcanism," says Gilmore, because volcanoes are powered by a planet's internal heat. Those volcanoes pump out the gasses that make up an atmosphere. But once that atmosphere is in place, a planet has to also be big enough that its gravity can actually "hold on to [that] blanket."

While Mars started out similar to Venus and Earth, its smaller size meant that its gravity wasn't strong enough to hold onto the small amount of atmosphere it had developed when it had oceans in its early life. And as Mars cooled down its volcanic activity slowed down ... and eventually stopped.

The problem is, planets actually need volcanoes to constantly replenish their atmospheres, because those atmospheres are constantly being lost to space, Gilmore explains.

"At the top of our atmosphere right now, there are all kinds of nasty rays that are eroding the atmosphere away, like cosmic rays and solar rays," she says.

So Mars's atmosphere was slowly eaten away — becoming so thin that water could no longer remain liquid on the surface. Some of it escaped to space, and some of it ended up frozen in ice.

Meanwhile, Venus was so close to the sun that its oceans boiled away. Volcanoes also continued to pump out stuff like carbon dioxide — which is a potent greenhouse gas.

Earth's atmosphere also has carbon dioxide, but our oceans help moderate its heat-trapping effects by sucking up excess carbon and eventually turning it into rock. That's why it's so crucial to take care of our oceans, Gilmore says.

"Once you get rid of an ocean, you turn off the major mechanism to store carbon dioxide in rock," Gilmore says. "And therefore, it just stays in the atmosphere. And the greenhouse effect takes over, and you get a super, super hot blanket."

The runaway greenhouse effect that makes Venus uninhabitable to life as we know it on Earth is something scientists worry about when studying the effects of climate change. Currently humans pump out 100 times more greenhouse gasses into the atmosphere than volcanoes do annually.

Studying Venus could give scientists insights of what a world looks like when there are no carbon sinks left. But it also happens to be the closest 'Earth-like' planet that researchers know of.

Hundreds of planets found outside of our solar system are Earth-sized and may be habitable. But these planets are so far away that sending spacecraft to investigate them will not be feasible for many generations.

Venus, Gilmore says, is much closer by. It's somewhere that we visited before, and can visit again. Earth is scientists' first data point on what a habitable planet can look like — but Venus's past could give us a glimpse of another planet that was once habitable before it was altered forever.

Regina G. Barber

Author

Earth, Mars and Venus all looked pretty similar when they first formed. Today, Mars is dry, cold, and dusty; Venus has a hot, crushing atmosphere. Why did these sibling planets turn out so different?

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An animation of the formation of Venus and Earth

Venus and Earth used to look like 'twin' planets. What happened?

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