It’s Cold Outside, but Earth Is at Its Closest Approach to the Sun

Sunset over the Atlantic Ocean, viewed from the International Space Station in 2014.

This story was updated to reflect 2020’s perihelion.

In the wee hours of Sunday (2:47 a.m. Eastern time, to be exact), Earth will make its closest approach to the sun and reach a point in its orbit known as perihelion. Chilly as winter may feel in the Northern Hemisphere, we’re more than three million miles closer to our fiery star than we were in the dead of summer.

The change in distance occurs because our planet’s orbit is stretched into an ellipse — so Earth snuggles up to the Sun every January and dips farther out into the outer solar system every July, at a point known as aphelion.

Although three million miles sounds vast, it’s not much on the scale of our solar system. In fact, despite the planet’s elliptical path through the heavens, most astronomers say that Earth’s orbit is basically circular. On a scale of 0 to 100 percent, where 0 is a perfect circle and nearly 100 is a very thin oval, Earth only scores a 1.7.

It’s a defining trait that keeps our planet at roughly the same distance from our sun, and keeps the climate relatively stable. This has led many astronomers to wonder whether a circular orbit just might be a crucial ingredient in the cocktail of life — and a key factor to consider as they search for signs of alien life around the thousands of exoplanets known to be circling other stars within the galaxy.

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Consider the exoplanet known as HD 20782 b, which boasts the highest eccentricity yet discovered — a whopping 97 percent. Although the alien world is likely more akin to Jupiter than Earth in mass, it’s easy to imagine what might happen to a wildly eccentric Earthlike planet.

At its closest approach to the star, the planet would face an explosion of heat that would evaporate the planet’s oceans and strip the planet’s atmosphere, sending crucial molecules such as oxygen streaming into space. The planet itself would also undergo dramatic tides, causing it to change shape, stretched by the star’s enormous gravity, and produce violent volcanic eruptions. Later, at its farthest distance from the star, the planet would undergo a long and deep freeze.

“This incredible seesaw of extreme conditions would devastate the planet, and I expect that it would not take very long for the planet to become a desiccated, barren rock,” said Stephen Kane, an astronomer at the University of California, Riverside, who described his own research as obsessed with eccentricity.

Such a dire portrait could suggest that life prefers a circle. But do most exoplanets orbit their host stars in bands similar to the Earth, or are their orbits more like that of HD 20782 b? In 2012, Dr. Kane and his colleagues analyzed the planets detected by NASA’s Kepler space telescope and found that smaller, rocky worlds tend to reside in circular orbits, whereas large, gaseous planets do not. This suggests that most of these rocky worlds might be able to host a stable climate and, therefore, life.

But Dr. Kane and other researchers warned against dismissing the possibility of life forming on worlds with highly-eccentric orbits.

“We should be very careful not to be too quick to exclude the possibility that these planets might have life,” said Vincent Van Eylen, an astronomer at Princeton University.

In fact, several recent studies have shown that a planet’s eccentricity can be nudged fairly high before that world becomes toxic.

Last year, Dr. Kane and his colleagues estimated that a planet with an eccentricity as high as 30 or even 40 percent could remain habitable — that is, it could have a temperature that supports liquid water — even as it swings toward and away from its host star. To pull off such a trick, the planet would need to host vast oceans of water and a thick atmosphere. Because water takes much longer to heat up and cool down than one might expect, the planet could avoid overheating in the summer and freezing in the winter. Likewise, a thick atmosphere would help keep a planet cool when it’s close to its host star, and serve as a blanket when it’s far away.

But there’s a limit to the amount of mayhem that these oceans and atmospheres can absorb.

While Dr. Kane says that an eccentricity of 30 to 40 percent is the upper limit, Sean Raymond, an astronomer at the University of Bordeaux in France, has argued that even a planet with near 60 percent eccentricity might be able to support life. Above this boundary, planets will have climates like that of HD 20782 b — one that gets thrown from the deep fryer to the freezer and back again.

But even then, the galaxy and its many orbiting planets could surprise us. “I think it’s often assumed that life is wimpy and needs ‘just so’ conditions to exist,” Dr. Raymond said. “But life on Earth did indeed survive some tough times.”

Roughly 650 million years ago, for instance, vast sheets of glaciers entombed our planet in a frozen skin that lingered for millions of years. It was a deep freeze, similar to what happens when these elliptical planets approach aphelion, that should have been deadly.

Yet life managed to survive here, as it might elsewhere.

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