ANOTHER EARTH

An MIT astrophysicist explains what we’d see on a trip to Earth-like planet Proxima b

Since at least the time of the Greek philosophers, humans have speculated about the existence of other worlds. Today we know our night sky is literally teeming with exoplanets—planets that orbit the stars outside our solar system. And it was with great excitement that the world greeted the announcement on Aug. 24 that scientists had detected a planet orbiting not just any star, but the star nearest to our own sun, Proxima Centauri.

The discovery was particularly exciting because of the possibility that the planet, known as Proxima Centauri b, might have the conditions necessary to support life. But in order to understand the import of the news, it is useful to take a look beyond the fanfare and examine the facts.

Scientists haven’t actually seen the planet directly. Instead, it was discovered indirectly via a technique that extensively, and precisely, observed the star’s tiny motion due to the gravitational effect of the planet orbiting it. In the formal publication describing the Proxima Centauri b discovery, astronomers conclude that the data is best explained by the presence of a planet, having ruled out any other phenomena (such as star spots) that might have induced a false alarm. Because the authors followed standard protocol using state-of-the art instrumentation and data analysis techniques, and the signal is strong, astronomers world-wide—including myself—endorsed the discovery.

Here’s what we know so far about this planet, which is approximately 25 trillion miles away. We know that its mass is at least 30% greater than Earth’s mass. The planet’s orbit (or year) is equal to eleven Earth days, and this orbit means that the planet is very close to its star. Based on what we know, the planet receives about the same total amount of energy from its star as Mars does from our sun. Proxima Centauri is a small red dwarf star with low energy output. So a planet in orbit very close to its star might have the right surface temperature to support life.

Based on this information, we can speculate about what a trip to Proxima Centauri b might look like. The most fascinating aspect to me is that on the planet, the alien sun would be in the same place in the sky at all times. We could choose to visit a spot on the planet where it is always daylight, or where it is always nighttime, or even at a location where the sun is always setting.

The reason why the planet would have a permanent day and night is because of its proximity to the star. The huge gravitational force from the star over time would have forced the nearby planet into a “tidally-locked” state, where the planet shows the same face to the star at all time, just as the moon does with Earth.

Your birthday on this planet would come about every eleven days. Depending on the planet’s actual mass and size, it may have a higher surface gravity than our world, making it very hard to stand up straight or do work. It might also be hard to see on the planet, because its star is dim and red.

For the average earthling, visiting this planet might not be much fun. We wouldn’t be able to use a mobile phone or play video games, and we would need to wear powerful sunscreen all the time. This is because the harmful ultraviolet radiation and huge flares of energy from the star would constantly be bathing the planet’s surface, disabling electronics and even destroying biological cells. We might have to spend our trip protecting ourselves on the permanently dark side of the planet.

But back here on Earth, we astronomers are pumped about the possibility that Proxima Cenaturi b might actually have the right surface temperature to support life. The planet’s surface temperature is controlled by the atmosphere’s as-yet-unknown inventory of greenhouse gases. At the moment, astronomers are abuzz with ideas about how to study the planet’s atmosphere, either later this decade after the launch of the James Webb Space Telescope or further in the future with the giant ground-based telescopes (with mirrors 20 to 40 meters in diameter) currently under construction in Chile and Hawaii.

When the Proxima Cenaturi b discovery was announced, I was up at my summer cottage on one of the Great Lakes. Outside, in the summer night sky, an absolutely spectacular number of stars greeted me. I knew that Proxima Centauri is only visible in the Southern hemisphere, and is too faint to see with the naked eye. But that didn’t stop me from wondering, “What—and who—is out there?” I like to imagine that on a planet orbiting one of those stars, there may be aliens looking out at our own far-away sun, wondering the same thing.

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