Above: An artist’s impression of a habitable exoplanet orbiting a red dwarf star (Ph03nix1986, Wikimedia Commons)
How many planets can you name? Seven? Eight? You don’t have to stop after Uranus and Neptune. Scientists have found thousands of planets that orbit other stars, just like Earth orbits the Sun.
Scientists call these distant planets extrasolar planets, or exoplanets for short. “Extrasolar” just means ”outside the solar system”. Humans have observed the stars for thousands of years, but the first exoplanet wasn’t confirmed until 1992. Since then, scientists have discovered thousands more.
Astronomers are especially interested in finding exoplanets that might be able to support life. In 2016, one such planet was discovered in the planetary system closest to ours. Scientists are already hard at work figuring out how to build spaceships that could travel there.
Finding “invisible” exoplanets
The Sun’s nearest neighbour is a star called Proxima Centauri. So far, astronomers have discovered one exoplanet orbiting this star. If you’ve ever seen Venus—the nearest planet to Earth—in the night sky, you know that it looks like a star. But depending on where Venus is in its orbit, Proxima Centauri is a whopping 200,000 to 1,000,000 times farther away from Earth!
If Venus looks like a star when you observe it from Earth, then moving it 1,000,000 times further away would make it invisible. So how do astronomers manage to find exoplanets orbiting around Proxima Centauri and beyond?
One important tool they use is called the transit method. Think of a solar eclipse. The moon passes between the Earth and the Sun, blocking part of the Sun’s light. In the same way, exoplanets sometime pass between their host stars and Earth, blocking part of the star’s light. This causes the star’s brightness to dim slightly when viewed from Earth. Human eyes can’t detect these changes, but modern telescopes and computers can.
Did you know? Some exoplanets are called rogue planets. These bodies wander the cosmos, not orbiting any star at all.
Liquid water means a planet could be “just right” for life
Most exoplanets have been found using NASA’s Kepler Space Telescope. It uses the transit method to search our galaxy, the Milky Way. Scientists using Kepler are trying to find out how many Earth-like planets there are in the galaxy. Earth-like planets are exoplanets that are about the size of Earth and are in their star’s habitable or ‘Goldilocks’ zone.
Why is it called the Goldilocks zone? If a planet is too close to its star, it would be too hot. Any water on the surface would evaporate, and the planet would suffer from a runaway greenhouse effect. If a planet is too far from its star, it would be too cold. Any surface water would be frozen solid. But if the distance between a planet and its star is just right, water would be liquid. Just like on Earth, which is in the Sun’s Goldilocks zone!
Liquid water is essential for life as we know it to exist. That’s why scientists pay so much attention to the Goldilocks zone. However, being in the Goldilocks zone doesn’t guarantee that a planet can host life.
For example, solar flares could make a planet in the Goldilocks zone uninhabitable. Solar flares are violent bursts of energy that come from a star. If the exoplanet’s parent star has strong solar flares, they could tear the planet’s atmosphere away and cause any liquid water to be lost to space. In fact, this is what scientists think happened to Mars four billion years ago, back when the Sun’s storms were far more violent.
Did you know? Using data from the Kepler Space Telescope, scientists have estimated that there are tens of billions of Earth-sized planets in their star’s habitable zone. And that’s just in the Milky Way galaxy!
Proxima b: The new exoplanet on the block
In the summer of 2016, astronomers announced big news about the planetary system next door. Proxima Centauri has a planet in its habitable zone!
The planet is called Proxima b. However, it’s not necessarily a second Earth. Like many Goldilocks zone exoplanets, Proxima b orbits a red dwarf star. Red dwarfs like Proxima Centauri are smaller and colder than the Sun. This means their habitable zones are very close to the star.
Because Proxima b orbits tightly to Proxima Centauri, a year on the planet is really short: only 11 days! This closeness suggests that Proxima b is tidally locked. Tidal locking means that one of the planet’s sides permanently faces the scorching star, while the other freezes in eternal darkness. And since it’s so near its star, Proxima b is also at risk of those atmosphere-destroying solar flares.
But the planet may have its own Goldilocks zone. There could be a band of perpetual twilight between the hot and cold faces, where the temperature is just right for life. And if life can exist on Proxima b, it will have a much brighter future than life on Earth. The Sun will die in around five billion years, but a red dwarf like Proxima Centauri still has trillions of years left to burn!
Did you know? Discoveries announced in 2016 nearly doubled the number of known exoplanets.
Breakthrough Starshot: Setting a course for Proxima b
One day, Earth might no longer be habitable. If this happened, could your descendants blast off to Proxima b? Scientists would have to learn more about this planet first.
You might also think sending anything that far would be impossible, but scientists like Stephen Hawking would disagree. Earlier this year, Hawking and entrepreneur Yuri Milner announced Breakthrough Starshot, a project to develop tiny probes and send them to Proxima Centauri within the next 20 years. Each of these little probes would only be the size of a postage stamp. They would be accelerated to a fifth of the speed of light using huge lasers, reaching Proxima Centauri after a 20-year journey. This is an ambitious goal: to travel at a fifth of the speed of light, the probes will be moving away from the solar system 600 times faster than any spaceship ever built!
Is Breakthrough Starshot possible? Maybe. There are huge technical obstacles to overcome, inventions to invent, problems to solve. But once the technology exists, Proxima Centauri wouldn’t be the limit. Scientists could explore the hidden corners of the solar system and countless other planetary systems.
Will Starshot find life out there? Is there life out there? In 40 years, humanity may know. And by then, you might even be the scientist discovering it.
About finding exoplanets:
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