Searching for exoplanets around the star next door

How many planets can you name? Seven? Eight? You don’t have to stop after Uranus and Neptune. Scientists estimate that there could be billions 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 1991. Since then, scientists have discovered thousands more.

Finding “invisible” exoplanets

Our solar system’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 early evening or early morning sky, you would know that it is recognizably bright. 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 than Venus is!

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 to the naked eye. So how do astronomers manage to find exoplanets orbiting around Proxima Centauri and beyond?

One important tool scientists 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 sometimes 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 seem to 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. A new telescope called TESS (the Transiting Exoplanet Survey Satellite) was also launched in 2018. TESS uses the same transit method to search for Earth-like planets that are located around stars nearby to Earth.

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!

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 could 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 host 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, NASA’s MAVEN mission in 2014 has led scientists to believe this is what happened to Mars approximately four billion years ago, back when the Sun’s storms were far more violent.

Did you know?

As of January 2020, astronomers have confirmed the existence of 4 116 exoplanets.

Meet our neighbour, Proxima b

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 dwarf stars like Proxima Centauri are smaller and colder than the Sun. This means their habitable zones are very close to their stars.

Because Proxima b is in a tight orbit with Proxima Centauri, a year on the planet is really short: only 11 days! This closeness suggests that Proxima b is tidally locked to Proxima Centauri. Tidal locking occurs when a planet or moon’s orbital period is the same as its rotational period. For instance, the Moon is tidally locked to Earth, so the same side of the Moon always faces us on Earth. If Proxima b is truly tidally locked, that means one side of the planet permanently faces the scorching star, while the other side 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!

In 2019, astronomers discovered that Proxima b might have a big sister! They detected Proxima c, a candidate planet orbiting a lot further from the star. Proxima c would be too cold for life. However, it may be an ideal planet for direct imaging. This is a method in which telescopes take photos of the planet itself, instead of studying it indirectly through methods like transits

Did you know?

2016 was a big year for exoplanet research. Discoveries announced in that year 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 some scientists would disagree. One example is the late scientist Stephen Hawking. In 2016, Hawking and entrepreneur Yuri Milner announced Breakthrough Starshot, a project to develop and send tiny space probes 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.