How do astronomers hunt for exoplanets?
For centuries, humans have looked to the skies and wondered what and where we came from. What lies beyond the world beneath our feet? Are planets common in the cosmos or is our solar system unique? Are there other worlds like Earth out there?
Today, modern astronomy has the capacity to gaze outside our own solar system and spot planets whizzing around other stars. The first confirmed exoplanet was discovered in 1992, and since then tools for exoplanet discovery have been rapidly improving and multiplying, allowing us to begin to form the shape of answers to those questions.
“Finding the first of something is really hard, because we don’t really know how to do it – it’s pushing the boundaries of technology,” explains Jonti Horner, an exoplanet researcher at the University of Southern Queensland. “But once you’ve found one thing, you’ll find tens, you’ll find hundreds, you’ll find thousands.”
At last count, we’d found 4,803 planets across 3,553 planetary systems.
“There’s a lot of stuff out there, but until your technology is good enough to find it, you don’t pick it up,” Horner says.
Turns out, finding exoplanets is pretty hard – not only are other star systems trillions of kilometres away, but planets are comparatively small and dim so can be drowned out by their star’s light.
So how do we find other worlds?
The most successful exoplanet-hunting technique so far is the transit method, which essentially searches for shadows.
When a planet crosses in front of its star during its orbit, it causes the star’s brightness to dim by a tiny but measurable amount. Since a planet’s transit will always last the same length of time and will always cause the same change in brightness, watching the brightness of stars over time is a reliable method for confirming the existence of planets.
This technique was used by NASA’s Kepler’s Mission, the planet-hunting powerhouse that was retired in 2018. It spent nearly 10 years in orbit around the Earth, observed over half a million stars, and found more than 2,600 exoplanets.
This method watches for a “wobble” in the orbit of a star. Though we say that a planet orbits a star, in reality a planet and a star are orbiting each other in a gravitational tug-of-war – the star’s mass is just so much bigger that it barely moves.
The radial velocity method spots the “wobble” of a star in its light by looking for a Doppler shift. As the star moves away from us, the waves of light stretch out slightly and become redshifted; when it moves towards us, the waves bunch up and become blueshifted. By watching for regular shifts in the star’s light spectrum and seeing how its velocity changes, the signals of a companion planet can be seen.
Similar to the transit method, gravitational microlensing involves looking for how the light of a star varies over time. But instead of a star dimming, the method watches them get brighter and then fainter again.
It’s not looking for a planet around that star – it’s actually looking for a cosmic coincidence, when a planet closer to Earth (orbiting around a different star) just happens to pass in front of a more distant one.
“The gravity of the planet in the foreground bends the light from the star, acting as a lens, and focuses the light too so that background star gets a little bit brighter,” Horner explains.
“This is a technique that once upon a time was expected to find gazillions of planets, and it’s found a handful. It’s not been as successful as people expected it to be, but that’s because it’s really hard.”
But this is the only real way we have of finding rogue planets – lonely worlds free-floating through the galaxy, without a star to call home.
This method works exactly as the name implies – while other methods can only infer the existence of an exoplanet, direct imaging takes snapshots of the planets themselves.
“The poster child for this is the star HR 8799, which has four planets going around it which have all been seen directly,” Horner says.
We don’t “see” the planet in detail like we do in images of Jupiter or Venus – exoplanets appear just as points of light – but it’s impressively difficult to parse out this tiny speck in comparison to the overwhelming glare of its parent star.
“It’s like trying to look at a moth flying around the light of a lighthouse from tens of kilometres away,” Horner says. “But with the biggest telescopes, you can do clever things to block out light from the star.”
These methods are the ones that have thus far actually spotted planets, but Horner says that another method that will be a big player in the future is astrometry.
“This is a technique with a really long heritage – it was actually used in the 1800s to discover binary stars,” he says.
Astrometry measures the position of the star in the sky. A star’s proper motion is normally in a straight line, but if there’s an unseen planet around it, this will cause the star to wobble in relation to the background stars.
“It’s quite possible that in the coming decade we’ll discover a large number of planets using this astrometry method,” Horner says – largely from ESA’s Gaia mission, which is up in orbit making precise measurements of hundreds of thousands of stars.
This method will enable us to find different types of exoplanets, too.
So far, Horner notes, we’ve been biased towards finding both big exoplanets and those that are very close to their star.
“We found a huge number of what we call hot and warm Jupiters,” he says, “hugging their star on orbits that are much closer than Mercury is to the Sun. They’re weird – we’ve got no equivalent to that in the solar system.
“That doesn’t mean they’re the most common kind of planet, it just means that they’re the easiest to find.”
This is because the transit and radial velocity methods are both more sensitive to these kinds of planets; astrometry, on the other hand, is sensitive to the opposite – planets far from their stars.
Using many methods combined will provide a more accurate astronomical census of the diversity of exoplanets in the universe, and therefore give us a better idea of whether our solar system is unique.
What’s our place in the universe?
Horner says that even in his career, exoplanet research has revolutionised our view of the cosmos and our understanding of our place within it.
“I grew up in a world where we only knew one planetary system,” he explains.
Today, we know that planets are ubiquitous across the cosmos, with most stars playing host to other worlds.
“Nowadays we take all this stuff for granted…’we found another exoplanet, woo!’ But hang on, we’re discovering alien worlds around distant stars. How can that ever become run-of-the-mill or ordinary?”
The discoveries are only going to continue to grow exponentially – and we’re not only finding planets but beginning to study them in-depth, in particular their atmospheres.
The successor to NASA’s Kepler mission is now up in orbit – the Transiting Exoplanet Survey Satellite, or TESS, which is hunting for alien planets around 200,000 of the nearest and brightest stars.
Specifically, TESS is trying to find Earth-like worlds: terrestrial planets in the habitable zone.
“To me, we’ve found nothing that is really Earth-like yet,” Horner says. But he hopes one day we’ll find a planet that is like our own not only in terms of size and distance from its star, but also atmospheric composition.
Horner works with MINERVA-Australis, the only dedicated follow-up facility for TESS in the southern hemisphere. Hosted by the University of Southern Queensland’s Mount Kent Observatory, it’s receiving a stream of data from TESS and scouring the southern skies to confirm these new worlds.
When we find a truly Earth-like planet, Horner says, “that’s when we can really start trying to get a handle over whether life is common in the cosmos or whether we’re alone”.
The answer, he reckons, might come within our lifetime.
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