How do we find an exoplanet?
Hot Jupiters, super-Earths, mini-Neptunes, ice giants. You must have heard of these strange worlds.
They make us dream.
They make us burn with curiosity.
They make our mind travel through the galaxy and imagine how they might look like and what they might be hiding.
But have you ever wondered how astronomers detect these distant planets? How do they know their size and composition? Let me take you on a voyage.
Imagine you’re leaving the solar system in the direction of the constellation Aquarius. Flying pass Jupiter, Saturn’s rings, leaving Pluto behind, you are now just a ball of energy, swooshing through space. At about 40 light-years from Earth: a dim reddish glow disturbs the pitch-black darkness. That’s TRAPPIST-1.
This ultra-cool red dwarf star is barely bigger than Jupiter, although it’s a lot more massive. Crammed around it are seven terrestrial planets orbiting so close around their star that they could all fit into Mercury’s orbit. How did astronomers detect these seven worlds? Well, they used a method that has proven very successful at discovering exoplanets — the Transit method.
Before we get into more details, let’s take a step back and take a look at what exoplanets actually are.
The idea of distant worlds
We know the 8 planets that orbit our Sun, including Earth. Exoplanets are planets that orbit other stars, outside our solar system.
The first speculations of exoplanets date back to the 16th century when Giordano Bruno suggested that other stars are like our Sun and must have their own planets. A similar possibility was evoked in the 18th century by Isaac Newton.
In 1917, the first evidence of a possible exoplanet was announced, but it was not recognized as such. The first real confirmation was made in 1992, with the discovery of several terrestrial-mass planets orbiting a pulsar 2,300 light-years from Earth.
The first confirmation of an exoplanet orbiting a Sun-like star came in 1995, when a giant planet was found orbiting the nearby star 51 Pegasi, about 50 light-years from us.
According to NASA, there are 4,276 confirmed exoplanets to this day. So what methods do scientists use to detect these distant worlds?
5 methods to detect exoplanets
Exoplanets are very difficult to detect with telescopes because they’re hidden by the bright glare of their star.
Astronomers have to use indirect methods to find these worlds. This means that they don’t observe the planet directly, but they try to detect the effect the planet has on its surroundings and in particular its star.
1. Transit method
By far the most prolific method is the transit method. Between 2009 and 2013, NASA’s Keppler mission detected thousands of possible exoplanets using this method.
The transit method works a bit like a solar eclipse. When the Moon passes directly in front of the Sun, it blocks some or all of its light. The same thing happens when a planet passes in front of its star — a distant observer will notice that the planet blocks some of the star’s light.
For a brief moment, that star will become a bit dimmer and this is the first hint that there might be a planet orbiting it.
The size and length of a transit can tell us a lot about the planet we’re observing. Bigger planets block out more light. Planets that are farther away have longer orbits. So, the longer the transit lasts, the farther away that planet is from its star.
The transit method can also provide information about the composition of a planet’s atmosphere or its temperature. When a planet passes in front of its star, some of the starlight passes through its atmosphere. We can analyze the colors of this light to get clues about its composition. This is how we can detect methane or water vapor on other planets.
2. Wobble method
The star and its planets constantly play the gravity game: the massive star has a very strong gravitational field and the smaller planet has a lot less gravity. Of course, the star always wins this game, that’s why planets orbit stars and not the other way around.
But the planet still has some gravitational effect on its star. This force causes the star to “wobble” a bit. Bigger planets cause stars to wobble a lot more than smaller planets. This wobble can tell us if a star has planets, how many, and how big they are.
The wobbling of the star is detected by a method called redshift. The light emitted by the star travels in waves. These waves are stretched out and squeezed according to the movement of the star.
When a star moves closer to us, its light waves squish together. To the observer, these waves will look bluer in color. When its moving away from us, the waves stretch out and make the light seem more reddish.
The wobble method is very productive at detecting exoplanets. It’s often used to confirm planets found with other methods. Two observatories which use this method with a lot of success are the Keck Telescopes in Hawaii and the La Silla Observatory in Chile.
3. Gravitational microlensing
Gravity, once considered a mysterious attraction between objects, was redefined by Einstein in his theory of general relativity as a geometric property of spacetime.
Big objects warp the fabric of spacetime. This causes light to distort and change direction when affected by the gravity of a massive object, like a star or a planet.
Gravitational microlensing happens when a star or planet’s gravity focuses the light of another, more distant star, in a way that makes it temporarily seem brighter.
To an astronomer, a lensing event looks like a star that gets gradually brighter and then fades away. If a planet is lensed, it looks like a brief blip of light that happens during this brightening and dimming process.
Astronomers can’t predict when or where these events will happen. They have to watch large parts of the sky over a long period of time.
About a hundred exoplanets so far have been detected using gravitational microlensing.
4. Direct imaging
Exoplanets are far away, and they orbit stars that are millions of times brighter than they are. So, taking pictures of them like you’d take pictures of Mars or Saturn is extremely hard.
New technologies are nonetheless making direct imaging possible. They use various techniques to block out the light of the star. Once the glare of the star is reduced, we can better see objects around the star, like planets.
One of the methods astronomers use to block the star’s light is called coronography. It uses a device inside a telescope to block light from a star before it reaches the telescope’s detector. Coronagraphs are used to directly image exoplanets from ground observatories.
Another method would be to use a ‘starshade’, a sort of spacecraft positioned at the right distance and angle from a space telescope to block starlight from the star we want to observe.
Direct imaging is in its starting phases as a method to detect exoplanets, but it has high potential. Future instruments might be able to take direct photos of exoplanets allowing us to identify atmospheric patterns, oceans, and landmasses.
5. Astrometry
Redshift isn’t the only way we can find stars that are wobbling due to the gravity of their planets. The wobble can also be visible as changes in the star’s apparent position in the sky. Astronomers can detect the star’s position wiggling around in space.
This method is called astrometry. As it’s amazingly difficult, so far we have found only one exoplanet using it. Stars wobble such a tiny amount that it’s very difficult to accurately detect this wobble.
To track the movement of these stars, scientists take a series of images of a star and of other stars that are near it in the sky. They then compare the distances between these reference stars and the target star. If the target star has moved in relation to the other stars, we can analyze that movement in search of exoplanets.
Astrometry requires very precise optics and is especially hard to do from Earth because our atmosphere distorts light.
Bottom line
Now you know that direct imaging of exoplanets is very hard, though not impossible. Most planets so far have been detected when a planet transits in front of its star and dims out some of its light.
Although small, planets have some gravitational influence on their star. This effect makes the star “wobble” a tiny bit and gives us a hint that there is a planet orbiting it.
Far-away worlds have fascinated humankind for centuries. But only recently has our technology allowed us to have a glimpse of these planets. And they don’t cease to astound us.
4,000 confirmed exoplanets might seem like a lot, but must be a tiny fraction of what’s really out there. There are an estimated 300 billion stars in our galaxy, and 2 trillion galaxies in the universe. Even if we don’t know if there are planets orbiting every star, the number of possible exoplanets is beyond our imagination.
When James Webb Space Telescope, the most powerful telescope ever built, will be launched in near future, which new wonders will we uncover? Strange places we’ve never imagined or Earth-like planets orbiting in the habitable zone around their stars? Does one of them host the building blocks of life? Or even life itself? Time alone will tell.
Thank you for reading my story!
