Exoplanet Detection Methods | The Schools' Observatory

Scientists use several methods to find exoplanets. Some are more effective than others.

Some of these methods are biased towards finding specific types of exoplanets, such as the transit and radial velocity methods. Both are better at finding giant planets close to their stars than other kinds of exoplanets.

Click on each method in the list to learn more details.

Astrometry

Astrometry is the science of making precise measurements of the position of an object in the night sky. This method looks for small changes in a star's position.

Objects in the night sky appear to move over time because the Earth rotates on its axis and orbits the Sun. Astronomers use special systems of coordinates that takes this into account. One of the most common sky coordinate systems uses Right Ascension (RA) and Declination (Dec).

Knowing the exact position of a star allows astronomers to look for small changes in its coordinates. If a star appears to 'wobble', this may be due to a nearby planet. But why does this happen? Let's think about our own Solar System.

You may know that everything in our Solar System orbits the Sun. But that is not quite true. Everything in our Solar System orbits the centre of all the mass in our Solar System. The Sun also orbits around this centre of mass, which gives it a very small wobble. However, since the vast majority of our Solar System's mass is in our Sun, the effect is quite small.

We can use this knowledge when we look for exoplanets. The amount that a star wobbles is related to the mass, number, and proximity of its planets. The bigger the planets are compared to the stars, the more noticeable the wobble is. This is because as this ratio increases, the centre of mass moves out from the centre of the star. A planet with a longer orbit also produces a bigger shift in the centre of mass.

This work "Zhatt" by Zhatt is licensed under Creative Commons Zero v1.0 Universal

Of course, many stars can be seen to move against a fixed background of stars and galaxies. If we see repeated changes in a star's position over time, the effect is due to an exoplanet.

Astrometry is a good method for finding exoplanets that (from our viewpoint on Earth) do not cross in front of their star. It can also be used to find planets located at large distances from their star. This makes astrometry a good method for looking for Earth-like exoplanets.

This method has only found a handful of exoplanets so far. However, it is useful way for confirming the presence of previously discovered exoplanets.

Astrometry can also be used to calculate an exoplanet's mass. It gives more accurate results than the radial velocity method.

Direct Imaging

Planets do not make their own light. They only shine because the light from their star hits them and bounces off. This light includes visible light and other wavelengths, like infrared.

The amount of light a planet re-emits reflect depends on the albedo of the planet. Most planets do not give off much light, especially at visible wavelengths. Compared to their parent stars, exoplanets are very faint.

This method is like looking for a lit match in the middle of a floodlit sports stadium. Fewer than 100 exoplanets have been discovered using direct imaging.

Astronomers use a circular plate called a coronagraph to block the light from the star. This reveals faint objects around the brighter star. The further an exoplanet is from its star, the easier it should be to separate light from the star and the planet. However, the greater the distance, the less light the planet receives from the star, so it is much fainter.

A large, blue sphere is in the centre of the image against a black background. There is a fuzzy glow around it. In the lower left corner there is a smaller, red sphere. — Infrared image of exoplanet 2M1207b (the red spot in the lower left) orbiting the brown dwarf 2M1207 (centre)

Sometimes, though, it is possible to detect the heat of a distant planet. We call this its 'thermal emission'. Using Wien’s Law, we know that a planet like our Earth re-emits most of its light as infrared waves.

Microlensing

The microlensing method looks for the effects of gravity on the light from a star. It is a good way of finding ver faint objects.

Gravity comes from mass warping and curving the 'surface' of time and space. You can think of it like a bowling ball on a trampoline. The ball will distort the material.

Light travels in a straight line. Gravitational lensing takes place when light follows the curve of the fabric of time and space. The light is bent and focused. The result is like looking through a giant magnifying glass. Really massive objects, like black holes, produce the biggest effect. However, less massive objects like stars and planets can also cause a small impact called microlensing.

We see microlensing events when the gravity of an exoplanet and its star acts like a lens. As the planet and its star pass between Earth and a more distant star, light from the more distant star is bent and magnified.

If the nearer star has an exoplanet, then the planet's gravity will add to the lensing effect. It will create a specific 'blip' in the light from the distant star.

Towards the upper left, there is an orange sphere. Below this, there is a yellow sphere. Faint yellow lines form an elongated oval around the yellow sphere, and an arrow points to the right. There is a graph beneath this showing "Brightness" and "Time" with a curve displayed. This whole set-up is repeated on the right of the image, except the yellow sphere has a small, brown sphere next to it and a dashed outline of a circle to show its in orbit. The graphs curve now displays a small blip on the peaks right — On the left: a star with no exoplanet. On the right: a star with an exoplanet.

This method needs the 2 stars to be almost exactly aligned as seen from Earth. The chances of this for individual stars are very low. These alignments are once-in-a-lifetime events that only last for days or sometimes weeks. So, to find an exoplanet, we must monitor a huge number of stars. The chances of success increase if we look for planets between Earth and the centre of the Milky Way. This is because there are more background stars towards the centre of the galaxy.

According to NASA's Exoplanet Census, about 3% of known exoplanets were found using microlensing.

Pulsar Timing

Radio waves from pulsars reach Earth as regular pulses. A planet orbiting a pulsar can create small, regular changes in the timing of the pulses.

The very first exoplanet was found using this method. But how does the method work? And what is a pulsar?

Though stars shine for thousands, millions, or billions of years, they do not last forever. What happens when a star eventually runs out of fuel depends on how the star's mass. Stars at least 8 times more massive than the Sun explode as supernovae. Some of these explosions leave behind a small but very dense star called a neutron star.

Neutron stars spin very fast and have a powerful magnetic field. They also emit beams of radiation from their poles. As the neutron star spins, the beams sweep past the Earth at regular intervals. It's a bit like seeing the flashes of light from a lighthouse. We can detect these flashes, or pulses, of radio waves and call them life. Neutron stars have magnetic fields that are trillions of times stronger than the Earth's.

Radial Velocity

This method looks for shifts in the spectrum of light from a star. Like astrometry, it uses the wobbles in a star's position caused by planets orbiting around it.

If a star is wobbling towards and away from the Earth, we can see this using spectroscopy. When the star moves away from us, its light is "stretched" slightly and appears redder than it would otherwise. We call this redshift. When the star moves towards us, its light is "squashed" slightly and appears a bit bluer than it would otherwise. We call this blueshift. Repeated shifts from red to blue are due to one or more exoplanets.

This work by NASA is licensed under Creative Commons Zero v1.0 Universal

The shifts in the spectrum can be used to measure the relative velocity of the star. This can tell us how long it takes for the planet takes to orbit its star, and then we can work out the planet's mass and composition.

Transit

A transit event takes place when a planet passes in front of a star. As it does so, it blocks some of the star's light. Astronomers look for regular dips in light from a star. These can be made by a giant planet passing in front of it.

There is a large yellow sphere in the upper middle. A white dashed arrow goes through it, pointing right. There is a black sphere repeated 4 times along the arrow: once before the yellow sphere, once on its edge, once on its other edge, and once after it. A graph beneath shows how the brightness changes over time. It is a straight line with a dip whenever the black sphere passes in front of the yellow one. — The characteristic dip in brightness as an exoplanet crosses the face of its parent star

If the planet is giant and close to the star, this dip in brightness can be as large as 2% of the total brightness of the star. It will also occur every few days. This makes this method good for finding planets with short orbital periods. Finding a planet which takes tens or hundreds of years to orbit its star would take too long!

We know that there are millions of stars in our galaxy. However, the transit method only works if we have a certain view of the planet and its star. If we look from above the planet's orbit, we will never see the planet pass between us and the star. It is thought that around 1% of stars might have a transiting planet, although catching one in the act is difficult.

This method lends itself to searches that use telescopes to survey regions of the sky. According to NASA's Exoplanet Census, over 75% of known exoplanets were found using the transit method.

Part of

Exoplanet

Exoplanets

About Us