How to find them?

It is very difficult to observe exoplanets directly since they emit little to no light themselves.  Even the brightest planets are outshined by their stars that are millions of times brighter than them.  To discover and study exoplanets, powerful instruments and clever detection methods are required.  Astronomers use five main methods to reveal them:

• Velocimetry (or radial velocity): Look for stars that dance and change colour because of the gravitational effect of their planets.
• Transit Method: Look for stars that blink.
• Direct Imaging: Take pictures of the planets directly.
• Gravitational Microlensing: Observe small increases in the light of distant sources caused by the gravity of stars and their planets.
• Astrometry: Spot the tiny movement of stars caused by their planets.

Velocimetry (radial velocity)

The presence of a planet around a star makes it dance, which changes the colour of the star that astronomers observe with their telescopes.

An animation explaining the velocimetry method (also called the radial velocity method). On top, we see a star and a planet orbiting around their common centre of mass. At the bottom, we see the change of position of the spectral lines of the star caused by the dance of the star, which is caused by the gravitational effect of its planet (Credit: ESO/L. Calçada)

The stars that dance

It is often said that planets orbit around their stars.  This is not 100% true.  Because planets also have gravity, they pull on the star just like the star pulls on the planets. In reality, both the star and the planets in a system orbit around a point called their centre of mass.  Because the star is much more massive than the planet, the centre of mass is usually very close to the centre of the star. Consequently, it usually looks like the star is not moving very much. The bigger the planet, the more it makes its star dance. A planet like Jupiter has a bigger effect on the Sun than a planet like the Earth.

A star is much brighter than its planet, but if we could somehow detect if that star is wobbling, we could detect its planet despite not being able to see it directly! So, how do we detect a wobbling star?

A star’s spectrum

The spectrum of the Sun, including several dark absorption lines associated with certain elements in its atmosphere (Credit: N.A.Sharp/NOAO/NSO/Kitt Peak FTS/AURA/NSF)

We can use a method called spectroscopy. We obtain the spectrum of a star’s light by breaking apart this light into all its constituent colours, like a rainbow. A star’s rainbow spectrum is usually has a few missing colours or gaps. These dark lines are caused by certain elements in the star’s atmosphere absorbing those very specific colours. We call these spectral lines.  Every star has a special pattern of these lines that tell us what the star is made of.

The Doppler Effect

There is one more puzzle piece needed to detect the wobble of a star, and this is called the Doppler effect. You have probably experienced the Doppler effect in your own life. As an ambulance moves towards you, it will sound higher pitched because the sound waves it is creating will become compressed, or squished. As it moves away from you, it will sound lower pitched, since its sound waves are now being stretched out.

An animation demonstrating the Doppler effect. The colour of an object producing light becomes redder as it moves away from the observer and bluer as it moves closer. (Credit: F. Baron)

The same effect happens with light, which is just another kind of wave. When the star moves towards us, its light looks a little bluer, and when it moves away from us, it looks a little redder.

We now have all the pieces required to detect a planet! By observing a star’s spectrum over a period of time, we can see if its spectral lines move back and forth because of the effect of a planet orbiting the star.

You can now see why we call this technique velocimetry, also known as the radial velocity method. By measuring the Doppler shift of a star’s light, we are actually measuring its velocity. And by measuring the amplitude of the velocity, we can actually determine the mass of the star’s orbiting exoplanet without even having seen it!

Velocimetry and iREx

At iREx, several researchers use the velocimetry method to find new planets or to confirm the presence of planets found by other methods. The SPIRou and NIRPS instruments, designed in part at the iREx, use the velocimetry method to study exoplanets.

Transit Method

When a planet passes directly between its star and an observer on Earth, the amount of light received by astronomical instruments decreases.

A cosmic wink

An image of the Sun during the transit of Venus in 2012. Venus is the small black circle that appears on the surface of the Sun at the top left. (Credit: SDO/NASA)

The transit method is one of the simplest detection methods to understand, but it can be extremely powerful. Imagine a planet orbiting a star. If we look at this system from the right angle (i.e. in the plane of the orbit), the planet will pass in front of the surface of the star once every orbit. This is called a transit event. This even happens within our own Solar System! From our point of view on Earth, we can sometimes see Venus and Mercury pass in front of the Sun.

An animation of a transit of a small rocky planet followed by a transit of a gas giant planet. The light curves appear at the bottom of the animation. The horizontal axis represents time, the vertical axis represents the brightness of the star. (Credit: NASA GSFC)

During a transit event, the star appears a little less bright since part of its disc is blocked by the planet. It’s sort of like a little cosmic wink telling us there’s an exoplanet in the system. A bigger planet blocks more light than a smaller planet. We can thus determine the size of the planet by measuring how much light is lost during a transit!

The shape, depth, and timing of the transit event can tell us all kinds of details about the planet: its orbit, its period, and sometimes even if there are other planets in the system.

Transit spectroscopy

A representation of the transit spectroscopy method. The light from the star passing through the exoplanet’s atmosphere contains signatures of the elements and molecules contained in this atmosphere. (Credit: ESO/D. Sing)

One of the best ways we have of studying the an exoplanet’s atmosphere is to combine the transit method with spectroscopy (we discuss spectroscopy in the velocimetry section above). When the planet is in transit, some starlight shines through the planet’s atmosphere. The different atoms and molecules in the planet’s atmosphere all leave specific signatures, like a fingerprint, in the light we receive on Earth. By carefully disentangling the planet’s signal from the star’s signal, we can determine the composition of the exoplanet’s atmosphere. This method is called transit spectroscopy.

The transit method and iREx

At iREx, several researchers use the transit method to find new planets or transit spectroscopy to study the atmospheres of exoplanets in depth. The PESTO instrument at the OMM, built by iREx researchers, uses the transit method. The Canadian instrument NIRISS, developed in part by iREx researchers, on board the James Webb Space Telescope has a mode that allows transit spectroscopy.

Direct Imaging

It is possible to take a picture of an exoplanet if we manage to block the blinding light of its star or if the planet is very far from its star.

Take a picture

An animation of a sequence of direct images taken by the Gemini-North Observatory of the HR 8799 system containing four exoplanets. The light from the central star has been blocked by a coronagraph. (Credit: J. Wang/Caltech/C. Marois/NRC-Herzberg)

It seems like an obvious question: Why don’t we just take a picture of the exoplanet?  The main problem is that planets are generally close to their host stars which are millions or even billions of times brighter than them. A good comparison would be to try and spot a tiny firefly buzzing around a giant lighthouse as seen from miles away. This is nearly impossible, but can sometimes be done in the right conditions and using very special tools and techniques.

The most important thing to do is to block as much light from the star as possible when taking the image using special devices called coronagraphs. The remaining light is carefully processed to remove as much of the noise as possible. The exoplanet’s orbit must also be located quite far from the star, or else the exoplanet itself will be blocked by the coronograph. With the right planetary system and a good coronograph, it is thus sometimes possible to take a real image and detect the exoplanet directly!

Exoplanets shine too!

A direct image of a planet forming around the star PDS 70 (whose light is blocked by a coronagraph) taken by the SPHERE instrument at the ALMA telescope. (Credit: ESO/A. Müller et al.)

A telescope taking a direct image of a planetary system in visible light is seeing the central star’s light being reflected off the exoplanet’s surface. A telescope looking at infrared light, however, is mostly looking at at light and radiation from the exoplanet itself! In both cases, it is easier to see larger planets because they reflect and emit more light.

For infrared telescopes, it also helps to observe young planets. As material collapses and squeezes together to form a planet, this generates a lot of heat and infrared light. Stars also tend to be fainter in infrared light, so the coronagraph doesn’t have to block as much light.  The easiest planets to directly image are therefore young and hot planets, on very wide orbits, around fainter stars, using infrared telescopes.

Direct imaging and iREx

The direct imaging method helped cement Quebec’s expertise in exoplanets with the first image taken of an exoplanetary system in 2008 by iREx researchers. In addition, the GPI imager, developed in part by iREx researchers, uses the direct imaging method.

Gravitational Microlensing

The light from a distant star is bent and magnified towards the Earth because of the presence of a closer star that passes between the Earth and the star.

Warped Space

Massive bodies like stars and planets deform spacetime, the fabric of the universe, through the force of gravity. The more massive an object, the greater of an effect it has. (Credit: T. Pyle/Caltech/MIT/LIGO Lab)

The most surprising method of exoplanet detection might just be gravitational microlensing. Einstein’s theory of relativity states that massive objects bend space and time around them. This is how gravity holds things together. You can imagine a bowling ball on a rubber sheet representing our Sun in the Solar System. The mass of the bowling ball will warp the sheet and create a dip. Other balls, representing the planets, will be attracted to the central ball and become trapped in this dip in the same way that the planets orbit the Sun.

Interestingly, the warped space can also cause light to bend around it. In this way, a massive object in space can act just like a big magnifying lens. When two stars line up perfectly in the sky, our telescopes may detect an increase in the brightness of the background star as it gets magnified. Even if we cannot see the object acting as a “lens”, we can determine its mass based on how the brightness changes over time.

Exoplanet Spikes

A gravitational microlensing event caused by a star-exoplanet system passing a distant star. The horizontal axis represents time, and the vertical axis represents the brightness of the distant star. The presence of the exoplanet causes a small additional peak in the brightness magnification of the distant star. (Credit: ESO/L. Calçada)

This “lensing” can happen with any kind of massive object. We generally use the term microlensing for smaller celestial objects like planets and stars (as opposed to galaxies). Astronomers can indirectly detect an exoplanet by looking for very specific lensing event signatures. In those cases, they would see the regular magnification from the star, as well as an additional spike in brightness due to the exoplanet acting like an extra magnifying lens.

Unfortunately, we cannot predict when two stars will perfectly align in the sky as they move around. To catch a microlensing event, we must observe thousands of stars at the same time and hope we’ll get lucky. Every once and a while, we get extra lucky and see evidence of an exoplanet as well.

Gravitational microlensing and iREx

Our postdoctoral researcher Lisa Dang is Canada’s leading expert in the use of gravitational microlensing for the detection and study of exoplanets.

Astrometry

Although it is often said that a planet revolves around its star, we should rather say that the planet and the star revolve around each other. The presence of the planet also makes its star move.

An animation of the small motion of a star caused by the gravitational effect of the exoplanet that orbits it. An exoplanet can be detected by observing and measuring this motion using the technique of astrometry. (Credit: NASA)

The term astrometry means to precisely measure the position and movement of a star. As mentioned in the velocimetry section above, orbiting exoplanets cause their host stars to wobble in space as they all orbit their common centre of mass. If a star is close to Earth, and if the exoplanet is very massive (causing a larger wobble), we may be able to directly measure the motion of the star.

These kinds of motions are typically very small, so it is extremely challenging to detect planets this way. Astronomers in the middle of the 20th century tried to detect exoplanets this way, but their instruments were not precise enough yet. Only recently, thanks to the Gaia mission, have we started to use this method. The Gaia telescope is measuring the positions of more than a billion stars with extreme accuracy. This is allowing us to discover and confirm the existence of many exoplanets in our solar neighbourhood thanks to astrometry.

Astrometry and iREx

Jonathan Gagné, iREx member and adjunct professor at UdeM, and his team use astrometry, with data from the Gaia mission among others, to detect and study exoplanets and stellar and sub-stellar objects.