Planets Beyond Our Solar System: What Do Exoplanets Look Like?

Artist impression of an exoplanet system. Credit: ESA

Imagine a gaseous exoplanet five times the size of Jupiter but much closer to its star as Mercury is to our Sun. This planet orbits its star in just a couple of days and always shows the same face towards it. Now, imagine a tiny rocky planet, only a third of the size of Earth orbiting its star in only 4.5 hours. These kinds of worlds really exist. Exoplanets vary in size, orbit, composition, and more. But how can we know all these aspects?

Different characterization techniques have been developed and adopted by the European Space Agency’s (ESA) exoplanet missions. In this endeavor, thousands of exoplanets will be subject to a study of mass, size, density, composition, and age. Below you can find explanations of the various characterization techniques.

Transiting exoplanets are detected as they pass in front of – transit – their host star, causing a dip in the starlight as seen from the observer’s viewpoint. The transit repeats, with the time interval depending on the time it takes the exoplanet to orbit its star. For example, an observer of our own Solar System would have to wait a year to see a repeat of Earth transiting the Sun. Credit: ESA

Size – transit method

The transit method provides a way to learn about exoplanets. When a planet passes in front of their star (from the point of view of the observer), it causes some of the starlight to be blocked. The observer temporarily receives less light from the star. ESA’s mission Cheops will look at planets known to transit and decipher their size. Plato will look for new, unknown exoplanets using the transit method. The bigger the planet, the deeper the dip in star brightness it causes. Cheops focuses on planets with sizes between that of Earth and Neptune and shorter orbital periods (<50 days). The orbital period is the time it takes for a planet to complete one orbit around their host star. For transiting exoplanets, this orbital period is easy to determine as it is just the time between two consecutive dips in light. Plato will be able to observe Earth-sized planets with longer orbital periods (>90 days).

Exoplanets can be detected by measuring the ‘wobble’ in its star’s motion caused by the gravitational pull of a planet as the planet and star orbit around a common centre of mass. When viewed from afar, the star appears to move towards and away from the observer. This motion makes the light from the star appear slightly bluer when it is moving towards the observer, and slightly redder when moving away. This shift in frequency is known as the Doppler effect, the same effect as the change in pitch of an ambulance siren as it rushes past you. Most early exoplanet discoveries were made using this so-called radial velocity method.
Credit: ESA

Mass – radial velocity and transit time variations

The mass is a fundamental characteristic of an exoplanet. Scientists can learn a lot about how planets form around their stars by comparing how massive different exoplanets are. Two methods can provide us with information on the mass of exoplanets. One is the radial velocity method, used by several ground-based observatories. When a star has a planet, the system moves around a common point, called the mass center. During this orbit, the stars’ light appears bluer when it moves towards the observer and redder when it moves away. This shift in frequency is known as the Doppler effect, the same effect as the change in pitch of an ambulance siren as it rushes past you. By measuring the shift in light coming from the star, it is possible to determine the velocity – the speed and direction – by which the star moves around the center of mass. The velocity is directly correlated with the mass of the planet.

A variation on the transit technique to detect exoplanets – known as transit timing variation (TTV) – can also be used to find additional planets in a system. By measuring tiny variations in the timing of the transit of a known planet, astronomers can reveal the presence of potential other planets orbiting the same parent star. Credit: ESA, CC BY-SA 3.0 IGO

Another method that provides us with information on the mass of exoplanets is that of Transit Time Variations or TTV for short. This method works like the transit method for a planetary system with multiple planets. Normally the time between transits of the same planet is not expected to vary. When a planet is seen crossing the face of its star earlier or later than expected, the system likely has another planet gravitationally tugging or pushing on its neighbor. This method has already led to the discovery of more than 40 exoplanets. What is interesting, is that the time difference between the transits also reveals information on the masses of the planets. This technique is used by ESA’s missions Cheops and Plato.

Density – combine two methods

Once both the mass and size are known, the density of the exoplanet can be determined. This is essential information as it can reveal its nature: is the exoplanet rocky like Earth and Mars or of a gaseous nature such as Saturn and Jupiter? Studying the diversity in exoplanets can shine a light on the formation of planetary systems.

Spectroscopy is the technique of splitting received starlight into its different colors using a prism. Exoplanets orbit their stars, when they transit – pass by from our point of view – some of the starlight passes through the planet’s atmosphere. Particles in the atmosphere like water vapor, carbon dioxide, methane and others absorb some of that light. This absorption happens at specific wavelengths of light. By studying at which wavelengths the starlight is absorbed, we can determine what kind of particles are present in the atmosphere. The NASA/ESA/CSA James Webb Space Telescope uses this technique to characterize exoplanets and ESA’s Ariel mission will study the atmospheres of as many as 1000 exoplanets this way. Both missions focus on infrared light because the signatures of molecules are very prominent in those colors. Credit: ESA, CC BY-SA 3.0 IGO

Composition of atmosphere – transmission spectroscopy

With spectroscopy we can study what the atmospheres of exoplanets are made of. Spectroscopy is the technique of splitting received starlight into its different colors using a prism. During a transit of a planet some of the starlight passes through the planet’s atmosphere. Particles in the atmosphere like water vapor, carbon dioxide, methane, and others absorb some of that light. This absorption happens at specific wavelengths of light. By studying at which wavelengths the starlight is absorbed, we can determine what kind of particles are present in the atmosphere. Tracking changes in the atmosphere over time gives insight in processes occurring on the surface of these exoplanets. The NASA/ESA/CSA James Webb Space Telescope uses this technique to characterize exoplanets and ESA’s Ariel mission will study the atmospheres of as many as 1000 exoplanets this way.

The phase curve method to study extrasolar planets, or exoplanets. Depending on a planet’s position with respect to its host star, the total light collected by a telescope will include a varying fraction of light reflected off the planet, in a similar manner to how we experience the phases of the Moon.
The planet reflects no light during a phase known as secondary eclipse, when it is hidden from view, whereas it reflects some light shortly before and after this phase. In addition to that, the planet blocks a fraction of the light as it transits in front of the star.
The changes in starlight reflected by the planet as it orbits its star provide insight into the physical processes that drive the transport of heat from the hot day side to the cooler night side. Analysis of the phase curves also reveals details of the planet’s atmosphere, including the presence of clouds, and possibly even hints of the cloud composition.
Credit: ESA, CC BY-SA 3.0 IGO

Clouds and surface – phase curve

When a planet orbits its star, it reflects starlight just like our Moon does with sunlight. In the same way, exoplanets have different ‘phases’ where different fractions of their surface reflect light. By studying the small differences in received sunlight during the planet’s orbit, it is possible to determine how reflective the planet’s surface is. Interestingly, also the presence of clouds in the exoplanets’ atmosphere can be revealed this way. The missions Cheops and Plato, and also Ariel (in parallel to their transit spectroscopy, see above) will use this technique to reveal the surface colors of exoplanets.

Direct imaging relies on measuring light from the exoplanet itself. This is particularly challenging at optical wavelengths, because the relatively dim planet can be lost in the glare of the much brighter host star.
Credit: ESA

Structure of exoplanetary systems – direct imaging

The above methods gave various characteristics of individual exoplanets. If we want to learn more about exoplanetary systems as a whole, we can make a direct image of the system. Taking a picture of planets is hard because the light of stars outshines that of their planets. You need a very high-resolution camera or a way to block out the bright starlight and not all space missions are equipped for the task. The Hubble and James Webb space telescopes have the needed resolution and have been able to make images of planets around stars other than our Sun. With this technique, we can also learn about the planets’ orbital periods and the distances to their stars. The new space telescope Roman will be able to image Jupiter-sized planets on orbits similar to Jupiter around the Sun. The telescope will do this by blocking the light of the star and image the fainter light of the planets around it.