Since the first exoplanet was confirmed in 1992, over 4,000 planets have been discovered around other stars.
The Exoplanet Revolution
Observing planets orbiting other stars was until quite recently seen by astronomers as a fruitless endeavour. Planets are billions of times less bright than their parent star, and orbit within 1/10,000th of a degree on the sky for even the closest stars, rendering direct observation of exoplanets an extremely challenging endeavour. Fortunately, astronomers have developed a suite of indirect methods to detect exoplanets via the way they influence their host star.
An early successful method for detecting exoplanets is the radial velocity method. As a planet orbits a star, its gravitational pull causes the star to wobble back and forth. This motion causes absorption signatures in the star’s light to become redder, then bluer, due to the Doppler effect:
By measuring how far absorption lines shift, astronomers can use the laws of orbital mechanics to work out a lower limit on the mass of an exoplanet. This powerful technique led to the discovery of the first hot Jupiter in 1995 and super-Earths from 2005. However, while the radial velocity method lets us detect and weigh a planet, it doesn’t give information on the size of the planet.
Starting in 2000, astronomers demonstrated an independent method for detecting and studying exoplanets: the transit method. This technique involves watching the light from a star ‘dip’ as a planet passes in front of it:
By measuring the depth of the observed dip, astronomers can figure out the size of planet (bigger planets block more light than smaller planets). Since the launch of NASA’s Kepler space telescope in 2009, thousands of planets have been discovered via this method. As of 2018, the successor mission to Kepler, the Transiting Exoplanet Survey Satellite (TESS) has begun a full-sky search expected to find tens of thousands of new exoplanets over the next few years.
With the mass of a planet measured from its radial velocity, and the size measured from its transit, the planet’s density (=mass/volume) can be calculated. Comparing the density to that of gases, liquids, rocks, and metals, astronomers can then infer basic facts about what materials an exoplanet is made of. Density measurements are now revealing rocky planets in the habitable zone of nearby stars, such as the seven planets in the TRAPPIST-1 system 40 light years away.
However, density alone cannot tell us what these planets are really like (e.g. Earth and Venus have similar densities, but the later is a quite hostile place). To glimpse the true nature of exoplanets, we need to peer into their atmospheres.
One of the most powerful tools in the astronomer’s arsenal is spectroscopy - the splitting of light into its individual colours (wavelengths). On Earth, we all notice that the sky is blue even though sunlight is white, which is caused by molecules of air interacting with higher energy (blue) light more strongly than low energy (red) light and thus causing it to scatter in different directions. Similarly, different colours of light are treated differently by the gases making up exoplanet atmospheres, with some colours absorbed while others can pass through:
Astronomers have developed many clever ways to use spectroscopy to study what exoplanet atmospheres are made of. One of the most successful (and the focus of much of my research) is transmission spectroscopy. This involves watching an exoplanet transit in front of its star at many different wavelengths to probe how strongly star light is absorbed at each wavelength. When an atom or molecule strongly absorbs a given wavelength, the planet will appear to be slightly larger (as the atmosphere is opaque), while the planet will appear to be smaller at wavelengths where the atmosphere is transparent:
By measuring the amount of star light blocked by the planet as a function of wavelength, astronomers can then make a plot of the size of an exoplanet as a function of the wavelength - this is called a transmission spectrum. Wherever a bump appears in the spectrum, this tells us that something in the atmosphere is absorbing or scattering starlight, and hence stopping it reaching us:
Transmission spectra measured using visible light have already revealed atoms, such as sodium (Na) and potassium (K), while space-based infrared observations have revealed molecules like water (H2O) in many exoplanet atmospheres. Future telescopes, such as NASA’s James Webb Space Telescope (JWST) and ESA’s Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL) will use this technique to peer into atmospheres in unprecedented detail, revealing new molecules and insights into the composition of exoplanetary atmospheres.
For a detailed overview of the current state of the art in exoplanet atmosphere science, including other techniques used to study these exotic worlds, I have recorded the following video:
Using modern analysis tools, we can go beyond detecting gases in exoplanet atmospheres to measure how much of each chemical resides in the atmosphere. Extracting this detailed information from an observed spectrum is called atmospheric retrieval.
By measuring the quantity of each gas making up an atmosphere, we gain important insights into how these planets formed, the conditions in their atmospheres, and even their potential habitability. Atmospheric retrieval is a tricky endeavour though, as one needs to consider millions of potential combinations of gases, atmospheric temperatures, clouds, and other factors to figure out the composition of even one exoplanet atmosphere. You can find out more in this video:
I am the lead developer of POSEIDON - an efficient atmospheric retrieval code designed to extract the composition of exoplanet atmospheres from ground and space-based transmission spectra. This code has already been applied to multiple hot Jupiter exoplanets, revealing evidence of new chemistry and atmospheric phenomena. A summary of my research findings to date can be found on the next page:
Image and video credits: ESO / NASA Goddard / Ryan MacDonald