The fundamentals of astronomy for beginners
EXPLAINER
Exoplanets – 30 years of discovery
Penny Wozniakiewicz looks at advances in the field since the first worlds were announced
On 9 January 1992, astronomers Aleksander Wolszczan and Dale Frail introduced the world to the first two planets to be found outside the Solar System, alien worlds observed orbiting the pulsar PSR B1257+12, around 2,300 lightyears away. The hunt for ‘exoplanets’ – as they are also known – was then, as now, heavily driven by our quest to find out just how unique (or not) our Solar System is, and whether there is life beyond it.
Given the latter aim, finding exoplanets orbiting a pulsar – effectively a dead (no longer burning) star – is far from ideal. Rather fittingly, a competition run by the International Astronomical Union (IAU) resulted in the pulsar and its planets being named after various macabre characters from mythology and popular culture: Lich for the star, and Draugr, Poltergeist and Phobetor for its worlds (the third was discovered two years later). Nevertheless, the discovery spurred interest in strange alien worlds among the public and the scientific community, and now, after three decades of searching, some 4,500 confirmed exoplanets have been identified.
So many planets to choose from
And strange worlds they are indeed – the sheer variety observed among these exoplanets is vast. To date, they range from rocky terrestrial planets similar in size to Earth, to gas giants much larger than Jupiter; from extremely hot to exceedingly cold, with orbits taking them insanely close or excessively far from their stars. They have also been found around all sorts of stars – small, large, young, old and dead.
became the first to be directly imaged near its host star,
the brown dwarf 2M1207 (centre)
Some even orbit multiple star systems, while so-called ‘rogue planets’ do not even orbit a star.
Both ground and space-based telescopes have been instrumental in exoplanet discoveries, perhaps most notably the Kepler Space Telescope, which is credited with more than 2,600 confirmed finds. Although it is possible to directly image some exoplanets (particularly those that are bright, massive and orbit at large distances from their star), the vast majority of exoplanets have been identified through indirect methods – the planets are found by studying the effect they have on their stars.
For example, the ‘radial velocity’ and ‘astrometric methods’ both search for evidence of wobbles in the motions of stars caused by orbiting planets, while the ‘transit method’ studies the dip in the brightness of stars caused as planets pass in front of them.
Combined, these studies allow us to work out details of each planet’s orbit and size, and then determine whether it is a rocky terrestrial planet or a gas or ice giant, and whether it orbits within the star’s habitable zone. This is the region around a star where temperatures may permit the existence of liquid water on a planet’s surface – and from what we know about life, liquid water is an essential ingredient.
But finding a planet like Earth in the habitable zone of its star does not mean liquid water is present or that it will be habitable – key to the ability to host liquid water is the presence of an atmosphere to maintain sufficient surface pressure. Atmospheres also provide a means to explore the conditions present on these planets, as their compositions may reveal details of surface processes and even potentially the presence of life.
On the trail of exoplanets
Studies of exoplanet atmospheres are performed using spectroscopy. For a transiting exoplanet this involves measuring the intensity of the star’s light at different wavelengths as the planet passes in front. Gaps in the spectrum result from absorption by elements or molecules present in its atmosphere.
Several different atmospheric constituents have been identified in exoplanet atmospheres, including water vapour, carbon dioxide and methane, and scientists have even interpreted details such as the presence of clouds, rain and extremely high-speed winds on some.
Current studies estimate that trillions of planets could exist in our Galaxy, so it’s safe to say that we have barely touched the surface when it comes to finding and learning about exoplanets. As new telescopes come online from now and in forthcoming years, such as the long-awaited James Webb Space Telescope (JWST), the Nancy Grace Roman Space Telescope (2027) and the Atmospheric Remotesensing Infrared Exoplanet Large-survey (2028), they will surely identify more exoplanets and investigate their atmospheres – perhaps one day finding that exoplanet which is truly similar to Earth.
Exoplanets, great and small
The search for exoplanets has revealed a previously unimaginable variety. We reveal some notable examples:
WASP-12b is one of the largest gas giants observed, twice Jupiter’s size. Discovered in 2008, it orbits so close to its star that it’s being torn apart!
Mysterium Cosmographicum TOI-849b is an extremely dense rocky planet lacking atmosphere, which was discovered in 2015. It is thought to be the core of a gas giant.
Proxima Centauri b is our nearest neighbour at 4.22 lightyears away. Spotted in 2016, it’s a bit bigger than Earth and orbits a small dwarf star.
Kepler 452b is a contender for the title of ‘most Earth-like planet’, Kepler 452b (discovered in 2015) orbits a similar star to our Sun at approximately the same distance in about the same amount of time.
The Kepler-90 system is the largest system of planets outside our own Solar System, thanks to the discovery of an eighth planet around this star in 2017.
Dr Penny Wozniakiewicz is a planetary scientist and space dust expert based at the University of Kent