The field of exoplanet science is relatively young, but is evolving and growing very quickly. In recent years, exoplanet astronomers have made a number of amazing discoveries. Here are just a few notable milestones.
The very first exoplanet ever confirmed around a Sun-like star was 51 Pegasi b. It was discovered in 1995 using the velocimetry method. 51 Pegasi b is a planet unlike anything in our own Solar System. It is a gas giant planet that orbits its star in only 4 days. It is 20 times closer to its star than Earth is to our Sun and is thus over 1000 degrees Celsius!
Due to its size and temperature, 51 Pegasi b is a type of exoplanet called a Hot Jupiter. Discovering this planet taught astronomers that we should not assume that exoplanets will look like the Solar System’s planets. Since 1995, we have discovered many more Hot Jupiters.
Most exoplanets are detected and studied using indirect methods. This means that we can typically only detect an exoplanet by studying its effects on its central star. But with the right conditions, it is possible to block out the star’s light and directly observe an exoplanet!
The first exoplanet system ever directly imaged was HR 8799 in 2008 using the Gemini-North Observatory. This incredible feat was performed by Canadian astronomers Christian Marois (NRC-Herzberg), René Doyon (iREx/UdeM), and David Lafrenière (iREx/UdeM). They used a special instrument called a coronograph and developed innovative data processing techniques. The four exoplanets in the system are all gas giants, with masses ranging from 7 to 10 times that of Jupiter. They are also very far from their star, with orbital periods ranging from 45 to 460 years.
Direct imaging is still a very challenging technique, with only a few dozen systems ever directly imaged. A suite of new instruments and telescopes coming online in the next decade or so should greatly improve this field.
In the early days of exoplanet science, nearly all discoveries were being made using the velocimetry method. This was proving successful, but worked best for bright stars and required a lot of telescope time. Scientists soon realised that the transit method may be more efficient, since a telescope could then monitor many stars at once.
Using the transit method, the Kepler Space Telescope surveyed hundreds of thousands of stars to discover how common exoplanets are. During its operations from 2009 to 2018, Kepler discovered over 2600 new exoplanets! Thanks to Kepler’s contribution, astronomers could finally perform statistical studies on large numbers of exoplanets. We started to learn what kinds of planets are out there, and how common each kind is.
The TRAPPIST-1 planetary system, discovered in 2016, contains seven rocky exoplanets, three or four of which might be in the habitable zone. This is the region around a star where water could potentially exist in a liquid state. The habitable zone is a key component of our search for extraterrestrial life.
The TRAPPIST-1 planets orbit a star much fainter, redder, and cooler than our Sun called an ultra-cool red dwarf. Because red dwarfs emit much less energy than our Sun, planets can orbit much closer to them without getting too hot. These stars’ habitable zones are thus very close in compared to larger and hotter stars. The TRAPPIST-1 system is very compact: it can fit entirely within the orbit of Mercury!
Proxima Centauri, a faint red dwarf star located 4.2 light years away, is our closest neighbouring star (other than the Sun). Amazingly, astronomers discovered an exoplanet around this star in 2016 using the velocimetry method. This exoplanet, Proxima Centauri b, appears to be a rocky Earth-like planet in the habitable zone. Further studies revealed that there is a second larger planet on a very wide orbit, and, possibly, a tiny third planet inside Planet b’s orbit.
Despite being our closest neighbour, it would still take our fastest current probes thousands of years to reach Proxima Centauri. Certain projects, such as Breakthrough Starshot, are looking into ways to take a closer look at the system. This includes sending a small computer chip on a giant solar sail going about 10% the speed of light. Even then, it would take 40 years for the ship to reach the star.
The exoplanet K2-18 b was discovered in 2015 orbiting a red dwarf star located 124 light years away. In 2019, a team of astronomers led by Björn Benneke (iREx/UdeM) detected water vapor in K2-18 b’s atmosphere. This was the first time water was ever detected on a exoplanet found in the habitable zone. The team also found that clouds and an entire water cycle might exist in K2-18 b’s atmosphere.
Astronomers are still not quite sure what category of exoplanet to classify K2-18 b under. It could be a super-Earth or a mini-Neptune. The discovery of water on this world does not guarantee it harbors life or is habitable. It does represent, however, a huge step in better understanding planet formation and the search for extraterrestrial life.