by Rahul Jayaraman '19
It’s hard to believe that only about twenty years ago, we knew of less than fifty planets outside our solar system (exoplanets), none of which were known to contain water or compounds suitable for life. Now, we know of more than three thousand exoplanets, and researchers are able to analyze these planets in much finer detail to determine their atmospheric content, using a whole host of new (and old) tools to do so.
One of the most popular methods to analyze exoplanets is through transit and eclipse spectroscopy. We train telescopes on a star system suspected to contain a planet for a fixed period of time, usually on the order of hours or days. As an exoplanet passes in front of its host star in a transit, the amount of light (flux) we detect from the star decreases. Likewise, as an exoplanet passes behind its host star (in an eclipse), the amount of flux we detect from the star decreases as well, but in a much smaller amount (~100-1000x less).
Transits are the most useful type of occultation to detect atmospheric composition. As the light from a star passes through the planet’s atmosphere, some of it gets absorbed by the compounds in the atmosphere, exciting these compounds and adding a noticeable defect in the transit light curve. By measuring the size and precise location of these defects at different wavelengths and comparing them to known chemical data, researchers can constrain the contents and relative abundances of elements in exoplanetary atmospheres.
While the TRAPPIST-1 discovery from last February might be the one that has been in the news most often, it is by far one of the less interesting planetary systems out there. Hot gaseous planets, referred to as “Hot Jupiters” or “Hot Saturns,” provide a much more interesting object of study, and this is where the most strides have been made recently. For instance, a team of researchers just discovered an exoplanet (WASP-39b) with a surprisingly large amount of water -- around three times as much as expected. Such a discovery has large implications for the future of astrobiology, as the development of methods to identify the presence of water may lead us to identifying planets that are suitable for life.
But why do exoplanet discoveries matter to the general public? Some view exoplanetary detection and characterization as an esoteric, useless area of astrophysics; however, this claim is far from true. (The author would like to indicate here that he is involved in exoplanet research.) Why do people obsess over exoplanet metallicity and atmospheric composition, when we have issues with our own atmosphere?
Simple -- to study the evolution of the Universe. Many of these planetary systems are much newer than the Solar System, and many exhibit patterns of a new type of planetary formation. The accepted model for Solar System formation is the accretion model, in which portions of the dust and gas cloud surrounding the infant Sun clumped into rocky and gaseous bodies, forming the eight (nine, in my heart) planets we know today. However, based upon the data researchers have gathered about hot gaseous exoplanets over the past decade, it seems as though other planetary systems did not follow this model. By identifying the composition of these exoplanets, researchers can identify whether their discoveries agree with certain known models of exoplanetary formation, or whether they’ll need to develop an entirely new model to explain their findings.
It’s mind-blowing that we can find out, to a high certainty, the atmospheric composition of planets that are hundreds of light years -- and it’s exciting to see what more we can discover in the coming years with the launch of the James Webb Space Telescope, NASA’s new high-tech telescope slated for launch in 2019.