Coming soon: A revolution in the study of exoplanets!
centuries, philosophers and scholars have wondered if the stars in the night sky might have their own planets. And yet, it has only been in the past few decades that astronomers have been able to find any extrasolar planets (aka. exoplanets). In fact, it wasn’t until 1992 that the first two exoplanets were officially confirmed orbiting a pulsar (PSR B1257+12) 2,300 light-years away.
Since then, thousands of exoplanets have been detected and confirmed, most of them in just the past decade. All of this is due to a combination of technological advancements, refinements in exoplanet-hunting methods, and improved coordination between observatories worldwide.
In the coming years, we can expect many more discoveries as new observatories become operational, space telescopes are launched, and machine learning and citizen scientists get better at sorting through all the data provided. As always, the ultimate goal is to find planets that are habitable, and maybe even inhabited!
So far, most exoplanets have been discovered using indirect means. Among them, the most popular and effective is the Transit Method (aka. Transit Photometry). This consists of observing stars for periodic dips in brightness, which can be the result of a planet passing in front of the star (aka. transiting) relative to the observer.
These dips are used to determine the size of the transiting planet, as well as its orbital period. On rare occasions, light passing through the exoplanet’s atmosphere will provide astronomers with spectra, which can be used to tell what gases the atmosphere is composed of.
The second most popular means is the Radial Velocity Method (aka. Doppler Spectroscopy). This method consists of measuring the way a star moves back and forth relative to Earth, which indicates that there are gravitational forces acting on it - in other words, one or more planets in orbit. There’s also Gravitational Microlensing, which takes advantage of the effects predicted by Einstein’s Theory of General Relativity.
In essence, the gravity of a star will alter the curvature of spacetime around it, which is then used as a “gravitational lens” to enhance and magnify the light of a background star. Over time, distortions may occur that could be the result of planets passing through the lens.
Then there’s Direct Imaging. For this method, astronomers detect exoplanets by looking for light reflected by their atmospheres or surfaces. The reason why this method is so prized is because this light can also be used to obtain spectra on a planet’s atmosphere and/or determine what its surface might look like (i.e. landmasses, oceans, vegetation, etc.).
Unfortunately, this technique is used only in rare cases because of the limits of our current telescopes. Most of the time, any light reflected by an exoplanet will be easily drowned out by the much brighter light coming from its star. Hence, Direct Imaging has only been possible with massive planets (like gas giants) that have wide orbits.
Meanwhile, rocky planets that orbit closer to their stars (like Earth) cannot be seen because of all the light interference. Since it is these planets that astronomers and astrobiologists expect to be the most habitable, being forced to study them indirectly is quite limiting. However, that’s all likely to change in the near future.
In 2021, after multiple delays, the James Webb Space Telescope (JWST) will finally be launched. In terms of exoplanet studies, the JWST will rely on nextgeneration infrared optics to conduct follow-up studies of confirmed planets. This will allow it to observe smaller planets that orbit closer to their stars and detect chemical signatures in their atmospheres.
By the mid-2020s, the JWST will be joined by the successor to the Hubble Space Telescope. Known as the Nancy Grace Roman Space Telescope (formerly the WFIRST), this observatory will have 100 times the resolution of its predecessor. Combined with timeseries microlensing of the Milky Way’s central region (the “bulge”), Roman will be able to spot exoplanets the size of Mercury or Mars (“sub-Earths”).
Both observatories will also be equipped with coronagraphs, a special instrument that blocks the direct light from a star so that nearby objects – like a system of planets – are visible. NASA also has plans for a special class of “qwlter” spaceship that will act as a coronagraph for telescopes that don’t have their own.
As part of the proposed New Worlds Mission (NWM) project, these plans call for a spacecraft equipped with a large “starshade” to fly ahead of a space telescope. Once in position, the NWM spacecraft will deploy its shade - a flower-shaped structure – to block out the light coming from a specific star so the space telescopes can get a better look at its exoplanets.
There are also some powerful ground-based telescopes that will be operational by the 2020s. In 2025, the ESO’s Extremely Large Telescope (ELT) will gather light for the first time while the Thirty Meter Telescope (TMT) and Giant Magellan Telescope (GMT) will commence observations by the mid-2020s and 2029, respectively.
These telescopes have their own coronagraphs but will also employ what is known as Adaptive Optics (AO). This technique involves using computercontrolled deformable mirrors to correct for distortions caused by the Earth’s atmosphere in real-time.
“By the end of the decade, astronomers could be making direct observations of Earth-like exoplanets on a regular basis.”
Equipped with these instruments, astronomers will have the resolution and sensitivity they need to directly image thousands of exoplanets and (more importantly) characterize them.
From Discovery to Characterization
In recent years, the focus of astronomers has slowly shifted away from finding exoplanets (i.e. discovery) to conducting follow-up studies to see if they can support life (i.e. characterization). This trend is expected to pick up speed very soon thanks to the aforementioned telescopes and more sophisticated data-mining techniques.
By the end of the decade, astronomers could be making direct observations of Earth-like exoplanets on a regular basis. These include Proxima b, Ross 128 b, Teegarden’s Star b, 82 G. Eridani e, and the seven planets of TRAPPIST-1 – potentially-habitable worlds that are within 40 light-years of Earth.
The data we glean from observing these exoplanets will go a long way towards helping us decide which we should be sending probes to someday.
Who knows? Maybe we’ll even detect biosignatures on some of these planets (fingers crossed!); in the process, solving the mystery of whether or not we’re alone in the Universe at last!
STSCI science themes
NASA: Ways to find a planet
ESO: Adaptive optics technology
SETI: Future space telescopes
Universe Today: How the next generation of ground-based super telescopes will directly observe exoplanets