- Title: The Next Great Exoplanet Hunt
- Authors: Kevin Heng and Joshua Winn
- First Author’s Institution: University of Bern
- Published in American Scientist
How do you answer when the theorist frowns and says: “Exoplanetary science isn’t fundamental, it’s just applied physics—no offense, of course”. Your reply might be: “None taken Dr. X, yes, we are not expecting to gain insights into grand unified theories by hunting for exoplanets, but the stakes are nevertheless high. We are on the verge of the Copernican revolution all over again: we could find signs of life out there, however small, removing humanity from the center of the biological Universe. Are you with us?”
Indeed, the stakes are high, and the hunt is on. For the last three decades there has been a rush of activity to find exoplanets. We have found many. The most successful method to date is the transit method. Kepler, the most successful transit mission to date, has confirmed the existence of over 1000 exoplanets, two thirds of currently known planets.
According to Heng & Winn, the authors of today’s paper, the long-term strategy of exoplanet hunting is clear. First you find them. Second, you characterize them. Third, you search for biomarkers in their atmospheres. The first two steps both have a number of maturing methods, but we are still finding our footing with the third step. In this paper Heng & Winn largely focus on transiting planets—planets whose atmospheres we can study for biomarkers. Why? Read on.
Space-based transits versus ground based?
Initially, exoplanet transits were studied by ground based observatories. They have problems: the Sun is periodically in the way (usually during the day), and Earth’s atmosphere interferes with the observations. These problems can be circumvented by launching telescopes into space, see figure below. In space, the precision is only limited by fundamental photon counting noise; we can’t ask for anything better. Granted, launching things into space is expensive, but is getting cheaper through help from the private sector. Notwithstanding, ground-based surveys will still most likely continue to give you the most bang for your buck, and will continue to play a strong complementary role to space based transit missions in the future.
Characterizing exoplanetary atmospheres
The atmospheres of transiting planets can be studied and analyzed for biomarkers via transit spectroscopy. There are two main ways. First, during a transit, some of the starlight can shine through the atmosphere of the planet. We can then look for atmospheric absorption features, by contrasting the observed spectrum while transiting, and while not. The second way is to study occultations. The planet itself reflects light from it’s host star, which contains information about its atmospheric structure. We can infer how much light is reflected by detecting the drop in brightness as the planet travels behind the star.
However, not all exoplanets are created equal for atmospheric characterization. This characterization is easiest for Hot Jupiters, big Jupiter-size planets that orbit close to their host star, which tend to have puffy atmospheres. Heng & Winn note a stark contrast between the impressive sounding 1500 confirmed exoplanets, and the relatively small number of exoplanets we can currently meaningfully characterize the atmospheres of: only about a dozen (take a look at Figure 2). Most of them are hot gas giants, flaming puffy planets significantly larger than the Earth: not habitable. We want to change that, and go for the gold: habitable planets.
Planned Transit-Hunting Machines: Finders, and Characterizers
According to Heng & Winn, the path to finding habitable planets is clear. We need to detect Earth sized planets around the nearest, and brightest stars. These are the systems that maximize the atmospheric signal-to-noise ratio. We could then proceed to search for biosignatures with enough statistics to robustly probe for signs of life. This, however, requires new telescopes to first find them, and then characterize them. What is planned?
First are the transit-finders. Kepler changed the game, but left most of the sky relatively unexplored. Its success has inspired a fleet of transit-hunting successors, space missions along with complimentary efforts on the ground. Like Heng & Winn, we will focus here on the space missions, some of which are contrasted in the figure below. NASA’s TESS mission is scheduled to be launched in 2017, and will scan the entire sky in a systematic manner, specializing in finding nearby short period planets. Conversely, the European CHEOPS mission, also scheduled to fly in 2017, focuses on studying transits, one star at a time. Later on, in 2024, the PLATO mission is the most ambitious of all. It will borrow hunting strategies from Kepler, TESS and CHEOPS, and aims to build a catalog of true Earth analogs: Earth-like exoplanets orbiting within the habitable zones of Sun-like stars.
Then there are the atmospheric-characterizers. These are the big telescopes that will focus on recording transmission spectra during planetary transits. This will largely fall into the hands of the much anticipated JWST space telescope, and the upcoming extremely large ground based telescopes. Ideally, before these expensive shared-time observatories come online, we would want to have compiled a list of our best candidates: our top 10 sexiest transiting planets. Let’s get cracking!