You could say that the study of extrasolar planets is in a phase of transition of late. To date, 4,525 exoplanets have been confirmed in 3,357 systems, with another 7,761 candidates awaiting confirmation. As a result, exoplanet studies have been moving away from the discovery process and towards characterization, where follow-up observations of exoplanets are conducted to learn more about their atmospheres and environments.

In the process, exoplanet researchers hope to see if any of these planets possess the necessary ingredients for life as we know it. Recently, a pair of researchers from Northern Arizona University, with support from the NASA Astrobiology Institute’s Virtual Planetary Laboratory (VPL), developed a technique for finding oceans on exoplanets. The ability to find water on other planets, a key ingredient in life on Earth, will go a long way towards finding extraterrestrial life.

The research was conducted by postdoctoral researcher Dominick J. Ryan, a postdoctoral researcher at Northern Arizona University (NAU), and Tyler D. Robinson – an Assistant Professor of Astronomy and Planetary Science at NAU and the NASA Astrobiology Institute. The study that described their findings, titled “Detecting Oceans on Exoplanets with Phase-Dependent Spectral Principal Component Analysis,” recently appeared online and is being considered for publication by The Planetary Science Journal.

An artist's illustration of the exoplanet HR8799e. The ESO's GRAVITY instrument on its Very Large Telescope Interferometer made the first direct optical observation of this planet and its atmosphere. Image Credit: ESO/L. Calçada
An artist’s illustration of the exoplanet HR8799e. The ESO’s GRAVITY instrument on its Very Large Telescope Interferometer made the first direct optical observation of this planet and its atmosphere. Credit: ESO/L. Calçada

When it comes to exoplanet characterization, the most promising technique is the Transit Method (aka. Transit Photometry). This consists of monitoring stars for periodic dips in brightness, which are indications of planets passing in front of their parent stars (relative to the observer). At times, astronomers are also able to obtain spectra as light passes through the transiting planet’s atmosphere, revealing things about its chemical composition. But as Prof. Robinson told Universe Today via email, this method doesn’t allow for surface observations:

“For now, our best techniques for characterizing rocky exoplanets do not tell us much about the surface environments for these worlds (including whether liquid water is present). For Hubble (and the soon-to-launch JWST), we use transit spectroscopy to characterize the atmospheres of exoplanets – looking for very slight changes in the brightness and color of a host star when a planet traverses its disk. In this geometry/setup, the very long paths the light takes through the atmosphere (most analogous to viewing the Sun at sunset on Earth) means that the deep atmosphere (and surface) is obscured.”

In the near future, this situation is expected to change considerably, thanks to next-generation instruments like the James Webb Space Telescope (JWST), and ground-based observatories like the Extremely Large Telescope (ELT). Thanks to their sophisticated optics, coronographs, and spectrometers, these telescopes will be able to directly image smaller exoplanets that orbit more closely to their stars (which is where potentially habitable rocky planets are more likely to be found).

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