In the coming decades, exoplanet exploration is likely to become a key goal for next-generation telescopes. Astronomers and astrobiologists envisage that detecting oceans on terrestrial exoplanets may be the key to success in our search for habitable worlds beyond the Solar System.
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Since the first exoplanet discovery in 1992, more than 4,500 exoplanets have been confirmed in several thousand individual star systems. Another nearly 8,000 exoplanet candidates are still awaiting confirmation. The first planets found orbiting other stars were detected by the so-called radial velocity method, where the astronomers observe the star wobbling back and forth. These subtle changes in the radial (or line-of-sight) velocity of the star are caused by the eccentric motion of the star, as both the star and the planet orbit their common center of mass.
How Do Astronomers Discover Exoplanets?
The radial velocity method was the primary exoplanet detection technique until the start of this century when scientists from the Harvard-Smithsonian Center for Astrophysics successfully observed a periodic dip in the host star's brightness arising from the transit of a planet across the star disk. Since then, the transit method was employed to discover the vast majority (more than 3,500) of the known exoplanets.
Most of these exoplanets were discovered by the Kepler Space Telescope launched in 2009. The advanced optical instruments onboard the spacecraft were able to discern tiny variations in the transit times that can reveal the presence of multiple planets in the system. When an exoplanet transits in front of the host star, Kepler's spectrographs analyze the starlight passing through the planet's upper atmosphere, receiving valuable information about its composition. Using this method, scientists have found various compounds, such as methane or water vapor, in the atmosphere of several exoplanets.
However, the transient method, even in conjunction with the most sophisticated space- and ground-based instruments, such as the Hubble Space Telescope, Kepler, the Transiting Exoplanet Survey Satellite (TESS), and the Very Large Telescope (VLT) in Chile's Atacama Desert, cannot characterize the surface environment of these worlds (including the presence of liquid water).
The situation may improve considerably with the launch of next-generation instruments such as the James Webb Space Telescope (JWST) and commissioning of the ground-based Extremely Large Telescope (ELT, a 39-meter aperture telescope based in the European South Observatory in Chile). With the next-generation optics used in their sophisticated coronagraphs and spectrometers, these telescopes will directly image smaller exoplanets that orbit closer to their host stars in the potentially habitable zone.
Next-Generation Instruments Shift Exoplanetary Exploration from Discovery to Characterization
The direct imaging method is still in its early stage as an exoplanet exploration tool with only 54 exoplanets discovered using this method so far. However, the rapid technological advancements promise it will eventually become a key to finding and characterizing exoplanets. As a result, exoplanet studies are expected to move away from simple planet discovery towards comprehensive characterization, where detailed observations of exoplanets would allow us to learn more about their surface environments.
Scientists are working on the development of future direct-imaging-capable instruments that could take detailed images of distant exoplanets. They are also developing novel analytical methods that would allow us to identify atmospheric patterns, oceans, and landmasses in these images.
Astronomers from Northern Arizona University, with support from NASA's Virtual Planetary Laboratory, recently developed a technique for finding oceans on exoplanets. By observing the subtle modulations in the planet's albedo (or reflectivity) as it rotates on its axis, the scientists aim to extract information about the planet's surface feature as they rotate in and out of view.
Modeling Earth to Find Hidden Alien Oceans
An important part of the research consisted of the development of a plausible model of how a distant Earth-like planet would appear when imaged with the next-generation telescopes.
Since Earth is our only example of a habitable world, the scientists used our planet as a reference in a series of simulations of the Earth's brightness at different wavelengths and illuminations. These simulations accounted for all of the realistic effects caused by the reflection of sunlight by surface water, such as ocean surface reflection and cloud scattering. In particular, the results showed that when Earth is viewed at near-crescent phases (only partially illuminated by its star), it does appear red and more reflective due to the specular reflection from a liquid ocean.
Applying such models when analyzing the astronomical data obtained from future space-based instruments such as the Habitable Exoplanet Observatory, a space telescope designed for direct imaging of Earth-like planets, and the Large UV/Optical/IR Surveyor, a multi-wavelength space observatory planned to launch in the next decade, would help to identify the crescent-phase reddening, signifying the presence of oceans. Finding water on other planets, a key ingredient of life on Earth, will be an important step towards finding extra-terrestrial life.
References and Further Reading
M. Williams (2021) A technique to find oceans on other worlds [Online] www.phys.org Available at: https://phys.org/news/2021-10-technique-oceans-worlds.html (Accessed on 16 November 2021).
Ryan, D. J., Robinson, T. D. (2021) Detecting Oceans on Exoplanets with Phase-Dependent Spectral Principal Component Analysis. Preprint arXiv:2109.11062. Available at: https://arxiv.org/abs/2109.11062v1
Bowens, R., et al. (2021) Exoplanets with ELT-METIS I: Estimating the direct imaging exoplanet yield around stars within 6.5 parsecs. Preprint arXiv:2107.06375. Available at: https://arxiv.org/abs/2107.06375v2
Howell, S. B. (2020) The Grand Challenges of Exoplanets. Front. Astron. Space Sci. 7, 10. Available at: https://doi.org/10.3389/fspas.2020.00010
M. Bown (2020) How the Very Large Telescope Uses Direct Imaging to Find Exoplanets [Online] www.teledyneimaging.com Available at: https://possibility.teledyneimaging.com/how-the-very-large-telescope-uses-direct-imaging-to-find-exoplanets (Accessed on 16 November 2021).