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Starts With A Bang

Not Only Didn’t We Find Water On An Earth-Like Exoplanet, But We Can’t With Current Technology

It’s a great scientific opportunity of modern times, but we won’t get there with our current observatories.

One of the holy grails of modern science is to find a world, beyond Earth, with life on it.

Most of the planets we know of that are comparable to Earth in size have been found around cooler, smaller stars than the Sun. This makes sense with the limits of our instruments; these systems have larger planet-to-star size ratios than our Earth does with respect to the Sun. (NASA / AMES / JPL-CALTECH)

Perhaps the most exciting possibility is to discover a rocky exoplanet with liquid water on its surface and biosignatures in its atmosphere.

The ideal ‘Earth 2.0’ will be an Earth-sized, Earth-mass planet at a similar Earth-Sun distance from a star that’s very much like our own. We have yet to find such a world, but even if we do, we must take care that we distinguish between what we think of as biosignatures, like oxygen, produced by life versus that produced by inorganic processes. Many advances are required to reach that stage. (NASA AMES/JPL-CALTECH/T. PYLE)

Over the past few decades, astronomers have uncovered thousands of new exoplanets.

Today, we know of over 4,000 confirmed exoplanets, with more than 2,500 of those found in the Kepler data. These planets range in size from larger than Jupiter to smaller than Earth. Yet because of the limitations on the size of Kepler and the duration of the mission, the majority of planets are very hot and close to their star, at small angular separations. TESS has the same issue with the first planets it’s discovering: they’re preferentially hot and in close orbits. Only through dedicates, long-period observations (or direct imaging) will we be able to detect planets with longer period (i.e., multi-year) orbits. (NASA/AMES RESEARCH CENTER/JESSIE DOTSON AND WENDY STENZEL; MISSING EARTH-LIKE WORLDS BY E. SIEGEL)

Some of them are rocky; some are temperate; some have water.

This artist’s impression displays the star TRAPPIST-1, located approximately 40 light-years away, and its planets reflected in a surface. The potential for water on each of the worlds is also represented by the frost, water pools, and steam surrounding the scene. However, it is unknown whether any of these worlds actually still possess atmospheres, or if they’ve been blown away by their parent star. One thing is certain, however: we won’t know whether they’re inhabited or not unless we examine their properties in depth for ourselves, and that requires observatories beyond what we presently have at our disposal. (NASA/R. HURT/T. PYLE)

However, the idea that exoplanet K2–18b is rocky, Earth-like, and has liquid water is absurd, despite recent headlines.

The atmosphere of the exoplanet WASP-33b was examined as starlight filtered through the planet’s atmosphere before arriving at our eyes. Similar techniques could work for other exoplanets as well, but to image the atmosphere of Earth-sized planets, as opposed to Jupiter-sized WASP-33b, we require observatories that are larger and more advanced than the ones we have today. (NASA / GODDARD)

Light filters through K2–18b’s atmosphere when it passes in front of its star, enabling us to measure what’s absorbed.

When a planet transits in front of its parent star, some of the light is not only blocked, but if an atmosphere is present, filters through it, creating absorption or emission lines that a sophisticated-enough observatory could detect. If there are organic molecules or large amounts of molecular oxygen, we might be able to find that, too. at some point in the future. It’s important that we consider not only the signatures of life we know of, but of possible life that we don’t find here on Earth. (ESA / DAVID SING)

Based on those absorption lines, the presence of many chemicals can be inferred, including water.

One of the two teams that studied exoplanet K2–18b, which was discovered by Kepler’s K2 mission, was able to extract a water signal from the transit data. However, it is water vapor, not liquid water, and only under some (untested) atmospheric scenarios is liquid water on this world even a possibility. (B. BENNEKE ET AL. (2019), ARXIV:1989.04642)

K2–18b is, truly, the first known habitable-zone exoplanet to contain water.

The red dwarf star, K2–18, is located 110 light-years away in the constellation of Leo. There is a planet orbiting in its habitable zone (K2–18b), where temperatures are expected to be between 0 and 40 Celsius (32 and 104 Fahrenheit), but the planet is more than twice the radius of Earth and more than eight times Earth’s mass; it cannot be rocky. (MR. BILLION / WIKIMEDIA COMMONS; STELLARIUM)

However, it is not rocky; its mass and radius are too large, necessitating a large gas envelope around it.

The classification scheme of planets as either rocky, Neptune-like, Jupiter-like or stellar-like. The border between Earth-like and Neptune-like is murky, occurring at approximately 1.1-to-1.5 Earth radii. Direct imaging of candidate super-Earth worlds, which might be possible with the James Webb Space Telescope, should enable us to determine whether there’s a gas envelope around each planet in question or not. Note that there are four main classifications of ‘world’ here, and that the cutoff between rocky planets and those with a gas envelope occurs well-below the sizes of any planet whose atmosphere we’ve measured as of 2019. (CHEN AND KIPPING, 2016, VIA HTTPS://ARXIV.ORG/PDF/1603.08614V2.PDF)

If its atmosphere were like Earth’s, it would be undetectable by current instruments.

Both reflected sunlight on a planet and absorbed sunlight filtered through an atmosphere are two techniques humanity is presently developing to measure the atmospheric content and surface properties of distant worlds. In the future, this technique, which only works for certain molecular signatures around worlds larger than Earth, could be extended to include Earth-sized worlds and the search for organic signatures as well. (MELMAK / PIXABAY)

It’s a mini-Neptune: interesting, but not the habitable exoplanet we’re seeking.

Although many of the Earth-like candidates from Kepler are close to Earth in physical size, they may be more like Neptune than Earth if they have a thick H/He envelope around them. Additionally, they predominantly orbit dwarf stars, meaning it may be difficult for them to have atmospheres. K2–18b definitely has an atmosphere, but it’s much more “super” than is possible for a rocky planet. (NASA AMES / N. BATALHA AND W. STENZEL)

For that, we need new, larger, more sophisticated observatories.

This is an illustration of the different elements in NASA’s exoplanet program, including ground-based observatories, like the W. M. Keck Observatory, and space-based observatories, like Hubble, Spitzer, Kepler, Transiting Exoplanet Survey Satellite, James Webb Space Telescope, Wide Field Infrared Survey Telescope and future missions. The power of TESS and James Webb combined will reveal the most Moon-like exomoons to date, possibly even in their star’s habitable zone, while ground-based 30 meter telescopes, WFIRST, and possibly a next-generation space-based observatory like LUVOIR or HabEx is required to truly find what humanity has been dreaming of for so long: an inhabited world outside of our Solar System. (NASA)

Unless we build them, we’ll never find the Earth-like worlds we dream about.

The Starshade concept could enable direct exoplanet imaging as early as the 2020s. This concept drawing illustrates a telescope using a star shade, enabling us to image the planets that orbit a star while blocking the star’s light to better than one part in 10 billion. (NASA AND NORTHROP GRUMMAN)

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.


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