There are so Many Water-Worlds Out There

Ever since the first exoplanet was confirmed in 1992, astronomers have found thousands of worlds beyond our Solar System. With more and more discoveries happening all the time, the focus of exoplanet research has begun to slowly shift from exoplanet discovery to exoplanet characterization. Essentially, scientists are now looking to determine the composition of exoplanets to determine whether or not they could support life.

A key part of this process is figuring out how much water exists on exoplanets, which is essential to life as we know it. During a recent scientific conference, a team of scientists presented new research that indicates that water is likely to be a major component of those exoplanets which are between two to four times the size of Earth. These findings will have serious implications when it comes to the search for life beyond our Solar System.

The research was the subject of a presentation titled “Growth Model Interpretation of Planet Size Distribution“, which took place at the 2018 Goldschmidt Conference in Boston. During a session titled “The Role of Extreme Atmospheric Escape from Hot Exoplanets“, the team presented findings that indicated that water worlds may be more common than previously thought.

These findings were based on data from the Kepler Space Telescope and Gaia mission, which were analyzed by an international team of scientists led by Dr Li Zeng – a researcher from Harvard University’s Department of Earth and Planetary Sciences. As they indicated, the Kepler mission has precisely measured the radii of over 4000 exoplanet candidates, along with their orbital periods and other parameters.

These exoplanet candidates can be divided into two size categories: those that have 1.5 times the radius of Earth, and those that average around 2.5 Earth radii. Combined with mass and recent radius measurements from the Gaia mission, the team was able to develop a model of these planets’ internal structure. Whereas planets that fall into the former category are believed to be rocky, the latter ones are generally thought to range from super-Earths to Neptune-sized gas giants.

However, according to the model developed by Li and his colleagues, many of the confirmed exoplanets that are between two to four times the size of Earth may in fact be water worlds. On these planets, roughly 50% of the mass consists of water, whereas water makes up just 0.2% of the Earth’s mass. As Dr. Zeng explained during the course of the presentation:

“It was a huge surprise to realize that there must be so many water-worlds… We have looked at how mass relates to radius, and developed a model which might explain the relationship. The model indicates that those exoplanets which have a radius of around x1.5 Earth radius tend to be rocky planets (of typically x5 the mass of the Earth), while those with a radius of x2.5 Earth radius (with a mass around x10 that of the Earth) are probably water worlds”.

However, when one considers the orbital characteristics of these planets (i.e. how closely they orbit to their respective stars), a very interesting picture begins to emerge. As Li explained, these “water worlds” are not so much rocky planets that are covered in deep oceans, but an entirely new type of planet for which there is no equivalent in the Solar System.

“This is water, but not as commonly found here on Earth. Their surface temperature is expected to be in the 200 to 500 degree Celsius range,” he said. “Their surface may be shrouded in a water-vapor-dominated atmosphere, with a liquid water layer underneath. Moving deeper, one would expect to find this water transforms into high-pressure ices before we reaching the solid rocky core. The beauty of the model is that it explains just how composition relates to the known facts about these planets.”

Perhaps even more surprising was just how common these planets appear to be. According to their study, Li and his colleagues indicated that about 35% of all known exoplanets that are larger than Earth should be be water-rich. What’s more, they hypothesize that they likely formed in a way that is similar to how the cores of gas giants are believed to have formed – a rocky core surrounded by layers of volatile material made solid by pressure.

Naturally, this discovery has significant implications for when it comes to the search for life beyond our Solar System. Until now, the idea that water is essential to life as we know it has been a foregone conclusion to scientists. But if this study is correct, then it appears that water is far more plentiful on exoplanets than previously though, and could be an impediment to life as we know it.

If indeed water worlds consist of hot, steamy atmospheres, and dense ice layers closer to their cores, then it would be difficult for life to emerge on these worlds. Basically, the extreme heat and the a lack of access to sufficient sunlight, hydrothermal activity and land masses would make for a pretty hostile environment. Nevertheless, the study does offer some intriguing possibilities when it comes to characterizing exoplanets and seeing what’s out there.

Looking to the future, Li and his colleagues are hopeful that the newly-launched Transiting Exoplanet Survey Satellite (TESS) will find many more of these water worlds. This will then be followed up on by ground-based telescopes – and the soon-to-be-launched James Webb Space Telescope (JWST) – which will provide spectroscopic measurements that will help scientists characterize these planets’ compositions and atmospheres.

As Professor Sara Seager, a professor of Planetary Science at Massachusetts Institute of Technology (MIT), and the deputy science director of the TESS mission, said:

“It’s amazing to think that the enigmatic intermediate-size exoplanets could be water worlds with vast amounts of water. Hopefully atmosphere observations in the future–of thick steam atmospheres—can support or refute the new findings.”

In the meantime, there are still plenty of rocky worlds out there to be explored for signs of life!

Further Reading: Eureka Alert!, Goldschmidt2018