Neighboring Exoplanets may Hold Water

Observations and modeling suggest TRAPPIST-1 exoplanets may have held onto water, billions of years after their formation.

Neighboring Exoplanets may Hold Water
This artist’s impression shows the view from the surface of one of the planets in the TRAPPIST-1 system. At least seven planets orbit this ultracool dwarf star 40 light-years from Earth and they are all roughly the same size as the Earth. Several of the planets are at the right distances from their star for liquid water to exist on the surfaces. Image: ESO/N. Bartmann/

Seven Earth-sized exoplanets circle, the ultra cool dwarf star TRAPPIST-1 is just 40 light-years from our own blue planet. According to MIT scientists, these neighboring exoplanets in this circle hold significant stores of water. Even three of them considered within the habitable zone of the star.

These results are found by observations on the TRAPPIST-1 star made by the NASA/ESA Hubble Space Telescope. Scientists prepared the telescope on the star to measure its current ultraviolet radiation. They then used these measurements to estimate how the star’s energy changed over the course of billions of years.

According to scientists, these neighboring exoplanets originally formed farther out from their star. They formed within a very cold zone with crystals of water ice. As they come closer, they potentially create tremendous stores of water.

From their observations and modeling, the researchers conclude that, over the past 8 billion years, heat and radiation from the star may have caused the innermost planets to lose more than 20 times the amount of water in all of Earth’s oceans. Means, the external planets would have lost considerably less, proposing they could, in any case, hold some water on their surfaces and in their insides.

Julien de Wit, a postdoc in MIT’s Department of Earth, Atmospheric and Planetary Sciences said, “In terms of habitability, this is a positive step forward to say that hopes are still high. This concludes that a few of these outer planets could have been able to hold onto some water if they accumulated enough during their formation. But we need to gather more information and actually see a hint of water, which we haven’t found yet.”

In February 2016, scientists discovered the seven Earth-sized planets around TRAPPIST-1. They then used the Hubble Space Telescope Imaging Spectrograph (STIS) to measure the amount of ultraviolet radiation given off by the TRAPPIST-1 star then received by its planets. They even trained the telescope on the system over one observing run of five orbits for each planet, totaling eight hours, in which they gathered 4.5 hours of data. But the observations were inconclusive.

De Wit said, “We see this flux is actually changing, and we can use this change to backtrack and have an understanding of how much energy the star is putting on each planet over the course of the planets’ lives.”

Scientists have likewise beforehand watched that the planets’ orbital arrangements are with the end goal that they likely relocated together. As they migrated into the star’s warmer zone, the star’s ultraviolet radiation likely started to strip away and evaporate the planets’ water resources.

Now scientists using their estimates to estimate the amount of water the exoplanets likely lost. They plugged their estimates into two models: an atmospheric model that calculates the amount of water vapor that might be lost given a certain ultraviolet concentration, and a geophysical model that estimates how much water ice and other volatiles, buried deep in a planet’s interior.

They estimate that the innermost planets lost more than 20 times Earth’s current oceanic water stores over their 8-billion-year.

Wit said, “Earth-sized planets can capture hundreds of Earth-oceans’ worth of water when they form, but it’s highly dependent on so many factors and difficult to say. We can say the inner ones probably lost a huge amount of water, and the outer ones way less, allowing them to actually still have some water if they captured it when they first formed.”

“It depends a lot on their initial water content. If they formed as ocean planets, even the inner ones would likely still harbor a lot of water. We are still a long way to determining the habitability of these planets. Our results suggest that the outer ones might be the best targets to focus our future observations.”

de Wit said, “If the planet’s atmosphere holds water vapor, and it is losing hydrogen as it reacts with ultraviolet radiation, it will look a bit like a gigantic comet with a tail, or a sphere that’s 10 times bigger than the planet, filled with atomic hydrogen, that is slowly flowing out of the planet, forming a tail from the stellar wind. It’s amazing how quickly our perspective on this [system] has changed. It’s really a steep learning curve that is really exciting.”