Solving the mystery of missing exoplanet water

Professor David Sing is the lead author on a new research paper that has detailed the atmospheres of a number of planets outside our solar system for that first time. Here he tells us more about this ground-breaking research…
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A little over six years ago, the space shuttle Atlantis lifted off for one last repair mission to one of the greatest scientific instruments of all time. NASA/ESA Hubble Space Telescope spectrographic electronics had failed in 2004 and the spectrograph had been unusable since. A spectrograph analyses the light of the star as it passes through different chemicals and elements of the planet’s atmosphere.

I had been granted precious Hubble time to observe the atmosphere of a transiting exoplanet – ‘transiting’ meaning that, from our point of view on earth, it passes directly in front of its star, and ‘exoplanet’ referring to a planet outside our own solar system. My work was to analyse the spectrographic changes in order to classify the gaseous makeup of a planet’s atmosphere, still a relatively new concept in exoplanet science at the time. But in order to proceed with what I hoped would be groundbreaking science, the instrument had to first be repaired by an astronaut on a space walk.

In space, even the most routine tasks can prove challenging. A space walk can be downright dangerous. Via a live stream, I watched nervously here on Earth as astronaut Mike Massimino left the airlock of Atlantis in order to repair the Hubble’s spectrograph. But in order to install the new electronics board, a handrail that was in the way had to first be removed. The handrail had, of all things, a stripped bolt; it wouldn’t budge, rendering the spectrograph inaccessible.

As I watched, all I could think about was the lost telescope time, lost exoplanet science, and the possibility of the next decade devoid of meaningful scientific progress in my field, all because of a bolt. Astronauts must be intelligent and ingenious in space, but in the end Mike relied on brute strength to rip the bar free– a risky manoeuvre in space. It allowed access and successful repair of the instrument. With Hubble fully repaired and better than ever, not only could we once again look at the composition of exoplanets, but its new infrared capabilities opened a much broader view of their atmospheres.

Today, there are nearly two thousand exoplanets known. My team has just completed a large Hubble survey, the first of its kind, that compares exoplanetary spectra across ten very different planetary worlds.

Teasing out the spectra of a planet is a tricky business, as the star vastly outshines the planet, and there are still only a select few dozen that can be analysed spectroscopically in detail. The exoplanets we can study are nothing like Earth, or even any of the rest of the planets in our solar system. Our survey targeted Hot Jupiters, gaseous planets orbiting so close to their stars they are heated to thousands of degrees, creating immense winds moving thousands of kilometers per hour and pushing heat to the night side.

Unlike other astronomical objects such as stars, exoplanets show immense diversity from planet to planet, and two otherwise similar exoplanets can show very different characteristics. These are, after all, different planets around different stars with completely different formation and evolutionary histories. If you looked at ten planets for the first time, and they were of a type scientists didn’t even expect to exist, what would they look like, what would you discover? The first few glimpses from Hubble’s new infrared camera showed puzzling results; the large water vapor feature everyone expected to see was barely visible. This was quite a surprise as oxygen is one of the most abundant elements in the universe, and we would expect to see water vapour dominating the infrared spectra in all Hot Jupiters. Scientists theorised that the answer to the missing water vapour could be traced all the way back to the very formation of these planets.

Gas giant planets form far away from their stars where there is ample gas to accumulate. Far away from the star, ice forms, lowering the water vapour content in the gas. Thus, the theory holds that Hot Jupiter planets form in these far away regions, gathering gas from water-starved regions to form its planetary structure. The planet then migrates very close to the star through processes such as gravitational interactions with another planet or another nearby star. If this theory were true, it would be a major new insight into how planets form and a shakeup of conventional thinking.

We have, however, a competing theory to explain the lack of expected water features on these planets: clouds. Unlike clouds on Earth, the clouds in Hot Jupiters are expected to be quite exotic, made of small iron or silicate particles. It can rain glass on some of these planets, and shortly after Hubble was repaired, our initial observations indicated this was the case on at least one planet. But how common are these clouds, and could they explain low amounts of water vapour in Hot Jupiters simply by covering up their features?

From our Hubble telescope survey of ten planets, we now know the answer. Clouds do indeed hide the water vapour features in many Hot Jupiters, but not all. Using Hubble spectra at optical wavelengths, we can detect and study the light scattering caused by the clouds, and using infrared spectra from both Hubble and the Spitzer Space Telescope, we can simultaneously study the water vapour, comparing how the two interplay and manifest themselves across the different types of Hot Jupiters.

The diversity we found was unexpected, to say the least. No two planets look the same, ranging all the way from clear, pristine atmospheres to heavily clouded planets. The planets with clear atmospheres show very large water features just as expected, indicating Hot Jupiters are not so water poor after all. The diversity of clouds across the different planet types has also given us new insights into how clouds form in extreme conditions, and we have discovered the unique atmospheric structure of Hot Jupiters makes them particularly sensitive to cloud formation.

We may not yet entirely understand how these planets form, but we’ve devised several metrics to enable astronomers to separate the different planet types, such that clear-atmosphere planets can be specifically targeted to accurately measure their chemical abundances, which will soon provide new insights into planet formation.

As Hubble enters its last few years at full capacity – no other servicing missions are planned now that the space shuttle has retired – a new successor is on the horizon, the James Webb Space Telescope, set to launch in late 2018. It’s a telescope so powerful, it’s expected to revolutionise nearly all areas of astrophysics, particularly exoplanet science. Instead of just water vapour, we’ll be able to study the full, rich chemistry of an exoplanet’s atmosphere, and the makeup of their exotic clouds. The James Webb telescope will enable us to study much smaller planets, possibly even to study rocky and potentially habitable ones.

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