Up in the Air: Hot Jupiters Highlight Challenges in the Search for Life Beyond Earth

NASA scientists found faint signatures of water in the atmospheres of five distant planets orbiting three different stars. All five planets appear to be hazy. Credit: NASA Goddard Space Flight Center, NASA, ESA, A. Mandell (Goddard Space Flight Center), and D. Deming (University of Maryland, College Park)

This week, the Goddard Center for Astrobiology celebrated the confirmation of atmospheric water on five exoplanets. This discovery heralds a new era in planet hunting at the same time as it acknowledge two conundrums, one old and one new.

The new problem directly involves the water signals themselves. While undoubtedly present, on each of the five planets the signal appears to be actively distorted by clouds – clouds that are more than a trillion miles away. The second problem has been haunting planet hunters since the first exoplanets were discovered. It is this: none of these planets should exist. Hot Jupiters, so-called for their size and proximity to stars, have no currently understood mechanism for either formation in or migration to their present positions.

In theory, the majority of exoplanets should have strong water signals, just as most of the planets in our Solar System do. “We believe that all planets are formed with a significant amount of water in their atmospheres,” reported Dr. Avi Mandell, Research Scientist at the Planetary Systems Laboratory and one of the lead authors.

The physical forces that form planets form water as well. In systems with gaseous oxygen, carbon, hydrogen and nitrogen, CO2, N2, O2 and H2O quickly become dominant. Signature signals from this water should be abundant and detectable using the current methods.

As anticipated, the infrared signatures on planets XO-1b, WASP-12b, WASP-17b, WASP-19, and HD209458b definitively demonstrate energy absorption by water. The signal was less prominent than expected, though- sometimes much less prominent. While the three star systems are 848, 600 and 150 light-years from Earth, respectively, vast distances cannot account for the discrepancy.

The diminished signals are apparently due to atmospheric “haze”- in other words, clouds. “You can often look out the window [on Earth] and see zero blue sky,” continued Mandell, “We are really going to have to deal with clouds and haze.”

Artist’s view of extrasolar planet HD 209458b. Credit: NASA, ESA and G. Bacon (STScI)

At the same time, we will be contending with the existence of planets the size of Jupiter in orbits smaller than Mercury’s. So close, in fact, that a few of them have been found to be losing mass- boiling away, in other words. For this reason, and many others, it is certain that hot Jupiters could not have formed in their current positions. How these planets arrived at such close proximity to their parent stars is a problem being addressed by many groups, including Mendall’s.

With advent of newer and better detectors, both of these issues are likely to become more prominent.

As our optics improve, more Hot Jupiters will be added to the list of known exoplanets. As more large planets will be added, many more small ones will be as well. To date, as we zero in on smaller and smaller exoplanets, we seem to be hitting a wall in terms of the cloud layer. The smaller the planet, the more likely there are to be clouds blocking the water signal.

Super-Earth-sized IG1214, circling an M-class star similar to our Sun, is one example. “They have looked very hard for a water signal on that planet,” reported Mandell, “They think [the weak signal] is a signpost that clouds dominate the smaller sized planets.”

With the James Webb telescope slated to come online in 2018, according to what we’re seeing now, each new planet we discover will have with a high potential for haze. Though, as Dr. Mandell pointed out, “We don’t have a lot of understanding…of our inability to see deep into the atmospheres.”

What we do understand is that planets are common, perhaps as common as the stars themselves. We are learning that water may be nearly as is common as planets, if not as easy to discover. To know the extent of life in our universe, it seems our search will be not only for planets, but for planets with clear skies, in places previously believed to be impossible.