JWST Tries to Untangle the Signals of Water. Is it Coming From the Planet or the Star?

The number of known extrasolar planets has exploded in the past few decades, with 5,338 confirmed planets in 4,001 systems (and another 9,443 awaiting confirmation). When it comes to “Earth-like” planets (aka. rocky), the most likely place to find them is in orbit around M-type red dwarf stars. These account for between 75 and 80% of all stars in the known Universe, are several times smaller than the Sun and are quite cool and dim by comparison. They are also prone to flare activity and have very tight Habitable Zones (HZs), meaning that planets must orbit very closely to get enough heat and radiation.

In addition, red dwarfs are highly-active when they are young, exposing planets in their HZs to lots of ultraviolet and X-ray radiation. As such, whether planets orbiting these stars can maintain or reestablish their atmospheres over time is an open question. Using the James Webb Space Telescope (JWST), researchers from the Space Telescope Science Institute (STScI) observed an exoplanet known as GJ 486 b. As they stated in a recent study, the team detected traces of water vapor, though it is unclear if the signal was coming from the planet or its parent star.

The research team consisted of from the University of Arizona’s Lunar and Planetary Laboratory and Department of Planetary Sciences (LPL), the Johns Hopkins University Applied Physics Laboratory (JHUAPL), Imperial College London, the HH Wills Physics Laboratory at the University of Bristol, NASA’s Ames Research Center, the Center for Research and Exploration in Space Science and Technology (CRESST) at NASA Goddard, and multiple universities. The paper that describes their findings was recently accepted for publication in The Astrophysical Journal Letters.

Using data from Webb’s Near-Infrared Spectrograph (NIRSpec), the team examined GJ 486 b, a rocky planet roughly 30% larger than Earth and 2.82 times as massive (a “Super Earth”). It also orbits very closely to its star (0.01734 AU) with a period of 1.5 days, meaning that it is probably tidally locked with its parent star – i.e., one side is permanently facing toward it. Because of this, it is estimated that surface temperatures are as high as 430 °C (800 °F), making it the last place astronomers expected to find spectra indicating traces of water vapor.

This work was part of Cycle 1 of a program that relies on Webb’s NIRSpec and Near-Infrared Camera (NIRCam) to characterize the atmospheres of exoplanets around red dwarf stars. Study co-author Kevin Stevenson of the JHUAPL is the principal investigator of the program, named “Tell Me How I’m Supposed To Breathe With No Air: Measuring the Prevalence and Diversity of M-Dwarf Planet Atmospheres.” As Stevenson stated in a NASA press release, “Water vapor in an atmosphere on a hot rocky planet would represent a major breakthrough for exoplanet science. But we must be careful and make sure that the star is not the culprit.”

“We see a signal, and it’s almost certainly due to water,” said lead author Sarah Morton of the LPL. “But we can’t tell yet if that water is part of the planet’s atmosphere, meaning the planet has an atmosphere, or if we’re just seeing a water signature coming from the star.” NIRSpec detected the chemical signature of water vapor as GJ 486 b transited in front of its star relative to the telescope. If the planet has an atmosphere, starlight filtering through the atmosphere will produce spectra that show the atmosphere’s composition (known as transmission spectroscopy).

As part of the program, G J476 b was observed during two transits that lasted for about an hour each. The team examined the data using three different methods, and all showed a mostly flat spectrum with a rise at the shortest infrared wavelengths. The team then ran computer models that considered different molecules based on their spectral properties and concluded that water vapor was the most likely source of the signal. But while water vapor could indicate the presence of an atmosphere, it is equally possible that the signal was due to water vapor from the star.

This graphic shows the transmission spectrum obtained by Webb’s observations of rocky exoplanet GJ 486 b. Credits: NASA, ESA, CSA, Joseph Olmsted (STScI)

This sometimes occurs with the Sun, where water vapor can sometimes exist on sunspots because they are much cooler than the surrounding surface. Since GJ 486 b’s host star is much cooler than the Sun, more water vapor could be concentrated within its starspots that would be enough to produce the signal coming from the system. Said co-author Ryan MacDonald of the University of Michigan:

“We didn’t observe evidence of the planet crossing any starspots during the transits. But that doesn’t mean that there aren’t spots elsewhere on the star. And that’s exactly the physical scenario that would imprint this water signal into the data and could wind up looking like a planetary atmosphere.”

If the water is associated with the planet, this could indicate that it has a dense atmosphere despite its extremely hot temperatures. While water vapor has been detected in the atmospheres of gas giants (such as WASP-96 b), it has not yet been confirmed around a rocky exoplanet. For such an atmosphere to remain in place, it would need to be replenished by volcanic outgassing. Otherwise, it would gradually be stripped away by stellar heating and irradiation. At this juncture, more observations are needed to determine if the signal is coming from the planet and how much water is present.

Luckily, more observation time has been booked for Webb to study this system. For this upcoming program, “Constraining the Atmosphere of the Terrestrial Exoplanet Gl486b,” teams will use the Mid-Infrared Instrument (MIRI) to observe the planet’s day side. If the exoplanet has no atmosphere (or a thin one), the hottest part of the planet will be the part facing the parent star, whereas heat circulation would indicate the presence of a denser atmosphere. Further observations will be needed using the Near-Infrared Imager and Slitless Spectrograph (NIRISS) to differentiate between a possible exoplanet atmosphere and sunspots.

Determining that the water vapor is part of GJ 486 b’s atmosphere would be a groundbreaking accomplishment, placing astronomers one step closer to finding a true Earth analog.

Further Reading: NASA, The Astrophysical Journal Letters.