The search for habitable exoplanets is a key priority and sits at the pinnacle of exoplanet science. The science community stated that clearly in the 2020 Survey on Astronomy and Astrophysics 2020 (Astro2020). That survey identified the search for habitable worlds as a priority in their Pathways to Habitable Worlds report.
A critical part of understanding and identifying habitable worlds is understanding how their atmospheres evolve, including atmospheric escape. Atmospheric escape is a natural part of planetary evolution, and it has occurred throughout Earth's history. However, it was much more pronounced in Earth's early history, due to the Sun's energetic output and more frequent impacts from asteroids and comets. In current times, the escape is minimal, though steady.
Since atmospheric escape is much more pronounced in a planet's early years, it can shape its future habitability. That's at the heart of new research to be presented at the upcoming Towards the Habitable Worlds Observatory: Visionary Science and Transformational Technology meeting in Washington, DC.
The new research explains how exoplanet scientists can use the Habitable Worlds Observatory (HWO) to study and measure atmospheric escape in exoplanets. It's titled "Exoplanet Atmospheric Escape Observations with the Habitable Worlds Observatory," and the authors are Leonardo Dos Santos and Eric Lopez. Dos Santos is from the Space Telescope Science Institute and the Department of Physics and Astronomy and Johns Hopkins University, while Lopez is from NASA's Goddard Space Flight Center.
The HWO is only a proposal at this point in time, and considering the current political leadership in the USA, its future is in doubt. But scientists are still working on the concept, including the different ways its observational power can be leveraged. The HWO would be a space telescope that works in optical, infrared, and UV light and would have a mirror as large as 8 meters. It's design is still being determined and could include a starshade separated from the telescope by tens of thousands of kilometers.
"By leveraging the ultraviolet (UV) capabilities of the Habitable Worlds Observatory, we can use transit spectroscopy to observe atmospheric escape in exoplanets and explore the processes that shape their evolution, assess the ability of small planets to retain their atmospheres, and search for signs of Earth-like atmospheres," the authors write. The researchers make the case that the HWO would benefit from a UV spectrograph in the 100-300 nm range.
Our exoplanet detection methods are biased toward planets that orbit closely to their stars. In many cases, these planets are so close they're exposed to radiation levels that are thousands of times higher than Earth. This powerful radiation can evaporate their atmospheres into space, rendering them uninhabitable.
That leads to a fundamental question about exoplanet habitability: "How efficiently can planets retain their atmospheres and thus be habitable?" the authors ask.
"Unlike solar system planets, for which intense evaporation is no longer detectable, exoplanets represent our best opportunity to see the phenomenon occurring now," they write. The researchers say that what's needed is a survey of exoplanets experiencing atmospheric escape, which will lead to an accurate model of the process.
The survey would have two guiding objectives. The first is "Determine whether [transiting] rocky planets in habitable zones have exospheres similar to the modern Earth."
The exosphere is the outermost layer of a planetary atmosphere where the particle density is very low. It's so low that particle collisions are unlikely, yet dense enough to detect with a few spectral lines. Earth's exosphere is mostly neutral atoms of hydrogen (H), oxygen (O) and nitrogen (N), but Earth's exosphere is unusual compared to the Solar System's other rocky planets. "Thus, detecting a similarly large and H-rich exosphere around a transiting exoplanet could provide compelling evidence to corroborate whether this planet is similar to modern Earth," the researchers explain. Earth's atmosphere also extends well into space, increasing its detectability.
Atmospheric escape continually replenishes Earth's exosphere with hydrogen. It's produced by photodissociation of water molecules in lower atmospheric levels, and those molecules come from surface water, so finding an exoplanet with a hydrogen-rich exosphere could indicate the presence of an ocean, and potential habitability.
The second guiding objective is "Determine how efficiently hydrodynamic escape erodes H2-dominated primordial envelopes."
Atmospheric escape plays a major role in how we understand planet formation and exoplanet demographics. If exoplanet scientists can determine mass loss on present-day planets, they can reconstruct their original properties, like the masses of their primordial H/He atmospheres which are accreted from the primordial solar nebula. "However, despite being key to the atmospheric properties of planets, including potential habitability, the dependence of a planet’s atmospheric mass-loss on stellar (i.e., irradiating stellar UV flux and wind) and planetary (i.e., mass, radius, orbital separation) properties is far from understood," the authors explain.
What's needed is a survey that constrains the rate of atmospheric escape across a broad range of exoplanet masses and ages. UV transit spectroscopy is the primary way that astronomers measure atmospheric escape rates on short-period planets. "Due to its predicted effective area in the UV, HWO is uniquely poised to enhance our capabilities to observe photoevaporation in a large range of exoplanet masses," the researchers write.
To understand exoplanet atmospheric escape more clearly, the pair of researchers propose two separate approaches to make best use of the HWO's capabilities. One is a 'deep field' type of survey similar to those conducted by other space telescopes like the Hubble and the JWST but focused on high-profile exoplanets. The targets would be transiting, rocky exoplanets within their stars' habitable zones.
The other is a broader survey covering a wide range of exoplanets. It would cover at least 50 transiting exoplanets within a range, including everything from hot Jupiters to cool Neptunes. The researchers have already simulated some of those results and it looks promising.
In preparation for the eventual use of the HWO to study atmospheric escape, the researchers have identified what needs improving beforehand. They explain that we need a better understanding of sources of opacity at UV wavelengths in exoplanet atmospheres. For example, vaporized metals have strong UV absorption lines, and molecules like methane can contribute to UV opacity. Clouds can also contribute.
The HWO concept has been around for a while. It's based on LUVOIR, the Large UV/Optical/Infrared space telescope and the Habitable Exoplanet Observatory (HabEx). While funding risks, technological advancements, and even scheduling could delay the telescope, the real risk is political will. If recent actions are any indicator, the current US political leadership is tone deaf to science and not interested in maintaining a world-leading scientific community.
So, like a lot of things in the world right now, the HWO's future is uncertain.