How the James Webb Space Telescope Will Find Habitable Planets Outside the Solar System

Rohan Menezes, Physical Sciences, Fall 2021

Figure: This Image from NASA is an illustration of an exoplanet with an atmosphere, similar to those JWST will soon be analyzing.

Image Source: Public Domain

When the James Webb Space Telescope (JWST) launches in December this year, it will surpass Hubble as the world’s premier space telescope. It will provide us key new insights into our own solar system, as well as distant worlds around other stars (Pulliam, 2020; Gialluca et al., 2021). One of JWST’s research targets is TRAPPIST-1, an exoplanet system of seven earth-sized rocky planets around an ultra-cool dwarf star (i.e. a dwarf star with an effective temperature of less than 2700K, which is approximately 4400 °F) (Pulliam, 2020; NASA, 2021). Three of these planets are within the habitable zone of the system’s star — the zone within which planetary surface temperatures are capable of sustaining liquid water (NASA, 2017).

However, being in the habitable zone alone does not mean these planets are capable of sustaining life. That is dependent on several indicators — called “biomarkers” in astronomy (Fraknoi et al., 2016). The most important of these biomarkers is the composition of their atmospheres (if they have atmospheres), which would need to have significant quantities of methane, oxygen, water vapor, carbon dioxide, and other gases in similar proportions to those observed on earth (Gialluca et al., 2021).

Finding these atmospheric biomarkers is one of the tasks JWST will try to accomplish. The nearest potentially habitable planets, however, are tens of light years away, even from where JWST will be stationed. So how will we analyze their atmospheres?

The answer lies in the stars – literally. When a planet is between our observation point and the exoplanet system’s host star, we can use sensitive telescopes to detect how much of the star’s light is blocked by the planet and its atmosphere before it reaches us. The proportion by which the light dips is called the transit depth and will differ by wavelength, since gaseous elements each selectively absorb certain wavelengths of light more than others. Using a specialized telescope called a spectrometer telescope, we can analyze light by wavelength to see which signals are lacking compared to when the star is not being transited (Fraknoi et al., 2016). Therefore, we can tell which kinds of gases are most likely prevalent in the atmosphere. This technique is called transit spectroscopy.

While it may seem high-tech, transit spectroscopy was first used through Hubble in 2002, and has been in common use for over a decade (Deming, 2016). So, why haven’t we already found life sustaining planets among the stars? What’s so special about JWST that only it can analyze the atmospheres of obvious candidates for habitability like TRAPPIST-1?

Well, certain steps, such as resolving the star angularly and spectrally (i.e., being able to separate it and the light it emits from the multitude of other extrasolar objects), resolving the planet angularly, and determining the size of the star and planet are possible with existing technology. However, the most important measurement – the transit depth – faces two major limiting factors that have restricted prior research. First, planets are very small relative to their stars. Even dwarf stars like TRAPPIST-1 are larger than Jupiter, which is itself over 11 times the size the earth-like planets that are the prime candidates for habitability (NASA: Jet Propulsion Laboratory, 2017; NASA, 2003). Therefore, the “dip” in brightness (transit depth) observed when a planet transits the surface of a star dozens of light years away is quite small. As such, measuring the miniscule amount by which this transit depth shifts by wavelength, which will be determined by the planet’s atmospheric composition, is even more difficult and requires very sensitive telescopes to detect.

Transit spectroscopy has primarily been used to detect the atmospheric composition of exoplanets much larger than earth with orbits very close to their stars (usually dwarf stars) — such as the so-called “hot Jupiters” and “hot Neptunes” — since the changes in transit depth, though still small, are easier to detect (Seager and Deming, 2010). No telescope has ever been able to detect or analyze the atmospheres of earth-sized exoplanets in their stars’ habitable zones before (Seager and Deming, 2010).

An additional challenge is that the light we receive may be partially blocked (i.e., have certain wavelengths absorbed) by objects in space between our telescope and the star. The most significant of these are the huge clouds of gases (mostly hydrogen) and tiny solid particles we call interstellar dust. This dust fills much of the space between stars, distorting most wavelengths of light as they travel through space towards our telescopes from around the universe (Fraknoi et al., 2016). A key exception to this rule is infrared light, which can penetrate through most dust unfiltered with their long wavelengths (NASA, 2021). However, there is a major challenge to analyzing light in the infrared — every object, including the telescope machinery, emits infrared radiation if their temperature is above freezing (National Aeronautics and Space Administration, Science Mission Directorate, 2010). To keep this interference down such that it does not impede infrared measurements from extrasolar objects, the telescope temperature must be kept near freezing (Villard, 2017).

While this is next to impossible on earth, in the vacuum of space, it is possible to maintain such a temperature relatively efficiently. JWST has also been equipped with specialized equipment for detecting and analyzing radiation in the near- and mid- infrared range such as the Near Infra-Red Spectrometer (NIRSpec) and the Mid Infra-Red Instrument (MIRI) (James Webb Space Telescope User Documentation, 2021; Rieke et al., 2015).

The combination of its location and advanced instruments grants JWST an infrared sensitivity greater than any previous telescope, which will allow it to characterize the atmospheres of earth-size exoplanet systems like TRAPPIST-1 for the first time. Future space telescopes are also being designed with these capabilities firmly in mind, such as the Habitable Exoplanet Observatory (HabEx) and the Large Ultraviolet Optical Infrared Surveyor (LUVOIR). Both are among four options to become NASA’s flagship project for the next twenty years (Billings, 2021). HabEx’s primary purpose would be to characterize exoplanet atmospheres to determine habitability, utilizing its high sensitivity to near-IR radiation, as well as optical and UV radiation (NASA, 2021). The Habex concept also includes the use of a flower-shaped spacecraft called a “starshade,” which could be moved to hide the light of stars and allow the telescope to analyze the exoplanets orbiting them in even greater depth than JWST (Billings, 2021). If JWST can confirm these exoplanets have atmospheres like Earth’s, this would indicate for the first time that exoplanets are potentially capable of sustaining life, and shape the future of astronomy for decades, if not generations to come.

References

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