How the next generation of space telescopes will hunt for habitable exoplanets

In recent years, we have discovered an astonishing array of planets outside our own solar system. In addition to those that are potentially habitable, we’ve also found exoplanets that are hotter than stars, have iron rain and yellow skies, and that have the density of cotton candy. But we’ve still just barely scratched the surface of what’s out there.

The next generation of planet-hunting missions will go even further, identifying exoplanets and determining their habitability even from thousands of light-years away. To learn more about how you go about hunting the needle of a planet in the haystack of our galaxy, we spoke to three experts working on cutting-edge exoplanet projects.

An artist's illustration of the super-hot exoplanet WASP-79b
An artist’s illustration of the super-hot exoplanet WASP-79b, located 780 light-years away. NASA, ESA, and L. Hustak (STScI)

An explosion of exoplanets

The first exoplanets were discovered in 1992, and in less than three decades the number of known planets outside of our solar system has exploded. NASA estimates that the number of known exoplanets roughly doubles every 27 months.

Exoplanet discovery began using ground-based telescopes, such as the famous discovery of the exoplanet 51 Peg b in 1995, for which two Swiss astronomers received the Nobel Prize. But exoplanet hunting really kicked into high gear with the advent of space-based planet-hunting telescopes like NASA’s Kepler and TESS missions.

Now, new missions from NASA and ESA (European Space Agency) are identifying and examining distant exoplanets in more detail than ever before.

Finding exoplanets in our galaxy

PLATO is ESA’s next-generation planet-hunting space telescope, and it is currently being built with an aim for a 2026 launch. The mission will concentrate on bright stars that are relatively nearby to us in the galaxy, typically in the region of between 300 and 1,000 light-years away, looking at each area for at least two years.

The mission will search for habitable worlds using the transit method, in which researchers measure the brightness of a distant star. If the brightness of the star dips at regular intervals, that implies that a planet is passing between us and the star, blocking out some of the light given off by the star and causing the dip in brightness. Measuring this dip precisely allows instruments like PLATO to very accurately calculate the size of the planet.

The two-year observation period allows the scientists to search for longer-period planets. So while a mission like Kepler looked at a small area of the sky for a long period of time, and TESS looks at large regions for the sky for a short period of time, PLATO will look both at a large region and for a long time.

 Searching for exoplanetary systems
The PLATO (PLAnetary Transits and Oscillations of stars) mission will assemble the first catalog of confirmed and characterized planets with known mean densities, compositions, and evolutionary ages/stages, including planets in the habitable zone of their host stars. ESA - C. Carreau

We’ll need instruments with a longer observation period than previous missions to spot planets like our own, Ana Heras, project scientist for PLATO, explained to Digital Trends in an interview. “We want to detect Earth-like planets, and this means if you want to see a planet similar to the Earth in the habitable zone, it will have an orbital period of one year,” she said. “So we have to observe for at least two years, because we want to see at least two transits.”

Current models suggest that observing two transits of a given star should provide enough data to identify and to some extent characterize an exoplanet, but there’s the possibility that PLATO might observe the same area for three or even four years if necessary.

“This will allow us to advance, in a fantastic way, understanding of stellar evolution and general knowledge about stellar physics”

In addition to these Earth-like planets, PLATO will also look at cooler red dwarf stars, which could potentially have habitable exoplanets orbiting them. The telescope’s highly accurate photometer can also measure information about the oscillations of stars being observed, which can tell scientists about their internal structure and ages. “This will allow us to advance, in a fantastic way, understanding of stellar evolution and general knowledge about stellar physics,” Heras said.

One of the most exciting possibilities of PLATO is that it is so accurate, it may even be able to detect moons orbiting around exoplanets, called exomoons. It stands to reason that moons exist outside of our solar system, but current methods have not yet definitively confirmed the detection of one.

The chance that PLATO could find such a moon opens up the possibility of searching for different types of habitable environment — not only Earth-like planets, but also moons similar to those like Saturn’s moon Enceladus which is one of the most promising potentially habitable non-Earth locations in our solar system.

How many planets are there in our galaxy?

We’ve discovered approximately 4,200 exoplanets so far, with more being announced practically every month. But an open question remains regarding exactly how many planets there are in our galaxy. Using methods like the transit method only reveals planets in particular configurations — particularly those which are in close orbits to their stars — so we need an overall view of the galaxy to get a better idea of how many planets are out there in total.

NASA’s Nancy Grace Roman Space Telescope
NASA’s Nancy Grace Roman Space Telescope, named after NASA’s first Chief of Astronomy. NASA

That’s what NASA’s upcoming Nancy Grace Roman Space Telescope, or simply Roman, aims to discover. The telescope is currently being built and, once it is launched in late 2025 or early 2026, it will begin a survey of the night sky called the Roman Galactic Exoplanet Survey (RGES).

The aim of this survey is not to discover or investigate exoplanets per se, but rather to gain a big-picture view of how many stars in our galaxy host planetary systems, and how these systems are distributed.

Detecting planets by bending light

To perform its sky survey, Roman will use a technique called microlensing, which can pick out exoplanets but mostly tells scientists about the stars around which planets orbit.

“Microlensing is unique in a lot of ways,” principal investigator for RGES, Scott Gaudi, told Digital Trends in an interview. It’s based on a process called gravitational lensing, which is used to detect stars. “The way it works is if you stare at a star for long enough (about 500,000 years) then just by chance another foreground star will float close enough to your line of sight that background star to split the light from that background star into two images,” he explained.

“The background source star is brightened as the foreground star comes in front of it, because the foreground star’s gravity bends light rays that would have been going away from the line of sight.” This means that if scientists observe a background star get brighter and then get fainter, they can infer that another star has passed between it and us.

This technique can be further refined to detect exoplanets. “If that foreground star happens to have a planet, then that planet has mass, which means it can gravitationally lens that star as well,” Gaudi said. “So if one of those two images of that background star created by the foreground host star happens to pass close to the planet, it will cause a brief additional brightening or dimming, which last between a few hours, in the case of an Earth-mass planet, to a few days, in the case of a Jupiter-mass planet.”

The problem is that these events, in which planets and stars line up just so, are rare and unpredictable. So to capture them, astronomers need to watch a huge number of stars. “You get one lensing event per star per 500,000 years, so that’s a long time to wait,” Gaudi said. “So instead we monitor roughly 100 million stars in the galactic bulge [a densely packed area of stars in the middle of our galaxy] and at any given time, many thousands are being lensed.”

Roman will be particularly suited to this type of investigation as it has a very large field of view, allowing it to observe a large chunk of the galactic bulge. It can also monitor these millions of stars on a timescale of 15 minutes, allowing the researchers to capture these lensing events as they happen.

Complementary missions

The primary data we have so far on how many exoplanets might exist in our galaxy comes from the now-retired Kepler Space Telescope, which surveyed the sky between 2009 and 2018, measuring the brightness of some 150,000 stars to search for exoplanets using the transit method.

This mission laid the groundwork for exoplanet research today. However, due to the method used by Kepler, there are still many exoplanets that it might have missed. The Roman project aims to extend and complement this work by using a different method.

Illustration of star Kepler 51 and three orbiting planets.
This illustration depicts the Sun-like star Kepler 51, and three giant planets that NASA’s Kepler space telescope discovered in 2012-2014. NASA , ESA , and L. Hustak, J. Olmsted, D. Player and F. Summers

“The RGES survey is important because it will be complementary to Kepler,” Gaudi explained. “The microlensing method is intrinsically sensitive to planets which are further out, so planets with orbits roughly greater than that of the Earth.” If this method was used by distant aliens to observe our solar system, for example, it would be able to detect all of the planets except for Mercury.

“Whereas Kepler was only barely sensitive to Earth-mass planets. So we really need to do the RGES survey to do this statistical census of exoplanets in the galaxy,” Gaudi said.

Microlensing also isn’t dependent on bright light from the stars being observed, so it allows scientists to observe systems which are both near to us and as far away as the center of the galaxy. Roman will allow researchers to gain a statistical understanding of how planetary systems are distributed throughout our galaxy, Gaudi said: “So we can actually determine the galactic distribution of exoplanetary systems, which is basically impossible with any other technique.”

Characterizing exoplanets using transits

The PLATO and Roman telescopes will be invaluable for discovering new exoplanets and estimating how many exoplanets exist in total in our galaxy. But once we know how many planets there are and where they are located, we need new tools to learn more about these planets — investigating characteristics such as their mass, size, and age. This information can help us see what kind of planets are out there, whether they’re gas giants like Jupiter or Saturn or rocky worlds like Earth and Mars.

ESA recently launched a new space-based telescope called CHEOPS (CHaracterising ExOPlanets Satellite) which is investigating exoplanets from orbit. The CHEOPS project will likely find some new exoplanets during its tenure, but its main goal is to investigate exoplanets found by other surveys in more detail using the transit method.

“We are, in effect, a follow-up mission,” Kate Isaak, project scientist on CHEOPS, explained to Digital Trends in an interview. “We’re following up to find the sizes, among other things, of known exoplanets.”

Artist's impression of Cheops, ESA's Characterising Exoplanet Satellite, in orbit above Earth.
Artist’s impression of Cheops, ESA’s Characterising Exoplanet Satellite, in orbit above Earth. In this view the satellite’s telescope cover is open. ESA / ATG medialab

This means that the scientists on this project have an advantage in their observations, as they already have the information they need about when a transit will occur. They can point the instrument toward the target planet just at the right moment as it is transiting in order to capture information about it.

CHEOPS was only launched a few months ago but it has already discovered new information about the planet KELT-11 b, finding that this quirky planet is so low density that it “would float on water in a big enough swimming pool,” according to a statement by the researchers.

Looking for Earth 2

Detection and study of exoplanets isn’t just about finding strange worlds like KELT-9 b or AU Mic b though. It’s also about the biggest of questions: Whether or not life exists outside of Earth. The work being done by astronomers now is beginning to investigate the questions of not only where planets are, but also whether they could be habitable. Eventually, they could help determine if these distant planets do in fact host life.

This illustration shows how planet KELT-9 b sees its host star
This illustration shows how planet KELT-9 b sees its host star. Over the course of a single orbit, the planet twice experiences cycles of heating and cooling caused by the star’s unusual pattern of surface temperatures. Between the star’s hot poles and cool equator, temperatures vary by about 1,500 F (800 C). This produces a “summer” when the planet faces a pole and a “winter” when it faces the cooler midsection. So every 36 hours, KELT-9 b experiences two summers and two winters. NASA's Goddard Space Flight Center/Chris Smith (USRA)

“One of the holy grails of exoplanet science is looking for life,” Isaak said. “One of the things that people are looking for is an Earth-like planet. An Earth 2, you could say.” That involves looking for a rocky planet within the habitable zone of a star — the distance from a star at which liquid water can exist on the planet’s surface. Future missions like the upcoming James Webb Space Telescope will even be able to investigate whether distant exoplanets have an atmosphere.

Heras, the PLATO project scientist, concurred with the importance of the search for habitability. “The study of possibly habitable exoplanets is really the next step in order to understand not only how planets evolve, but also maybe how life appeared,” she said. “After all we have learned about exoplanets, the next step is going to be learning more about the development of life and how life started.”

There’s also a big open question about whether there are other solar systems out there similar to our own. “We would also like to know how unique our planet is,” Heras said. She explained that even with the thousands of exoplanets discovered, very few of these are within the habitable zone of their stars. “So we really don’t know yet, with our knowledge, how unique our solar system is and how unique Earth is.”

The ultimate question

This link between exoplanet discovery and the search for life drives both the scientists working on these projects and the public’s appetite for learning about distant worlds. It’s impossible to hear about bizarre exoplanets and not to imagine what it would be like to live in these strange places.

“Exoplanets are fascinating, if nothing else because they’re easy to understand,” Isaak said. “We live on a planet. The question of whether we’re alone is a profound one — philosophically, physically, psychologically — it’s a fascinating question and one we can easily understand. Searching for and studying exoplanets are steps towards the question of are we alone… With CHEOPS, we’re not going to find life. We won’t end the mission saying we’ve discovered little green men on Planet X. But what we will do is to contribute to the process by which you could do that in the longer term.”

Even if the search for life turns up nothing, that would still be a profound finding. And the search itself can spur scientific investigation and deep contemplation of our place in the universe.

“I think we all are searching for meaning,” Gaudi said. “If we could somehow have an idea about whether or not life, even simple life, arose on another planet independently from life on Earth — or if not and we’re cosmically lonely — either one would have a very profound impact on our view of ourselves and our place in the universe. It’s that meaning that drives me personally to study the search for habitability and potentially life.”

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