Exoplanets: Alien worlds beyond our solar system

Exoplanets have long occupied the thoughts scientists and dreamers. Ever since humanity first discovered that the stars in the night sky were bodies similar to our own sun, we have imagined and speculated about the worlds that could orbit these stars. 

Would these exoplanets be rocky terrestrial bodies similar to Earth? Could they possess liquid water? Could the presence of this vital life-sustaining element on other worlds mean that we are not alone in the Universe?

"For millennia, humans have been asking the question of whether we are alone. And tied to that question are other planets anywhere else?" Nikku Madhusudhan, a professor of astrophysics and exoplanetary science at the Institute of Astronomy, University of Cambridge, told LiveScience. "So, it's just very fundamental to being human to ask the question if there are planets elsewhere." 

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With this considered, it is almost shocking to consider that before the 1990s, astronomers weren’t even certain that stars outside the solar system even possessed their own planets.

Related: Is there water on Mars?

There was no evidence to suggest that extrasolar planets, or exoplanets for short, didn’t exist, nor were there hints that the solar system was in any way unique in the Milky Way. But until the very end of the 20th century, astronomers had been frustrated by the lack of direct evidence of worlds beyond the influence of our star.

This is because exoplanets are notoriously difficult to detect, according o the University of Colorado Boulder. Historically, the most successful exoplanet detection methods have worked by inferring the tiny effect that planets have on their parent stars, like tiny dips in light or the near imperceptible "wobble" they cause in their star's motion.

"Until 30 years ago, we didn't know of any planets outside the solar system, all we knew of were the planets in the solar system," said Madhusudhan, "But, as soon as exoplanets were discovered, that opened an entirely new window, into the Universe and its other planetary systems."

Since this point, improved technology and cunning detection techniques have resulted in a bulging exoplanet catalog containing over 4,800 distant worlds.

“The first big milestone in the study of exoplanets was the realization of just how common exoplanets are," said Madhusudhan, who developed a technique of atmospheric retrieval to infer the compositions of exoplanets. "But also, that those exoplanets are extremely diverse. Exoplanets come in all sorts of masses, sizes, temperatures."

When it comes to the categorization of these objects, humanity’s solar system bias is evident. That means worlds outside the solar system are labeled as "Super-Earths, hot Jupiters, and sub-Neptunes" but these planets can be radically different from those of our planetary systems, meaning that they can come in a startling array of forms.

If the discovery of thousands of exoplanets has shown anything, it is that our solar system is reassuringly and almost uniquely mundane. 

First exoplanet discovery

The first exoplanet discovered outside the solar system was an example of an object conspicuously absent from the solar system. It was discovered by Aleksander Wolszczan and Dale Frail in Jan. 1992. The duo discovered the rocky exoplanet orbiting a binary PSR B1620−2 6, consisting of a white dwarf and a pulsar located over 12,000 light-years away.

The following year, a second planet was discovered in the same system, also a terrestrial world. These planets, the two outermost planets of the system, were given the names Poltergeist and Phobetor, and represented the first examples of so-called "super-Earths."

These Super-Earths are planets are defined by their masses, which are greater than our planet’s mass but still less than those of the solar system’s ice giants, Uranus and Neptune. The upper limit for the mass of a Super-Earth is generally considered to be ten-times that of our planet.

You shouldn’t be fooled into thinking that Super-Earths bear any other similarities to our planet. The term doesn’t say anything about an exoplanet’s surface conditions or habitability.

As a striking example of this, researchers quickly determined that neither Poltergeist nor Phobetor could support life as they were being blasted by harsh radiation from the pulsar they orbited.

The search for a planet around a star similar to the sun hit paydirt in 1995 when Michel Mayor, Professor at the Observatory of the Faculty of Science of the University of Geneva (UNIGE), Switzerland, and his then doctoral student Didier Queloz discovered 51 Pegasi b, or Dimidium, a planet in orbit around a star that resembled our sun. In October 2019, the Nobel Committee awarded the Nobel Prize in Physics to the duo for their discovery of the planet.

Though the star it orbits, 51 Pegasi, is sun-like, that doesn’t mean its planetary system resembles the solar system. This discovery marked the first detection of a "hot Jupiter" — a planet with the size and composition of the solar system’s gas giant but located scorchingly close to its parent star.

"These planets are at an orbital distance closer than Mercury is from the sun," Romain Allart, a postdoctoral Trottier fellow at the University of Montréal, Canada, and a team member at the Institute for Research on Exoplanets, told LiveScience. "That means hot Jupiters complete their orbits in only a few days, and to their location close to their host stars, they are highly irradiated with temperatures of 2000K or more."

Not only was 51 Pegasi b an early hint to astronomers that the Universe is a wilder and more varied place when it comes to planets than they may have previously suspected, but hot Jupiters would also become mainstays of the exoplanet catalog.

"Hot Jupiters are actually not so common in the Universe, but due to instrumental biases, they are extremely common in the current exoplanet catalog," Allart, who was part of the team that investigated the hot Jupiter WASP-76b, explains. “Because they are close, large, and massive the radial velocity and transit techniques [see side bars] are efficient to detect hot Jupiters and these two techniques have discovered almost all exoplanets up until now!"

In terms of exoplanet populations, Madhusudhan says that sub-Neptunes — which are planets with a smaller radius than Neptune but a larger mass, or one with a smaller mass than Neptune but a larger radius — seem to dominate the Milky Way.

"The realization that small planets that are extremely common elsewhere is another major milestone," Madhusudhan adds.

One milestone in exoplanet research that is currently ongoing and will develop exponentially in the future, the astrophysicist says, is the investigation of these more diminutive planets’ atmospheres and the search for water.

Not too Cold, not too hot… just right

An exoplanet transiting the face of its host isn’t just a great way for astronomers to spot such a world by the dip in light output from the stars that it causes. The transit method has also proved a good way of assessing the composition of a planet’s atmosphere.

This is because atoms and molecules absorb light at characteristic wavelengths. So, by observing the gaps in the light signatures of stars as they shine through planets’ atmospheres, astronomers can see what elements make up these gaseous envelopes.

In 1999, Greg Henry and David Charbonneau used the transit method to detect and observe an exoplanet as it passed in front of the star HD 209458. This revealed that the planet, named HD 209458 b, had an atmosphere of oxygen, nitrogen, carbon, and importantly, water. This atmosphere is being stripped away from this world, leaving a trail behind it that is similar to that of a comet.

According to Madhusudhan, since 1999 and particularly in the past decade, atmospheric observations of exoplanets have taken off in a big way, with the first robust measurements of water vapor in the atmospheres of these planets being made. 

Unfortunately, as was the case with HD 209458 b, many of these detections tell us little about the possibility of life existing there.

"Hot, giant planets are where we have detected water, for the most part as water vapor. And there is no scope of life on these planets," Madhusudhan says.

Excitingly, however, this is beginning to change. Madhusudhan is the editor of Exofrontiers, which collects pioneering work from the exoplanet science community: he points out that our methods of examining atmospheres have improved to the point where we are now able to detect chemical elements around much smaller planets. 

This includes Earth-like worlds in the so-called "Goldilocks" habitable zones of planets where conditions are just right to allow for the existence of liquid water.

"We are able to detect small Earth-sized planets in the habitable zones of their host stars around nearby stars. And this is especially true for small stars called M dwarfs," Madhusudhan says, referencing, in particular, the planets in the TRAPPIST-1 system.

Discovered in 2017, the system contains seven rocky terrestrial worlds all of which exist at a suitable distance from their red dwarf to facilitate the existence of water at their surface. "These are all small, rocky, Earth-like rocky planets at the right distances for habitability around their host stars."

Observations of the TRAPPIST-1 planets conducted in Feb. 2018 revealed that some of them may even be able to harbor more liquid water and wider oceans than Earth.

This makes the system one of the prime targets for atmospheric investigations by future telescopes, including the James Webb Space Telescope (JWST).

The search for exoplanets

This life-searching, atmosphere-investigating aspect wasn’t part of the JWST’s mission when the plans for a 32-foot (ten-meter), passively-cooled, near-infrared telescope in a high-Earth orbit was initially floated in 1989.

In the last year of the 1980s, astronomers had not even discovered planets around other stars and the Hubble Space Telescope, which would make an important contribution to this search, was still a year from launch.

Various teams of astronomers are chomping at the bit for observation time with the new space telescope so they can investigate planets outside the solar system. This includes Madhusudhan, who will be leading a team working with the JWST to investigate exoplanet atmospheres in unprecedented detail, “We are indeed in the golden age of exoplanet science, but we are also on the verge of a major revolution in modern astronomy.” 

And while even the $10 billion JWST won’t be able to conclusively tell if a planet is hosting life, its observing power brings humanity tantalizingly close to the detection of molecules that hint at the presence of living organisms. This will lay further groundwork for future missions.

“We are the fortunate generation that might witness the discovery of life elsewhere, within this generation,” Madhusudhan says. “We have been dreaming of that for thousands of years and we happen to be that blink of an eye generation in which that momentous discovery is going to happen. "To me that is huge."

Madhusudhan is part of research into so-called hycean worlds — water-rich planets with surfaces covered almost entirely in oceans and with atmospheres made up of mostly molecular hydrogen. These hypothetical worlds could potentially redefine the limits of what we consider the habitable zone. This gives researchers targets outside the traditional habitable zone to include in the search for the telltale signatures of life.

And nothing says “casting a wider net” like the revelation this year that astronomers may have caught a hint of the first exoplanet planet ever to be detected outside the Milky Way. The team, including Nia Imara from the University of California, may have detected a Saturn-sized exoplanet 28 million light-years from Earth in the galaxy Messier 51. This extragalactic exoplanet seems to be orbiting a high mass compact object such as a neutron star or a black hole.

"Surprisingly, we are only scratching the surface as we now think that almost one star in every two hosts a planet, and there are hundreds of billions of stars in our galaxy, and there are billions of galaxies in the Universe," Allart adds. "Exoplanet diversity is already so rich that even the best sci-fi authors could not have imagined it.  "It is amazing to discover more and more strange exoplanet systems and worlds."

According to Allart, despite this wealth of planets and our increasing knowledge of them, protecting our own world is still of paramount importance, "The solar system and in particular, the Earth remains unique in the diversity of exoplanets. Therefore, it is important to understand that there is no planet B."

Additional resources

For more information about exoplanets check out "The Planet Factory: Exoplanets and the Search for a Second Earth" by Elizabeth Tasker and "Exoplanets" by John W. Mason. If you want hunt for exoplanets check out NASA’s many citizen science projects.

Bibliography

• ESA, "The future of Exoplanet research", March 2022. 
• NASA, "Exoplanet Exploration: Planets Beyond Our Solar System", March 2022. 
• Nikku Madhusudhan, "ExoFrontiers: Big questions in exoplanetary science", IOP Publishing Ltd, October 2021. 
• David Spiegel, et al "Structure of exoplanets", PNAS, Volume 111, December 2013, https://doi.org/10.1073/pnas.1304206111. 
• Geoffrey Marcy, et al, "Observed Properties of Exoplanets: Masses, Orbits, and Metallicities", Progress of Theoretical Physics Supplement, Volume 158, February 2005, https://doi.org/10.1143/PTPS.158.24.
• Brendan Crill, et al, "Key Technology Challenges for the Study of Exoplanets and the Search for Habitable Worlds", arXiv, March 2018, https://doi.org/10.48550/arXiv.1803.04457.