It’s so…black! You can hardly make out its shape…light just seems to fall into it! That is the word of David Kipping of the Harvard-Smithsonian Center for Astrophysics, and he is referring to TrES-2b, the darkest known exoplanet. Having an albedo of less than 1%, this enigmatic world, 750 light-years from us in the constellation Draco, absorbs all visible light short of a few photons, darker even than coal or black acrylic paint. But TrES-2b is just one of the many extraterrestrial planets that are forcing our assumptions about planet diversity and origin into a reevaluation.
Over 5,600 exoplanets have now been found by astronomers, each one more staggering than the last. All made possible by advances in telescope technology, like the James Webb Space Telescope (JWST). From iron rain raining like liquid to potential signs of life, the variety of these worlds is science fiction at its best.
Take WASP-76 b, a gas giant 640 light-years from our planet. It is a tidally locked planet with a scorching-hot dayside 4,350 degrees Fahrenheit (2,400 degrees Celsius) that reduces metals like iron to vapor. Turbulent winds force the boiled-off iron to the night side, where it rains liquid iron. WASP-76 b is a good representative of the extreme weather that predominates on most otherworldly planets.
And then there’s 55 Cancri e, the “hell planet.” Only 41 light-years from our planet, this super-Earth is under attack by tidal waves of mobile lava, and scientists have even estimated that it could potentially get lava. But closer analysis of Spitzer Space Telescope data shows that the planet may be enveloped by a dense atmosphere of nitrogen, water, and oxygen. Renyu Hu, a NASA Jet Propulsion Laboratory astronomer, had this to say about the planet: “If there is lava on this planet, it would need to cover the entire surface,” but the lava would be hidden from our view by the thick atmosphere. One would ask why the atmosphere has not yet been stripped away despite the host star’s intense radiation.
Not all exoplanets are hostile. K2-18b, which is 124 light-years away from us, makes its orbit in the habitable zone of its star, where temperatures may be perfect to support liquid water survival. Sneaking a peek with the JWST in recent observations revealed methane, carbon dioxide, and dimethyl sulfide (DMS)-a molecule produced by phytoplankton in our oceans-to exist. While scientists, like University of Cambridge’s Nikku Madhusudhan, are hopeful, he added, “Given everything we know about this planet, a Hycean world with an ocean that is teeming with life is the scenario that best fits the data we have.” Further observations, however, need to be done in order to be able to determine if DMS actually occurs in the K2-18b atmosphere or if abiotic processes are at play.
To science fiction fans, Kepler-16b is a real “Tatooine.” It’s a circumbinary planet that orbits two stars, with double sunsets that will be reminiscent of the Star Wars universe. This gas planet, discovered through the radial velocity method, is the same size as Saturn and 245 light-years away from Earth. Although its cold climate renders it lifeless, the discovery that it is found negates classical assumptions about the development of planets. Amaury Triaud, an exoplanet researcher, explained, “The presence of two stars interferes with the protoplanetary disk, and this prevents dust from agglomerating into planets, a process called accretion.” Kepler-16b might have originally formed along distance from its stars and migrated inward, offering new information about how planets form in binary systems.
WASP-107b, or the “fluffy planet.” is another fascinating discovery. The hot Neptune-like world 200 light-years from our own has a very low density and is likened to a toasted marshmallow. Silicate dust particle clouds observed in JWST observations made them think that it could actually rain sand. The planet’s puffed-up atmosphere goes against existing models of planet formation and structure and makes it an interesting topic for future research.
Even the rogue planets the ones that aren’t tethered to a star add to the richness of other-worlds diversity. Hubble and JWST were used by astronomers to discover hundreds of free-floating worlds in the Orion Nebula. And about 80 of them come in binary pairs with each other, cruising about the cosmos as twins. These “Jupiter-mass binary objects” remain mysteries, their makers stumped as to why they were expelled from their birth systems or how they create companions.
The search for life beyond Earth drives current exoplanet science. Even if they are not capable of supporting life, the majority of exoplanets are hiding secrets regarding planet formation, evolution, and climate. As Madhusudhan remarked, “Our ultimate goal is the identification of life on a habitable exoplanet, which would transform our understanding of our place in the universe.”
From rain of iron in liquid to binary sunsets, such extraterrestrial worlds remind us that the universe is stranger than we ever had the courage to imagine. And with each new discovery, we move a step closer to answering the question that has intrigued us for centuries: Are we alone?
