Exoplanets Like Jupiter
An artist’s impression of ten hot planets similar to Jupiter outside our solar system that scientists have detected. Creative liberties were taken for the colors of the planets, which are currently unknown. The exception is the top-right one, which is known to sport a blue exterior.
Are we alone in the universe? While no one can say for sure, space scientists know where to start looking—exoplanets. An exoplanet is a planet beyond the cradle of Earth’s eight-membered solar system. Hundreds of billions of these extraterrestrial kingdoms swarm our galaxy, and hundreds of sextillions of others exist beyond that.
While scientists know innumerous foreign worlds are out there, researchers have only pinpointed the existence of a tiny fraction. Being generally smaller, colder and much darker than a star, exoplanets are also harder to detect, so specialized techniques are required for capturing their faint signals. Planets exert gentle influence on nearby stars, so by watching the wobble, dimming or distortion of starlight, scientists can infer the presence of these elusive targets.
Since the turn of the millennium, thanks to advancing technology, we have been in a golden age of exoplanet discovery. Space-based telescopes such as Kepler and the Transiting Exoplanet Survey Satellite (TESS) have helped comb through patches of sky to identify interesting targets, while the James Webb Space Telescope and ground-based observatories allow for a deeper second look. As a result, we now know that the universe is littered with other worlds—they even surpass the number of stars. Understanding each new cosmic island in the dark ocean of space will help us answer important questions, such as how Earth came to be the way it is. Each exoplanet is a celestial companion that may reveal something about our place in the vast cosmos. Here are some of the most exciting of Earth’s distant neighbors.
The exoplanet with volcanic rage
Hot and Volcanic Planet
An artistic illustration of a hot and volcanic planet at the doorstep of its star Mark Garlick / Science Photo Library via Getty Images
A planetary sibling fight has driven one of the most volcanic worlds discovered to date. Twenty parsecs away from Earth, three planets encircle the star HD 104067, but the innermost one, TOI-6713.01, has been bossed around by the larger outer planets, to fiery consequences. Planets naturally prefer a circular orbit around their star, but in the case of TOI-6713.01, the outer planets’ gravitational influence bends it onto a more eccentric path. The result of this gravitational tug-of-war is that the planet is internally heated, so that excess heat bubbles out as active volcanism on the surface.
“It’s what I refer to as a tidal storm,” says Stephen Kane, a planetary astrophysicist at the University of California, Riverside, who discovered the planet from analysis of TESS’s registers. “Essentially, this planet is being squeezed so much that it has the surface temperature of a small star.”
Kane’s team calculated that temperatures soar to 2,600 Kelvin on TOI-6713.01’s surface. “This thing must be erupting volcanoes all over the place,” Kane says.
The exoplanet with a strange course
D 20782 b
An artist’s rendering of the eccentric HD 20782 b getting flash-heated as it makes its closest approach to its star NASA’s Goddard Space Flight Center
The crown for the planet with the most eccentric orbit goes to HD 20782 b. On a scale of zero to one, where zero denotes a perfectly circular trajectory and one denotes an object’s tight slingshot around its star before leaving for good, HD 20782 b’s orbit scores a 0.96. By contrast, Earth’s eccentricity is 0.0167. HD 20782 b barely hangs on by the gravitational leash of its star for most of its orbit, yet when it makes a closest approach it swings by so closely that it likely skims through the star’s atmosphere and gets flash-heated. “It must go through an incredible stress when it passes close to the star,” says Kane, before it goes off into the cold void.
Exactly how HD 20782 b came to be so eccentric is a mystery. Left untampered with, new planets usually form circular and symmetric orbits. And unlike the volcanic TOI-6713.01, HD 20782 b is the sole planetary occupant in its solar system. “Something very bad happened,” Kane suspects of its history. One theory suggests that in the past HD 20782 b had a close encounter with a passer-by star that yanked the planet out of harmony. Or the eccentric planet threw all its other planetary siblings out of the solar system, but not without suffering some long-term gravitational trauma.
“After a planet forms from the disk around the star, that is clearly not the end of the story,” Kane says. “It makes you realize how fortunate we are in [our] solar system to have orbits that are reasonably OK.”
The exoplanet that led to an age of discovery
HD 209458 b
An illustration of HD 209458 b, an exoplanet that lies close to its star NASA / ESA / Alfred Vidal-Madjar (Institut d’Astrophysique de Paris, CNRS)
HD 209458 b is a gas giant larger than Jupiter spinning 158 light-years away from Earth. The exoplanet has a 3.5-day orbital period, presses close to its star and runs hot—and all these factors make it easy to spot. This planet isn’t exactly peculiar, but it’s significant because space scientists owe humankind’s explosion of exoplanet discovery in the last decade to HD 209458 b.
In the early 1990s, an astronomer named William Borucki was struggling to get NASA to greenlight the construction of a space telescope exclusively in the business of seeking out new exoplanets. He was pushing for a brand-new technique for detecting exoplanets by measuring the blip in light when a planet passes in front of a star, known as the transit method. But many scientists were skeptical whether the technique would pay off. For two decades Borucki refined his proposals, only to meet with multiple rejections. Only after HD 209458 b, a known exoplanet by then, became the first target to be detected by the transit method using ground-based telescopes did Borucki’s mission get off the ground. His relentless campaigning led to the design and construction of the space telescope Kepler, the most prolific exoplanet hunter to date.
From 2009 and 2018, Kepler revealed that the universe was full of exoplanets. Among the 5,500-plus worlds known to humankind so far, Kepler contributed to the discovery of half of that tally. Now, scientists agree that not launching Kepler would have been unthinkable to the progress of space science. “HD 209458 b changed the narrative forever,” Kane says. “That’s a really significant planet for the history of astronomy.”
The exoplanet with an ocean of lava
55 Cancri e
Lava pools on the scorching exoplanet 55 Cancri e, as shown in this artist’s concept. Stocktrek Images via Getty Images
No one would be eager to call 55 Cancri e home. The rocky world lies so close to its star that it only takes 0.7 Earth days to complete a full loop. Scientists estimate that the temperature on the sun-facing side is around 6,000 degrees Fahrenheit. “It’s hotter than what it would take to melt rocks,” says Renyu Hu, a planetary scientist and astronomer at NASA’s Jet Propulsion Laboratory. So, the surface is likely molten—an ocean of lava.
Still, 55 Cancri e has some tricks up its sleeve. Recently, a new study found that the sun-seared planet has somehow managed to retain an atmosphere. A planet so close to its star usually loses its gassy sheath to the barrage of energetic particles from the sun, collectively known as solar wind. But recent observations of55 Cancri eusing the James Webb Space Telescope show a bona fide atmosphere rich in carbon dioxide or carbon monoxide. “It’s actually the first confirmed observation of any rocky exoplanet having an atmosphere,” says Hu, who was a co-author of the study.
The exoplanets as puffy as cotton candy
Kepler-51
An illustration of the star Kepler-51 surrounded by three exoplanets that have extraordinarily low density NASA, ESA, and L. Hustak, J. Olmsted, D. Player and F. Summers (STScI)
Super-puff planets have tiny cores and thick gassy coats of hydrogen or helium, such that their overall density is as light as cotton candy. This type of exoplanet is relatively rare, but 2,615 light-years away, the star named Kepler-51 is surrounded by not one but three super-puffs that are the size of Saturn but weigh only slightly more than Earth. This past December, researchers announced that they may have discovered a fourth member in that ultra-light star system.
These low-density specimens draw curiosity because they’ve somehow hung onto their gases despite the intense radiation from the young and vibrant Kepler-51. As with all far-flung worlds that barely graze our vision, scientists are befuddled by the bizarre nature of the Kepler-51 system’s members and are struggling to explain it all. Perhaps their fluffed sizes are due to photochemical hazes drifting high up into the atmosphere and clouding our vision, making these planets look bigger than they actually are. “We don’t know why [they’re] so puffy,” says Xinting Yu, a planetary scientist at the University of Texas at San Antonio. “So that’s a mystery still.”
The exoplanet that looks like a comet
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Comets are icy objects that form tails once they near their star and it warms the ice off their surface. But growing a tail is possible for planets, too. A hot Jupiter-sized exoplanet 163 light-years from Earth called WASP-69 b has a luscious tail 350,000 miles long, at least seven times longer than the planet itself and about one and a half times the distance from Earth to the moon. The tail comes thanks to the excessive solar wind that pummels the planet, which resides close to the sun.
Scientific calculations estimate that WASP-69 b is losing 200,000 tons of gas every second, the mass equivalent of two Washington Monuments. But since the planet is quite hefty, about a third of the mass of Jupiter, it can afford to shed mass for a while.
The exoplanet that’s dark as night
TrES-2b
An artist’s depiction of TrES-2b JohnVanVliet via Wikipedia under CC By-SA 4.0
TrES-2 b was discovered in 2006 using several ground-based telescopes. One thing was clear right away about this exoplanet: It’s a dark one. Measurements indicate that the surface only reflects less than a hundredth of the light it receives, making it inkier than coal or black acrylic paint. The exoplanet is so dark that if you were traveling inside the atmosphere, you’d be flying blind. Deeper within the planet, though, the surface might be emanating a faint red, due to incandescence resulting from being baked by its star.
Researchers have yet to fully understand why the planet is so dark. Because the planet lies close to its sun and roils at above 1,800 degrees Fahrenheit, its skies are likely devoid of clouds, which tend to reflect sunlight and make planets bright. Scientists also think that the atmosphere is populated with light-absorbing metallic gases such as sodium and potassium that help shroud the planet in darkness.
The exoplanet that’s hotter than a star
KELT-9 b
An artist’s concept of KELT-9 b, a feverish exoplanet that owes its hot temperatures to its proximity to its young star NASA’s Goddard Space Flight Center / Chris Smith, USRA
The future is looking bright for KELT-9 b—perhaps, too bright. The planet lies so close to its star that its surface is being sizzled off by the heat. With dayside surface temperatures about 7,800 degrees Fahrenheit, KELT-9 b is the hottest exoplanet that scientists have discovered so far. The exoplanet is hotter than the K- and M-class stars, the coolest classes of stars. It doesn’t help that KELT-9b’s star, the 300-million-year-old KELT-9, is a young hothead compared with the rest of its peers across the universe. The star is roughly twice as large and hot as our sun, making for harsh conditions for any planet at its doorstep. The planet’s discovery team speculated that the star’s ultraviolet bombardment may completely evaporate the planet, if the star doesn’t swell into a red giant and engulf it first.
The exoplanet was first detected in 2016 when ground telescopes caught it zipping around KELT-9 and blotting out some starlight every 1.5 days. Researchers think that the atmosphere is too hot to contain the usual molecules such as water and carbon dioxide. Instead, its atmosphere is probably chock-full of vaporized atomic metals—like stars of similar temperatures.
The exoplanets that are wanderers
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Many planets aren’t bound to a star but instead roam freely across the stretches of space. The scientific community calls these cold, desolate bodies rogue planets. They form when their stellar orbits destabilize, ending with their exile from a solar system. Or they simply coalesce into existence from a swirl of gas and dust just as a star would, but they never quite make it so big.
Studies have found that untethered planets aren’t rare, only hard to detect. Without a background star to interact with regularly, rogue planets only reveal themselves if scientists happen to be looking at the right place at the right time. To suss them out, skywatchers stay hypervigilant for any one-off dip in starlight when a rogue planet crosses serendipitously in front of a star. James Webb’s sensitive thermal instruments can also directly catch the faint heat signals that rogue planets emit—last year, it found 540 of them in the Orion Nebula. The most promising technique for detecting these bodies is microlensing, whereby a rogue planet warps the light from a star in a telltale fashion to reveal its presence. The year 2027 promises to be a revolutionary one for the discovery of these free-floaters with the launch of NASA’s Nancy Grace Roman Space Telescope, a microlensing specialist that will survey the night sky from a perch beyond Earth’s distortive atmosphere.
A statistical analysis computed that trillions of orphan worlds populate the universe. In fact, they may outnumber star-bound bodies seven-to-one in our galaxy alone. So rogue planets aren’t the strange ones in our universe. The outliers are orbs like Earth—those twirling among a family of planets, at home in the warmth of a star.