The Planetary Report

June Solstice 2024

From Our Member Magazine

Meet the superlative exoplanets

School yearbooks have a tradition of awarding students superlative titles, recognizing those who were the best at something or those who exemplified a quality. In that same spirit, meet our picks for the galaxy’s superlative exoplanets.

MOST POPULAR: Kepler-186f
If an exoplanet could be voted prom king or queen, it would be Kepler-186f. It was one of only six exoplanets to be featured on NASA’s space tourism poster series because when it was discovered in 2014, it was the most Earthlike exoplanet ever found. Kepler-186f is about 10% larger and 70% more massive than Earth and almost certainly rocky. An orbital period of 130 days also puts Kepler-186f squarely in the habitable zone of its star. That star, however, is only about half the size and mass of the Sun, so the hunt for an Earth twin continued.

Kepler-186f tourism poster
Kepler-186f tourism poster This NASA space tourism poster invites people to imagine visiting Kepler-186f.Image: NASA / JPL

YZ Ceti b, the first rocky planet found with a possible magnetic field, is a clear vote for the planet with the best “hair.” A few planets with magnetic fields had previously been found, but they were all gas giants whose fields should be bigger and easier to detect. YZ Ceti b is a rocky planet a bit smaller and less dense than Earth, and it orbits its red dwarf host star every two days. YZ joined the exoplanet class when it was discovered in 2017, but we didn’t know about its potential magnetic field until astronomers noticed repeated radio signals coming from the planet in 2023. More observations are necessary to confirm the magnetic field, but if true, it could be a big step in the search for a habitable world because we know Earth’s magnetic field has been so beneficial to our survival.

YZ Ceti b
YZ Ceti b This artist’s impression shows YZ Ceti b, the first rocky exoplanet thought to have a magnetic field.Image: NASA

Among exoplanets, the Most Athletic award would surely have to go to the biggest, strongest, fastest one around: TOI 4201b. This planet was a latecomer to the class, just discovered in 2023, but its athletic prowess is impossible to deny. Weighing in at 2 1/2 times the mass of Jupiter, 4201b is one of the biggest planets found orbiting a red dwarf, yet it can still sprint a full lap in just 3 1/2 days. This planet’s size is especially impressive because long-lived red dwarf stars have less extra material and typically contain fewer heavy elements than more massive stars, which makes it harder to form big planets.

CLASS CLOWN: Proxima Centauri b
The nearest star to our Sun is Proxima Centauri, a small red dwarf about four light-years away that orbits a pair of Sunlike stars every half-million years. Even though it’s so close, Proxima is invisible to the unaided eye and wasn’t discovered until 1915. It took another century and year to find Proxima’s first planet, Proxima Centauri b. Our nearest neighbor planet is practically Earth-size, just a tad bigger and more massive. It orbits its star every 11 days, well inside the habitable zone, where astronomers had unsuccessfully searched for a planet for decades. Proxima b’s ability to evade detection (clearly learned from its parent star, so the family is full of tricksters) is the greatest joke a planet has ever played on planet hunters.

Discovered in 2012, Wasp J1407b is so far the only planet — or potentially brown dwarf — beyond our Solar System to be found with rings, and they put Saturn’s style to shame. This super Saturn transits a young star slightly less massive than the Sun, and data from the system match a model of 30 different rings, each tens of millions of miles wide — 200 times bigger than Saturn’s. J1407b even shows signs of an exomoon that has cleared a path in the rings. The varied accessories made this vote a no-brainer.

Super Saturn J1407b
Super Saturn J1407b This artist’s impression shows the exoplanet Wasp J1407b and its astounding set of rings.Image: NASA / Ron Miller

Most Eccentric may not be the yearbook award you’d want to win, but it’s a remarkable feat among planets. Eccentricity is a measure of how not round a planet’s orbit is, and most planets’ orbits are close to circular — not HD 20782b’s, though. It has the most eccentric orbit found to date since its discovery in 2006. The planet itself is bigger than Jupiter and orbits its Sunlike star every 1.6 years. On a scale from zero to one, this planet’s eccentricity is 0.95. Throughout its orbit, it can come as close as 105 million kilometers (65 million miles) to its star or as far as 400 million kilometers (250 million miles). This definitely creates drastic temperature and pressure variations on HD 20782b over the course of its eccentric orbit, most likely causing extreme weather in the planet’s atmosphere.

If success means the potential to host life, there are no obvious winners in this category, but one exoplanet seems to meet more criteria than the rest: K2 136b. It’s a dense super Earth that orbits a metal-rich K-type star every eight days. What earns it this title is a combination of its galactic location and host star properties. Planet hunters place a lot of emphasis on the habitable zone around a star, and there are hundreds of planets in their local “Goldilocks zone.” To stand out, an exoplanet has to be in the galactic habitable zone as well. This is the region in a galaxy that meets most or all of the large-scale criteria for life. Over the last few decades, the list of conditions has grown to include distance from extremely radiative events, like supernovae; availability of heavy elements; and a stable stellar orbit over billions of years. Recently, studies have suggested that K-type stars might be the best hosts of habitable planets because they’re longer-lived and more prevalent than Sunlike stars but often less magnetically active than M dwarfs. Let’s just hope K2 136b can live up to its potential.

With over 5,500 planets identified around other stars, you can find plenty of standouts in the exoplanet yearbook. But it’s the lessons we astronomers have learned from the class as a whole that really changed our understanding of the Universe. Now we know that most stars have planets and that rocky planets like Earth are common. We know that stellar systems are amazingly diverse and that planets can migrate away from where they formed. Best of all, we know the field of exoplanet research is just getting started.

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The Planetary Report • June Solstice

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