In the 1980s, we knew of no planets outside our Solar System. This week, NASA announced more than 5,000 exoplanets. It’s quite the leap from the discovery of an odd set of planets around a dead star in 1992 that slowly opened the floodgates to more and more planets — especially when, in 1995, the first exoplanet was confirmed around a Sun-like star.
Countless astronomers have spent countless hours getting us to the 5,005 mark across multiple observatories. Some have spent all entire careers finding worlds that once seemed elusive, out of reach, or even non-existent.
In celebration of the 5,000th (plus five) exoplanet, we asked a few exoplanet astronomers the impossible question: Which, of those 5,005, is your favorite exoplanet? Here are their top picks.
“It's hard to believe when I started in Astronomy, less than 20 years ago, there was only about 100 known, and now we're at 5,000,” Jeff Coughlin, a researcher at the SETI Institute and former director of the Kepler and K2 Science Office, tells Inverse. “I think my younger self would be in shock to hear that we would discover so many so fast.”
Coughlin worked intimately with the Kepler mission over the years. Launched in 2009, the telescope spent nearly four years in space looking for barely perceptible eclipses across stars caused by planets passing in front of them, all located in a small patch of sky. When a broken reaction wheel rendered the craft unable to hold still long enough to do long-term surveys, it turned to short-term targets in the K2 mission before ultimately running out of fuel in late 2018.
“I think my younger self would be in shock.”
The Kepler mission is responsible for finding more than 2,600 planets — a majority of the worlds we know. With thousands of worlds to choose from, Coughlin still can choose a favorite.
“My favorite by far is Kepler-1649c,” he says. “In terms of its size and habitable zone location, It's the most Earth-like planet ever found by Kepler, and one of the most Earth-like ever found to date.”
The planet also wasn’t found in the original run. It was discovered in 2020 using sophisticated algorithms and a little help from the amateur astronomy community.
“Who knows what other planets may be lurking in datasets just waiting to be found?” Coughlin says. “From Kepler's data we also now know that about half of stars have a rocky planet in their habitable zone, and planets like Kepler-1649c are abundant in the universe.”
“It's been the experience of a lifetime to have been part of both discoveries, and I have no doubt I'll be looking back 20 years from today equally in shock as we continue to discover thousands of more exoplanets and study their atmospheres.”
Jessie Christiansen is the lead scientist at the NASA Exoplanet Archive and was previously a staff scientist with the Kepler mission. She has also worked with the Transiting Exoplanet Sky Survey (TESS) mission scientists as part of the TESS Follow-up Working Group. Unlike Kepler, TESS looks at the whole sky rather than a narrow patch.
Christiansen’s work also reaches the public, which makes it difficult for her to pick just one exoplanet.
“My professional answer is ‘the next one,’ because every new planet is super cool and another world filling in the void around us,” she tells Inverse. “My personal answer is the K2-138 system, which is a six-planet system discovered by citizen scientists that plays music!”
K2-138, as the name implies, was discovered in K2 mission data. A group of citizen scientists found convincing evidence for five planets and strong evidence for a sixth, which Christiansen and her team validated. The tightly-packed, mostly-Neptune-sized worlds are in resonance with each other, meaning that for every three orbits one completes, the next one orbits twice.
This makes their orbits rhythmic, producing a sort of perfect cosmic music if converted to sound. It’s a little more John Cage than Johnny Cash, but beautiful nonetheless.
7. HIP 41378f
While Earth-sized worlds may get all the glory, there is a vast diversity of planets out there — weird shapes, weird sizes. Some giant worlds are remarkably weird. Take, for instance, HIP 41378f. It’s deceptively big, but that large size holds a planet that’s a bit puffy.
Mercedes López-Morales, an astronomer at the Center for Astrophysics, spends much of her time studying exoplanet atmospheres — a field of astronomy that will experience a boom under the James Webb Space Telescope. She tells Inverse that HIP 41378f could be hiding a secret that could make it an exo-Saturn.
“I have several favorites, but the planet I am currently most excited about is HIP 41378f,” López-Morales says. “This planet is the size of Saturn, but has about eight times less mass. At face value it appears it be super-puffy and we have no idea how a planet like that can form, unless ..... the planet has rings!”
Exoplanets with rings aren’t unprecedented. Astronomers have their suspicions that they exist, including around a world in-between the size of Earth and Neptune orbiting Proxima Centauri. But HIP 41378f’s rings might transit its star, allowing astronomers to glimpse the rings via Webb. But HIP 41378f might be the first exoplanet we confirm them around.
“We are right now tryin to test that theory with observations from Hubble and JWST,” López-Morales says. “If the planet indeed has rings, it will be the first time we detect rings around an exoplanet and that is super exciting.”
6. Proxima Centauri b
Abel Méndez, director of the Planetary Habitability Laboratory at the University of Puerto Rico Arecibo, is all about finding life beyond Earth. He regularly updates the Habitable Exoplanets Catalog, keeping track of worlds that are near enough to their star for liquid water to not evaporate, but not so far that the water would freeze — often called the habitable or Goldilocks zone.
To make the cut on the most stringent list in the catalog, planets have to be rocky (between half and 1.6 times the radius of Earth) and no heavier than three Earth masses. Only 21 of those planets make the cut. And while all eyes are on a system 39 light years away with seven Earth-sized worlds, Méndez likes to stay closer to home.
A technology called an occulter — which blocks out starlight and is in limited use in astronomy now but has yet to be paired with a space telescope — could block out the light of Proxima Centauri, a star just 4.24 light years away. The system is known to have at least three planets. One of those planets, Proxima Centauri b, is in the habitable zone of Proxima and thus able to be studied by this technology in tantalizing ways.
“Ground and space telescopes with an occulter would be able not only to sample the atmosphere but also [determine] land/ocean ratios, if any,” Méndez says.
5-4. Kepler-1708b or Kepler-1625b
David Kipping, an assistant professor of astronomy at Columbia University. In his work at the Cool Worlds Lab, he usually spends his time trying to find planets farther out from their star, which would be more in line with Jupiter or Saturn and find thus find systems more like our own Solar System.
Part of that work ends up looking at farther out gas giants, with the hope of finding not just planets, but the sorts of things that come with giant planets — including rings and moons.
“My favorite exoplanet is the next one!” Kipping tells Inverse, which we’ve heard a time or two before ... “But seriously I would pick Kepler-1708b or Kepler-1625b for their exciting nature of having the only exomoon candidates we know of.”
Kepler-1708b and Kepler-1625b are both gas giants quite a bit more massive than Jupiter in long-period orbits. But they have something that really excites astronomers: strong evidence for moons, as discovered by Kipping and his graduate students. The moons would be quite unusual by our Solar System standards — if they exist. Both are around the size and mass of Neptune and would be beyond a doubt planets if they had their own orbits.
3-1. When you can’t choose just one exoplanet
Cullen Blake, an associate professor of astronomy at the University of Pennsylvania, wants to find more planets like those around Proxima Centauri — Earth-like worlds around tiny stars. He’s been involved with various projects over the years, including the MINERVA telescopes and the NN-explore Exoplanet Investigations with Doppler spectroscopy instrument in development for the Kitt Peak Observatory, in the hopes of characterizing such worlds.
Blake studied under David Charbonneau and David W. Latham, two pioneers in the field of exoplanet research, while in grad school at Harvard University. As such, he’s been able to see the field evolve over the past couple of decades and had a few objects impressed upon him in that time. He shares with Inverse a few of them.
OGLE 2003-BLG-235 — While astronomers typically rely on transits or looking for planets indirectly via the way they tug on their star, one method that also works is microlensing. A massive object distorts space and time around it — and if it passes in front of a distant object, can act like a magnifying glass for the more distant of the two, revealing planets that might have otherwise been too far away.
“I remember seeing a plot of this light curve (this was the first planet discovered through microlensing) before it was announced and thinking ... wow, relativity is totally amazing!” Blake says. “Bohdan Paczynski showed me the plot. He was also totally amazing and made important contributions in many areas of astronomy.”
HD 114762 b — HD 114762b is not considered a likely planet anymore — but upon its discovery in 1989 it remained an enigma, straddling the boundary between a planet and a larger object called a brown dwarf.
“This discovery was published in Nature in 1989 and some argued it was the first exoplanet ever discovered. The term ‘giant planet’ is in the abstract,” Blake says. But, there was ambiguity in the mass (it could have also been a more massive brown dwarf). More recent analyses using Gaia data indicate that it is probably too massive to be a planet.”
“Dave Latham was the first author of the 1989 paper. Dave has had (and continues to have) an amazing career and was a true pioneer in the field of exoplanets,” he adds. “He is also an incredibly gracious and generous guy.”
HD 209458 b — Before there was transiting, there was radial velocity. It looks at the spectra of a star to find slight changes in the position of a star caused by a planet tugging on it. Astronomers had suspected since the 1940s that the transit method would work to find planets, but needed photometers (which measure changes in light) sensitive enough to detect them first, which didn’t come until the late 20th century. Astronomers looked at planet HD 209548b in radial velocity first before turning to the transit method — and opening up the doors to other research.
“I love that Dave Charbonneau, as a graduate student, discovered this planet using very modest equipment observing from the parking lot of an institute in Boulder,” Blake says. “When I first saw this paper, I was definitely motivated to pursue a career in exoplanets. Dave went on to make a number of very important discoveries and is now world leader in our field.”
These planets may remain these astronomers’ favorites ... you know, until the next one.
This post was updated on March 28, 2022 to add a selection by Center for Astrophysics astronomer Mercedes López-Morales. It also clarified Jeff Coughlin’s position at the SETI Institute.