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For 76 years, Pluto was the beloved ninth planet. No one cared that it was the runt of the solar system, with a moon, Charon, half its size. No one minded that it had a tilted, eccentric orbit. Pluto was a weirdo, but it was our weirdo.
“Children identify with its smallness,” wrote science writer Dava Sobel in her 2005 book The Planets. “Adults relate to its inadequacy, its marginal existence as a misfit.”
When Pluto was excluded from the planetary display in 2000 at the American Museum of Natural History in New York City, children sent hate mail to Neil deGrasse Tyson, director of the museum’s planetarium. Likewise, there was a popular uproar when 15 years ago, in August 2006, the International Astronomical Union, or IAU, wrote a new definition of “planet” that left Pluto out. The new definition required that a body 1) orbit the sun, 2) have enough mass to be spherical (or close) and 3) have cleared the neighborhood around its orbit of other bodies. Objects that meet the first two criteria but not the third, like Pluto, were designated “dwarf planets.”
Science is not sentimental. It doesn’t care what you’re fond of, or what mnemonic you learned in elementary school. Science appeared to have won the day. Scientists learned more about the solar system and revised their views accordingly.
“I believe that the decision taken was the correct one,” says astronomer Catherine Cesarsky of CEA Saclay in France, who was president of the IAU in 2006. “Pluto is very different from the eight solar system planets, and it would have been very difficult to keep changing the number of solar system planets as more massive [objects beyond Neptune] were being discovered. The intention was not at all to demote Pluto, but on the contrary to promote it as [a] prototype of a new class of solar system objects, of great importance and interest.”
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There was a problem signing you up.For a long time, I shared this view. I’ve been writing about Pluto since my very first newspaper gig at the Cornell Daily Sun, when I was a junior in college in 2006. I interviewed some of my professors about the IAU’s decision. One, planetary scientist Jean-Luc Margot, who is now at UCLA, called it “a triumph of science over emotion. Science is all about recognizing that earlier ideas may have been wrong,” he said at the time. “Pluto is finally where it belongs.”
But another, planetary scientist Jim Bell, now at Arizona State University in Tempe, thought the decision was a travesty. He still does. The idea that planets have to clear their orbits is particularly irksome, he says. The ability to collect or cast out all that debris doesn’t just depend on the body itself.
Everything with interesting geology should be a planet, Bell told me recently. “I’m a lumper, not a splitter,” he says. “It doesn’t matter where you are, it matters what you are.”
Not everyone agrees with him. “Fifteen years ago we finally got it right,” says planetary scientist Mike Brown of Caltech, who uses the Twitter handle @plutokiller because his research helped knock Pluto out of the planetary pantheon. “Pluto had been wrong all along.”
But since 2006, we’ve learned that Pluto has an atmosphere and maybe even clouds. It has mountains made of water ice, fields of frozen nitrogen, methane snow–capped peaks, and dunes and volcanoes. “It’s a dynamic, complex world unlike any other orbiting the sun,” journalist Christopher Crockett wrote in Science News in 2015 when NASA’s New Horizons spacecraft flew by Pluto.
Observations from NASA’s New Horizons mission showed that the surface of Pluto’s Sputnik Planitia region is covered in churning nitrogen ice “cells” (white polygonal blocks) that constantly bring fresh material up to the surface from below.JHU-APL, NASA, SWRI
Closer views highlight the rugged water-ice mountains that border some of these cells.JHU-APL, NASA, SWRI
The New Horizons mission showed that Pluto has fascinating and active geology to rival that of any rocky world in the inner solar system. And that solidified planetary scientist Philip Metzger’s view that the IAU definition missed the mark.
“There was an immediate reaction against the dumb definition” when it was proposed, says Metzger, of the University of Central Florida in Orlando. Since then, he and colleagues have been refining their views: “Why do we have this intuition that says that it’s dumb?”
Retelling the tale
It turns out that the “we just learned more” narrative isn’t really true, Metzger says. Though the official story is that Pluto was reclassified because new data came in, it’s not that simple. Teaching that narrative is bad for science, and for science education, he says.
The truth is, there’s no single definition of a planet — and I’m beginning to believe that’s a good thing.
For centuries, the word “planet” was a much more inclusive term. When Galileo turned his telescope at Jupiter, any largish moving body in the sky was considered a planet — including moons. When astronomers discovered the rocky bodies we now call asteroids in the 1800s, those too were called planets, at least at first.
Pluto was considered a planet from the very beginning. When Clyde Tombaugh, an amateur astronomer from Kansas newly recruited to the Lowell Observatory in Flagstaff, Ariz., spotted it in photos taken in January 1930, he rushed to the observatory director and declared: “I have found your Planet X.”
Clyde Tombaugh, shown here with a homemade telescope, discovered Pluto in 1930 when he was 24 years old.GL Archive/Alamy Stock Photo
The discovery was no accident. In 1903, U.S. astronomer Percival Lowell hypothesized that a hidden planet seven times the mass of Earth orbited 45 times farther from the sun. Lowell had searched for what he called Planet X until he died in 1916. The search continued without him.
The new planet was thought to be “black as coal, nearly as dense as iron, twice as dense as the heaviest earthly surface rocks,” Science News Letter, the predecessor of Science News, reported in 1930.
Further research showed Lowell had grossly overestimated Pluto’s mass: It’s more like one five-hundredth the mass of Earth. Things got even weirder when scientists realized Pluto wasn’t alone out there. In 1992, an object about a tenth the diameter of Pluto was found orbiting the sun “in the deep freeze of space well beyond the orbits of Pluto and Neptune,” as Science News described it.
Since then, more than 2,000 icy bodies have been found hiding in that frigid zone dubbed the Kuiper Belt, and there are many more out there. Awareness of Pluto’s neighbors brought new questions: What characteristics could unite these strange new worlds with the more familiar ones? And what sets them apart? With so many new objects coming into focus, there was a growing desire for a formal definition of “planet.”
In 2005, Brown spotted the first of the Kuiper Belt bodies that seemed to be larger than Pluto. If Pluto was the ninth planet, then surely the new discovery, nicknamed Xena (in honor of the TV show Xena: Warrior Princess), should be the 10th. But if Xena was an icy leftover from the formation of the solar system undeserving of the “planet” title, so too was Pluto.
Tensions over how to categorize Pluto and Xena came to a head in 2006 at a meeting in Prague of the IAU. On the final day, August 24, after much acrimonious debate, a new definition of “planet” was put to a vote. Pluto and Xena got the boot. Xena was aptly renamed Eris, the Greek goddess of discord.
On August 24, 2006, in Prague, members of the International Astronomical Union voted for a new definition of planet that redesignated Pluto and its neighbor Eris as dwarf planets, shrinking the total number of planets in the solar system to eight.Michal Cizek/AFP/Getty Images
Textbooks were revised, posters were reprinted, but many planetary scientists, especially those who study Pluto, never bothered to change. “Planetary scientists don’t use the IAU’s definition in publishing papers,” Metzger says. “We pretty much just ignore it.”
In part that might be cheek, or spite. But Metzger and colleagues think there’s good reason to reject the definition. Metzger, Bell and others — including Alan Stern of the Southwest Research Institute, the planetary scientist who led the New Horizons mission and has argued since before the discovery of the Kuiper Belt that the solar system contains hundreds of “planets” — make their case in a pair of recent papers, one published in 2019 in Icarus and one forthcoming.
After examining hundreds of scientific papers, textbooks and letters dating back centuries, the researchers show that the way scientists and the public have used the word “planet” has changed over time, but not in the way most people think.
In and out
Consider Ceres, the first of what are now known as dwarf planets to be discovered. Located in the asteroid belt between Mars and Jupiter, Ceres was considered a planet after its 1801 discovery, too. It’s often said that Ceres was demoted after astronomers found the rest of the bodies in the asteroid belt. By the end of the 1800s, with hundreds of asteroids piling up, Ceres was stripped of its planetary title thanks to its neighbors. In that sense, the story goes, Ceres and Pluto suffered the same fate.
But that’s not the real story, Metzger and colleagues found. Ceres and other asteroids were considered planets, sometimes dubbed “minor planets,” well into the 20th century. A 1951 article in Science News Letter declared that “thousands of planets are known to circle our sun,” although most are “small fry.” These “baby planets” can be as small as a city block or as wide as Pennsylvania.
The dwarf planet Ceres orbits in the asteroid belt. It was also once considered a planet. NASA’s Dawn mission visited the dwarf planet in 2015 and found that it is also a geologically interesting world.JPL-Caltech, NASA, UCLA, MPS, DLR, IDA
It wasn’t until the 1960s, when spacecraft offered better observations of these bodies, that the term “minor planets” fell out of fashion. While the largest asteroids still looked planetlike, most small asteroids turned out to be lumpy and irregular in shape, suggesting a different origin or different geophysics than bigger, rounder planets. The fact that asteroids didn’t “clear their orbits” had nothing to do with the name change, Metzger argues.
And what about moons? Scientists called them “planets” or “secondary planets” until the 1920s. Surprisingly, it was nonscientific publications, notably astrological almanacs that used the positions of celestial bodies for horoscope readings, that insisted on the simplicity of a limited number of planets moving through the fixed sphere of stars.
Metzger thinks that older definition of a planet that included moons was forgotten when planetary science went through a “Great Depression” between about 1910 and 1950. So many asteroids had been discovered that searching for new ones or refining their orbits was getting boring. Telescopes weren’t good enough to start exploring asteroids’ geology yet. Other parts of space science were way more exciting, so attention went there.
But new data that came with space travel brought moons back into the planetary fold. Starting in the 1960s, “planet” reappeared in the scientific literature as a description for satellites, at least the large, round ones.
Real-world usage
The planet definition that includes certain moons, asteroids and Kuiper Belt objects has had staying power because it’s useful, Metzger says. Planetary scientists’ work includes comparing a place like Mars (a planet) to Titan (a moon) to Triton (a moon that was probably born in the Kuiper Belt and captured by Neptune long ago) to Pluto (a dwarf planet). It’s scientifically useful to have a word to describe the cosmic bodies where interesting geophysics, including the conditions that enable life, occur, he says. There’s all sorts of extra complexity, from mountains to atmospheres to oceans and rivers, when rocky worlds grow big enough for their own gravity to make them spherical.
Pluto and hundreds or thousands of other objects that rival Pluto in size and interest orbit in the icy back of the solar system’s fridge, called the Kuiper Belt (white fuzzy ring).NASA
“We’re not claiming that we have the perfect definition of a planet and that all scientists ought to adopt our definition,” he adds. That’s the same mistake the IAU made. “We’re saying this is something that ought to be debated.”
A more inclusive definition of “planet” would also give a more accurate concept of what the solar system is. Emphasizing the eight major planets suggests that they dominate the solar system, when in fact the smaller stuff outnumbers those worlds tremendously. The major planets don’t even stay put in their orbits over long time-scales. The gas giants have shuffled around in the past. Teaching the view of the solar system that includes just eight static planets doesn’t do that dynamism justice.
Caltech’s Brown disagrees. Having the gravitational oomph to nudge other bodies in and out of line is an important feature of a world, he says. Plus, the eight planets clearly dominate our solar system, he says. “If you dropped me in the solar system for the first time, and I looked around and saw what was there, nobody would say anything other than, ‘Wow, there are these eight — choose your word — and a lot of other little things.’ ”
Pluto rises above the horizon of its largest moon, Charon, in this illustration.Mark Garlick/Science Photo Library/GettyImages Plus
Thinking of planets that way leads to big-picture questions about how the solar system put itself together.
One common argument in favor of the IAU’s definition is that it keeps the number of planets manageable. Can you imagine if there were hundreds or thousands of planets? How would the average person keep track of them all? What would we print on lunch boxes? I’m not making fun of this idea; as an astronomy writer who has been obsessed with space since I was 8, I would be reluctant to turn people off to the planets.
But Metzger thinks teaching just eight planets risks turning people off to all the rest of space. “Back in the early 2000s, there was a lot of excitement when astronomers were discovering new planets in our solar system,” he says. “All that excitement ended in 2006.” But those objects are still out there and are still worthy of interest. By now, there are at least 150 of these dwarf planets, and most people have no clue, he says.
This is the argument I find most compelling. Why do we need to limit the number of planets? Kids can memorize the names and characteristics of hundreds of dinosaurs, or Pokémon, for that matter. Why not encourage that for planets? Why not inspire students to rediscover and explore the space objects that most appeal to them?
I’ve come to think that what makes a planet may just be in the eye of the beholders. I may be a lumper, not a splitter, too.
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Pluto continues to charm us all, as shown in these 2015 interviews after New Horizons sent its images of the geologic richness of the dwarf planet. MoreBurning bits of ground-up meteorites may tell scientists what exoplanets’ early atmospheres are made of.
A set of experiments baking the pulverized space rocks suggests that rocky planets had early atmospheres full of water, astrophysicist Maggie Thompson of the University of California, Santa Cruz reported January 15 at the virtual meeting of the American Astronomical Society. The air could also have had carbon monoxide and carbon dioxide, with smaller amounts of hydrogen gas and hydrogen sulfide.
Astronomers have discovered thousands of planets orbiting other stars. Like the terrestrial planets in the solar system, many could have rocky surfaces beneath thin atmospheres. Existing and future space telescopes can peek at starlight filtering through those exoplanets’ atmospheres to figure out what chemicals they contain, and if any are hospitable to life (SN: 4/19/16).
Thompson and her colleagues are taking a different approach, working from the ground up. Instead of looking at the atmospheres themselves, she’s examining the rocky building blocks of planets to see what kind of atmospheres they can create (SN: 5/11/18).Sign Up For the Latest from Science News
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The researchers collected small samples, about three milligrams per experiment, of three different carbonaceous chondrite meteorites (SN: 8/27/20). These rocks are the first solids that condensed out of the disk of dust and gas that surrounded the young sun and ultimately formed the planets, scientists say. The meteorites form “a record of the original components that formed planetesimals and planets in our solar system,” Thompson said in a talk at the AAS meeting. Exoplanets probably formed from similar stuff.
The researchers ground the meteorites to powder, then heated the powder in a special furnace hooked up to a mass spectrometer that can detect trace amounts of different gases. As the powder warmed, the researchers could measure how much of each gas escaped.
That setup is analogous to how rocky planets formed their initial atmospheres after they solidified billions of years ago. Planets heated their original rocks with the decay of radioactive elements, collisions with asteroids or other planets, and with the leftover heat of their own formation. The warmed rocks let off gas. “Measuring the outgassing composition from meteorites can provide a range of atmospheric compositions for rocky exoplanets,” Thompson said.
All three meteorites mostly let off water vapor, which accounted for 62 percent of the gas emitted on average. The next most common gases were carbon monoxide and carbon dioxide, followed by hydrogen, hydrogen sulfide and some more complex gases that this early version of the experiment didn’t identify. Thompson says she hopes to identify those gases in future experimental runs.
The results indicate astronomers should expect water-rich steam atmospheres around young rocky exoplanets, at least as a first approximation. “In reality, the situation will be far more complicated,” Thompson said. Planets can be made of other kinds of rocks that would contribute other gases to their atmospheres, and geologic activity changes a planet’s atmosphere over time. After all, Earth’s breathable atmosphere is very different from Mars’ thin, carbon dioxide-rich air or Venus’s thick, hot, sulfurous soup (SN: 9/14/20).
Still, “this experimental framework takes an important step forward to connect rocky planet interiors and their early atmospheres,” she said.
This sort of basic research is useful because it “has put a quantitative compositional framework on what those planets might have looked like as they evolved,” says planetary scientist Kat Gardner-Vandy of Oklahoma State University in Stillwater, who was not involved in this new work. She studies meteorites too and is often asked whether experiments that crush the ancient, rare rocks are worth it.
“People inevitably will ask me, ‘Why would you take a piece of a meteorite and then ruin it?’” she says. “New knowledge from the study of meteorites is just as priceless as the meteorite itself.” MoreA surprisingly bright cosmic blast might have marked the birth of a magnetar. If so, it would be the first time that astronomers have witnessed the formation of this kind of rapidly spinning, extremely magnetized stellar corpse.
That dazzling flash of light was made when two neutron stars collided and merged into one massive object, astronomers report in an upcoming issue of the Astrophysical Journal. Though the especially bright light could mean that a magnetar was produced, other explanations are possible, the researchers say.
Astrophysicist Wen-fai Fong of Northwestern University in Evanston, Ill., and colleagues first spotted the site of the neutron star crash as a burst of gamma-ray light detected with NASA’s orbiting Neil Gehrels Swift Observatory on May 22. Follow-up observations in X-ray, visible and infrared wavelengths of light showed that the gamma rays were accompanied by a characteristic glow called a kilonova.
Kilonovas are thought to form after two neutron stars, the ultradense cores of dead stars, collide and merge. The merger sprays neutron-rich material “not seen anywhere else in the universe” around the collision site, Fong says. That material quickly produces unstable heavy elements, and those elements soon decay, heating the neutron cloud and making it glow in optical and infrared light (SN: 10/23/19).
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A new study finds that two neutron stars collided and merged, producing an especially bright flash of light and possibly creating a kind of rapidly spinning, extremely magnetized stellar corpse called a magnetar (shown in this animation).
Astronomers think that kilonovas form every time a pair of neutron stars merge. But mergers produce other, brighter light as well, which can swamp the kilonova signal. As a result, astronomers have seen only one definitive kilonova before, in August 2017, though there are other potential candidates (SN: 10/16/17).
The glow that Fong’s team saw, however, put the 2017 kilonova to shame. “It’s potentially the most luminous kilonova that we’ve ever seen,” she says. “It basically breaks our understanding of the luminosities and brightnesses that kilonovae are supposed to have.”
The biggest difference in brightness was in infrared light, measured by the Hubble Space Telescope about 3 and 16 days after the gamma-ray burst. That light was 10 times as bright as infrared light seen in previous neutron star mergers.
“That was the real eye-opening moment, and that’s when we scrambled to find an explanation,” Fong says. “We had to come up with an extra source [of energy] that was boosting that kilonova.”
Her favorite explanation is that the crash produced a magnetar, which is a type of neutron star. Normally, when neutron stars merge, the mega-neutron star that they produce is too heavy to survive. Almost immediately, the star succumbs to intense gravitational forces and produces a black hole.
But if the supermassive neutron star is spinning rapidly and is highly magnetically charged (in other words, is a magnetar), it could save itself from collapsing. Both the support of its own rotation and dumping energy, and thus some mass, into the surrounding neutron-rich cloud could keep the star from turning into a black hole, the researchers suggest. That extra energy in turn would make the cloud give off more light — the extra infrared glow that Hubble spotted.Sign Up For the Latest from Science News
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But there are other possible explanations for the extra bright light, Fong says. If the colliding neutron stars produced a black hole, that black hole could have launched a jet of charged plasma moving at nearly the speed of light (SN: 2/22/19). The details of how the jet interacts with the neutron-rich material surrounding the collision site could also explain the extra kilonova glow, she says.
If a magnetar was produced, “that could tell us something about the stability of neutron stars and how massive they can get,” Fong says. “We don’t know the maximum mass of neutron stars, but we do know that in most cases they would collapse into a black hole [after a merger]. If a neutron star did survive, it tells us about under what conditions a neutron star can exist.”
Finding a baby magnetar would be exciting, says astrophysicist Om Sharan Salafia of Italy’s National Institute for Astrophysics in Merate, who was not involved in the new research. “A newborn highly magnetized, highly rotating neutron star that forms from the merger of two neutron stars has never been observed before,” he says.
But he agrees that it’s too soon to rule out other explanations. What’s more, recent computer simulations suggest that it might be difficult to see a newborn magnetar even if it formed, he says. “I wouldn’t say this is settled.”
Observing how the object’s light behaves over the next four months to six years, Fong and her colleagues have calculated, will prove whether or not a magnetar was born.
Fong herself plans to keep following up on the mysterious object with existing and future observatories for a long time. “I’ll be tracking this till I’m old and grey, probably,” she says. “I’ll train my students to do it, and their students.” MorePASADENA, Calif. — The faint dwarf galaxies in a nearby galaxy group seem to have missed the memo. Instead of being dispersed evenly around the group’s most massive galaxy, which is what happens in our own galaxy group, these newly found dwarfs cluster in one region. And astronomers don’t know why.
“This satellite distribution is just weird,” astronomer Eric Bell said June 13 at the American Astronomical Society meeting.
Bell, of the University of Michigan in Ann Arbor, and colleagues used the Subaru telescope in Hawaii to hunt for faint clumps of stars, indicating dwarf galaxies, around the galaxy M81. This Milky Way–like galaxy is the most prominent member in a relatively nearby group of galaxies, all about 12 million light-years from Earth. The team found one definite dwarf galaxy and six possible fainter ones.
Most of the known satellite galaxies (circled in red) in the M81 galaxy group, along with seven newfound candidates (yellow), seem to cluster toward one side of the galaxy M81 (center).Sloan Digital Sky Survey
“The part that’s just bananas,” Bell said, is that the newfound satellite galaxies all sit on one side of M81.
Computer simulations of galaxy evolution suggest that the largest galaxies have many faint, small galaxies sprinkled uniformly throughout the outer part of the dominant galaxy’s diffuse cloudlike halo. Observations in our galaxy group back this up: The dozens of dwarf galaxies known to orbit in the Milky Way’s outskirts are distributed evenly around the galaxy, as are most of the dwarf galaxies seen around our nearest large neighbor, the Andromeda Galaxy (SN: 3/11/15; SN: 8/19/15).
But in the M81 group, the seven newly identified star clumps appear to surround a smaller member of that group, NGC 3077, which is about one-tenth the mass of M81. “The fact that the bigger thing doesn’t have more satellites,” Bell says, “nobody expects that.” More
A dense, scorched planet around a faraway star may be the naked core of a gas giant. Satellite and Earth-based telescope observations show that the newly discovered exoplanet has a radius nearly 3.5 times Earth’s and a mass about 39 times as big. Those dimensions combined point to a density roughly the same as Earth’s, […] More
