HOTTEST
Keep your eyes on the night sky this summer, scanning for the constellation Corona Borealis, and if you are lucky, you may glimpse what appears to be a new star winking on in the dark.
The brightening point of light will not be a new star, but a nova eruption about 3,000 light-years from Earth. There, a white dwarf star orbiting a red giant tears material from its larger companion. When enough mass collects on the white dwarf’s surface, the rising pressure and temperature will trigger a blast that can be seen from Earth with the naked eye — but for only a few days to a week. More
The finding suggests large structures in the cosmic web are magnetized MAGNETIC CONNECTION Two distant galaxy clusters, Abell 0399 (left) and Abell 0401 (right), are connected by magnetic fields containing high-energy electrons. X-rays from the cluster cores are shown in purple, and radio emission from the magnetic bridge forms the blue-toned smear in between. DSS, […] More
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. MoreGood news for late bloomers: Planets may have millions of years more time to arise around most stars than previously thought.
Planet-making disks around young stars typically last for 5 million to 10 million years, researchers report in a study posted October 6 at arXiv.org. That disk lifetime, based on a survey of nearby young star clusters, is a good deal longer than the previous estimate of 1 million to 3 million years.
“One to three megayears is a really strong constraint for forming planets,” says astrophysicist Susanne Pfalzner of Forschungszentrum Jülich in Germany. “Finding that we have a lot of time just relaxes everything” for building planets around young stars.
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Planets large and small develop in the disks of gas and dust that swirl around young stars (SN: 5/20/20). Once a disk vanishes, it’s too late to make any more new worlds.
Past studies have estimated disk lifetimes by looking at the fraction of young stars of different ages that still have disks — in particular, by observing star clusters with known ages. But Pfalzner and her colleagues discovered something odd: The farther a star cluster is from Earth, the shorter the estimated disk lifetime. That made no sense, she says, because why should the lifetime of a protoplanetary disk depend on how far it is from us?
The answer is quite simple: It doesn’t. But in clusters that are farther away, it’s harder to see most stars. “When you look at larger distances, you see higher-mass stars,” Pfalzner says, because those stars are brighter and easier to see. “You basically don’t see the low-mass stars.” But the lowest-mass stars constitute the vast majority. These stars, orange and red dwarfs, are cooler, smaller and fainter than the sun.
So Pfalzner and her colleagues examined only the nearest young star clusters, those within 650 light-years of Earth, and found that the fraction of stars with planet-making disks was much higher than that reported in previous studies. This analysis showed that “the low-mass stars have much longer disk lifetimes, between 5 and 10 megayears,” than astronomers realized, she says. In contrast, disks around higher-mass stars are known to disperse faster than this, perhaps because their suns’ brighter light pushes the gas and dust away more quickly.
“I wouldn’t say that this is definite proof” for such long disk lifetimes around orange and red dwarfs, says Álvaro Ribas, an astronomer at the University of Cambridge who was not involved with the work. “But it’s quite convincing.”
To bolster the result, he’d like to see observations of more distant star clusters — perhaps with the James Webb Space Telescope — to determine the fraction of the faintest stars that have preserved their planet-making disks between 5 million and 20 million years (SN: 10/11/22).
If the disks around the lowest mass stars do indeed have long lifetimes, that may explain a difference between our solar system and those of most red dwarfs, Pfalzner says. The latter often lack gas giants like Jupiter and Saturn, which are about 10 times the diameter of Earth. Instead, those stars frequently have numerous ice giants like Uranus and Neptune, about four times the diameter of Earth. Perhaps Neptune-sized planets arise in larger numbers when a planet-making disk lasts longer, Pfalzner says, accounting for why these worlds tend to abound around smaller stars. More
Earth’s deep stores of water may have been locally sourced rather than trucked in from far-flung regions of the solar system.
A new analysis of meteorites from the inner solar system — home to the four rocky planets — suggests that Earth’s building blocks delivered enough water to account for all the H2O buried within the planet. What’s more, the water produced by the local primordial building material likely shares a close chemical kinship with Earth’s deep-water reserves, thus strengthening the connection, researchers report in the Aug. 28 Science.
Earth is thought to have been born in an interplanetary desert, too close to the sun for water ice to survive. Many researchers suspect that ocean water got delivered toward the end of Earth’s formation by ice-laden asteroids that wandered in from cooler, more distant regions of the solar system (SN: 5/6/15). But the ocean isn’t the planet’s largest water reservoir. Researchers estimate that Earth’s interior holds several times as much water as is found at the surface.
To test whether or not the material that formed Earth could have delivered this deep water, cosmochemist Laurette Piani of the University of Lorraine in Vandœuvre-lès-Nancy, France, and colleagues analyzed meteorites known as enstatite chondrites. Thanks to many chemical similarities with Earth rocks, these relatively rare meteorites are widely thought to be good analogs of the dust and space rocks from the inner solar system that formed Earth’s building blocks, Piani says.Sign Up For the Latest from Science News
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She and her team measured the abundance of hydrogen in these meteorites — a proxy for how much H2O they could produce — and calculated that local interplanetary debris had the potential to deliver at least three times as much water as is found in all the oceans. The meteorites don’t contain water, Piani says. Rather, they house enough of the raw ingredients to create water when heated.
In the meteorites, the team also found a close match to the type of water found in Earth’s mantle. A smattering of all water molecules on Earth contain a heavy variant of hydrogen known as deuterium. The ratio of deuterium to hydrogen in the enstatite chondrites lies within the range measured in Earth’s deep water. That similarity, the team argues, makes a strong case for local building blocks being the source of much of the planet’s water.
“This work is something I wanted to do myself or had been waiting for someone to do,” says Lydia Hallis, a planetary scientist at the University of Glasgow in Scotland. In 2015, she led a team that measured the deuterium abundance in lava plumes that tap deep into Earth’s mantle (SN: 11/12/15). “I’m really happy that [the new data] sits within the region where our previous data from deep mantle samples is sitting.”
Hallis and others stress that these new measurements are difficult. Once the meteorites hit the ground, they quickly absorb hydrogen from Earth’s environment. “They did a really good job of picking the right meteorites and making the right measurements,” she says. “This is pretty convincing that this hydrogen that’s measured is from the enstatite chondrites rather than from terrestrial contamination.”
The enstatite chondrites could have also contributed a lot of water to the oceans as well — but they are not the full story. The deuterium-hydrogen ratio in ocean water, which is a bit higher than that of mantle water, is better matched to the ratio found in icy asteroids from the outer solar system. “We still need a bit of water coming from the outer solar system,” Piani says. So, while local materials may have delivered the bulk of Earth’s water, the oceans were likely topped off a bit later by collisions with remote space rocks. More
