Geoffrey Evatt was snowmobiling in Antarctica when he spotted an outlandish feature. A black rock stood so starkly against the diamantine ice that even the untrained eye would have known it was not from this world, but a meteorite. “You’ll never get over that high of finding the first one,” he says. Not that it […] More

When the first-ever image of a black hole was released in April 2019, it marked a powerful confirmation of Albert Einstein’s theory of gravity, or general relativity.
The theory not only describes the way matter warps spacetime, but it also predicts the very existence of black holes, including the size of the shadow cast by a black hole on the bright disk of material that swirls around some of the dense objects. That iconic image, of the supermassive black hole at the center of the galaxy M87 about 55 million light-years away, showed that the shadow closely matched general relativity’s predictions of its size (SN: 4/10/19). In other words, Einstein was right — again.
That result, reported by the Event Horizon Telescope Collaboration, answered one question: Is the size of M87’s black hole consistent with general relativity?
But “it is very difficult to answer the opposite question: How much can I tweak general relativity, and still be consistent with the [black hole] measurement?” says EHT team member Dimitrios Psaltis of the University of Arizona in Tucson. That question is key because it’s still possible that some other theory of gravity could describe the universe, but masquerade as general relativity near a black hole.

Sign Up For the Latest from Science News

Headlines and summaries of the latest Science News articles, delivered to your inbox

In a study published October 1 in Physical Review Letters, Psaltis and colleagues have used the shadow of M87’s black hole to take a major step toward ruling out those alternative theories.
Specifically, the researchers used the size of the black hole to perform what’s known as a “second-order” test of general relativity geared toward boosting confidence in the result. That “can’t really be done in the solar system” because the gravitational field is too weak, says EHT team member Lia Medeiros of the Institute for Advanced Study in Princeton, N.J.
So far so good for relativity, the researchers found when they performed this second-order test.
The results are on par with those from gravitational wave experiments like the Advanced Laser Interferometer Gravitational-Wave Observatory, which has detected ripples in spacetime from the merger of black holes smaller than M87’s (SN: 9/16/19). But the new study is interesting because “it’s the first attempt at constraining a [second-order] effect through a black hole observation,” says physicist Emanuele Berti of Johns Hopkins University, who was not involved in the new work.
Generally, physicists think of general relativity as a set of corrections or add-ons to Isaac Newton’s theory of gravity. General relativity predicts what those add-ons should be. If measurements of how gravity works in the universe deviate from those predictions, then physicists know general relativity is not the full story. The more add-ons or factors added to a test, the more confidence there is in a result.
In weak gravitational fields, like within the solar system, physicists can test whether “first-order” additions to Newton’s equations are consistent with general relativity or not. These additions are related to things like how light and mass travel in a warped spacetime, or how gravity makes time flow more slowly.
Those aspects of gravity have been tested with the way stars’ light is deflected during a solar eclipse for example, and the way laser light sent to spacecraft flying away from the sun takes longer than expected to return to Earth (SN: 5/29/19). General relativity has passed every time.
But it takes a strong gravitational field, like the one around M87’s black hole, to kick the tests up a notch.
The new result is slightly disappointing for the physicists hoping to find cracks in Einstein’s theory. Finding a deviation from general relativity could point the way to new physics. Or it could help unite general relativity, the physics of the very large, and quantum mechanics, the leading theory that describes the physics of the very small, like subatomic particles and atoms (SN: 3/30/20). The fact that general relativity still refuses to bend is “worrying for those of us who are old enough that we were hoping to get an answer in our lifetime,” Psaltis says.
But there is some hope that general relativity might still fail around black holes. The new study makes the box of possible ways for the theory to break down smaller, “but we haven’t made it infinitesimal,” Medeiros says. The study is “a proof of concept to show that the EHT could do this… But it’s really just step one of many.”  
Future observations from the EHT will make for even more precise tests of general relativity, she says, especially with yet-to-be-released images of Sgr A*, the black hole at the center of the Milky Way. With much more precise measurements of Sgr A*’s mass than any other supermassive black hole, that image may make the possible box around the theory even smaller — or blow it wide open. More

Future astronauts could make lunar buildings out of moon dust and pee. That’s the suggestion of chemist Anna-Lena Kjøniksen and her colleagues, who made a cement from urea — a major component of urine — and faux lunar soil. When humans take up long-term residence on other planets or the moon, they will need to […] More

The sun could come from a large, loose-knit clan or a small family that’s always fighting.
New computer simulations of young stars suggest two pathways to forming the solar system. The sun could have formed in a calm, large association of 10,000 stars or more, like NGC 2244 in the present-day Rosette Nebula, an idea that’s consistent with previous research. Or the sun could be from a violent, compact cluster with about 1,000 stars, like the Pleiades, researchers report July 2 in the Astrophysical Journal.Whether a star forms in a tight, rowdy cluster or a loose association can influence its future prospects. If a star is born surrounded by lots of massive siblings that explode as supernovas before a cluster spreads out, for example, that star will have more heavy elements to build planets with (SN: 8/9/19).
To nail down a stellar birthplace, astronomers have considered the solar system’s chemistry, its shape and many other factors. Most astronomers who study the sun’s birthplace think the gentle, large association scenario is most likely, says astrophysicist Fred Adams of the University of Michigan in Ann Arbor, who was not involved in the new work.
But most previous studies didn’t include stars’ motions over time. So astrophysicists Susanne Pfalzner and Kirsten Vincke, both of the Max Planck Institute for Radio Astronomy in Bonn, Germany, ran thousands of computer simulations to see how often different kinds of young stellar families produce solar systems like ours.

Sign Up For the Latest from Science News

Headlines and summaries of the latest Science News articles, delivered to your inbox

The main solar system feature that the pair looked for was the distance to the farthest planet from the star. Planet-forming disks can extend to hundreds of astronomical units, or AU, the distance between the Earth and the sun (SN: 7/16/19). Theoretically, planets should be able to form all the way to the edge. But the sun’s planetary material is mostly packed within the orbit of Neptune.
“You have a steep drop at 30 AU, where Neptune is,” Pfalzner says. “And this is not what you expect from a disk.”
In 2018, Pfalzner and her colleagues showed that a passing star could have truncated and warped the solar system’s outer edge long ago. If that’s what happened, it could help point to the sun’s birth environment, Pfalzner reasoned. The key was to simulate groupings dense enough that stellar flybys happen regularly, but not so dense that the encounters happen too often and destroy disks before planets can grow up.
“We were hoping we’d get one answer,” Pfalzner says. “It turned out there are two possibilities.” And they are wildly different from each other.
Large associations have more stars, but the stars are more spread out and generally leave each other alone. Those associations can stay together for up to 100 million years. Compact clusters, on the other hand, see more violent encounters between young stars and don’t last as long. The stars shove each other away within a few million years.
“This paper opens up another channel for what the sun’s birth environment looked like,” Adams says, referring to the violent cluster notion.
The new study doesn’t cover every aspect of how a tight cluster could have affected the nascent solar system. The findings don’t account for how radiation from other stars in the cluster could erode planet-forming disks, for example, which could have shrunk the sun’s disk or even prevented the solar system from forming. The study also doesn’t explain certain heavy elements found in meteorites, which are thought to come from a nearby supernova and so could require the sun come from a long-lived stellar family.
“I think [the research] is an interesting addition to the debate,” Adams says. “It remains to be seen how the pieces of the puzzle fit together.”
Pfalzner thinks that the star cluster would break apart before radiation made a big difference, and there are other explanations for the heavy elements apart from a single supernova. She hopes future studies will be able to use that sort of cosmic chemistry to narrow the sun’s birthplace down even further.
“For us humans, this is an important question,” Pfalzner says. “It’s part of our history.” 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.”

Sign Up For the Latest from Science News

Headlines and summaries of the latest Science News articles, delivered to your inbox

Thank you for signing up!
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.

[embedded content]
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. More