Space & Astronomy

A meandering trek taken by light from a remote supernova in the constellation Cetus may help researchers pin down how fast the universe expands — in another couple of decades.

About 10 billion years ago, a star exploded in a far-off galaxy named MRG-M0138. Some of the light from that explosion later encountered a gravitational lens, a cluster of galaxies whose gravity sent the light on multiple diverging paths. In 2016, the supernova appeared in Earth’s sky as three distinct points of light, each marking three different paths the light took to get here.

Now, researchers predict that the supernova will appear again in the late 2030s. The time delay — the longest ever seen from a gravitationally lensed supernova — could provide a more precise estimate for the distance to the supernova’s host galaxy, the team reports September 13 in Nature Astronomy. And that, in turn, may let astronomers refine estimates of the Hubble constant, the parameter that describes how fast the universe expands.

The original three points of light appeared in images from the Hubble Space Telescope. “It was purely an accident,” says astronomer Steve Rodney of the University of South Carolina in Columbia. Three years later, when Hubble reobserved the galaxy, astronomer Gabriel Brammer at the University of Copenhagen discovered that all three points of light had vanished, indicating a supernova.

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By calculating how the intervening cluster’s gravity alters the path the supernova’s light rays take, Rodney and his colleagues predict that the supernova will appear again in 2037, give or take a couple of years. Around that time, Hubble may burn up in the atmosphere, so Rodney’s team dubs the supernova “SN Requiem.”

“It’s a requiem for a dying star and a sort of elegy to the Hubble Space Telescope itself,” Rodney says. A fifth point of light, too faint to be seen, may also arrive around 2042, the team calculates.

In another Hubble image of the galaxy cluster MACS J0138.0-2155, the cluster split the light from a supernova into three points, SN1, SN2 and SN3. The other two points, SN4 and SN5, are predictions of where the light from the supernova will appear in future years.S. Rodney et al/Nature Astronomy 2021

The predicted 21-year time delay — from 2016 to 2037 — is a record for a supernova. In contrast, the first gravitational lens ever found — twin images of a quasar spotted in 1979  — has a time delay of only 1.1 years (SN: 11/10/1979).

Not everyone agrees with Rodney’s forecast. “It is very difficult to predict what the time delay will be,” says Rudolph Schild, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., who was the first to measure the double quasar’s time delay. The distribution of dark matter in the galaxy hosting the supernova and the cluster splitting the supernova’s light is so uncertain, Schild says, that the next image of SN Requiem could come outside the years Rodney’s team has specified.

In any case, when the supernova image does appear, “that would be a phenomenally precise measurement” of the time delay, says Patrick Kelly, an astronomer at the University of Minnesota in Minneapolis who was not involved with the new work. That’s because the uncertainty in the time delay will be tiny compared with the tremendous length of the time delay itself.

That delay, coupled with an accurate description of how light rays weave through the galaxy cluster, could affect the debate over the Hubble constant. Numerically, the Hubble constant is the speed a distant galaxy recedes from us divided by the distance to that galaxy. For a given galaxy with a known speed, a larger estimated distance therefore leads to a lower number for the Hubble constant.

This number was once in dispute by a factor of two. Today the range is much tighter, from 67 to 73 kilometers per second per megaparsec. But that spread still leaves the universe’s age uncertain. The frequently quoted age of 13.8 billion years corresponds to a Hubble constant of 67.4. But if the Hubble constant is higher, then the universe could be about a billion years younger.

The longer it takes for SN Requiem to reappear, the farther from Earth the host galaxy is — which means a lower Hubble constant and an older universe. So if the debate over the Hubble constant persists into the 2030s, the exact date the supernova springs back to life could help resolve the dispute and nail down a fundamental cosmological parameter. More

One can only imagine what Grote Reber’s neighbors thought when, in 1937, the amateur radio enthusiast erected in his yard a nearly 10-meter-wide shallow bowl of sheet metal, perched atop an adjustable scaffold and topped by an open pyramid of gangly towers. Little could his neighbors have known that they were witnessing the birth of a new way of looking at the cosmos.

Reber was building the world’s first dedicated radio telescope. Unlike traditional telescopes, which use lenses or mirrors to focus visible light, this contraption used metal and circuitry to collect interstellar radio waves, low frequency ripples of electromagnetic radiation. With his homemade device, Reber made the first map of the sky as seen with radio-sensitive eyes and kicked off the field of radio astronomy.

“Radio astronomy is as fundamental to our understanding of the universe as … optical astronomy,” says Karen O’Neil, site director at Green Bank Observatory in West Virginia. “If we want to understand the universe, we really need to make sure we have as many different types of eyes on the universe as we possibly can.”

When astronomers talk about radio waves from space, they aren’t (necessarily) referring to alien broadcasts. More often, they are interested in low-energy light that can emerge when molecules change up their rotation, for example, or when electrons twirl within a magnetic field. Tuning in to interstellar radio waves for the first time was akin to Galileo pointing a modified spyglass at the stars centuries earlier — we could see things in the sky we’d never seen before.

Today, radio astronomy is a global enterprise. More than 100 radio telescopes — from spidery antennas hunkered low to the ground to supersized versions of Reber’s dish that span hundreds of meters — dot the globe. These eyes on the sky have been so game-changing that they’ve been at the center of no fewer than three Nobel Prizes.

Not bad for a field that got started by accident.

In the early 1930s, an engineer at Bell Telephone Laboratories named Karl Jansky was tracking down sources of radio waves that interfered with wireless communication. He stumbled upon a hiss coming from somewhere in the constellation Sagittarius, in the direction of the center of the galaxy.

Karl Jansky, shown here with his rotating radio antenna, stumbled on a radio hiss coming from the direction of the center of the galaxy, marking the beginnings of radio astronomy.NRAO, AUI, NSF, Jeff Hellerman

“The basic discovery that there was radio radiation coming from interstellar space confounded theory,” says astronomer Jay Lockman, also of Green Bank. “There was no known way of getting that.”

Bell Labs moved Jansky on to other, more Earthly pursuits. But Reber, a fan of all things radio, read about Jansky’s discovery and wanted to know more. No one had ever built a radio telescope before, so Reber figured it out himself, basing his design on principles used to focus visible light in optical scopes. He improved upon Jansky’s antenna — a bunch of metal tubes held up by a pivoting wooden trestle — and fashioned a parabolic metal dish for focusing incoming radio waves to a point, where an amplifier boosted the feeble signal. The whole contraption sat atop a tilting wooden base that let him scan the sky by swinging the telescope up and down. The same basic design is used today for radio telescopes around the world.

For nearly a decade — thanks partly to the Great Depression and World War II — Reber was largely alone. The field didn’t flourish until after the war, with a crop of scientists brimming with new radio expertise from designing radar systems. Surprises have been coming ever since.

Grote Reber erected the world’s first dedicated radio telescope – shown here – in his yard in Wheaton, Ill.GBO, NSF, AUI

“The discovery of interstellar molecules, that’s a big one,” says Lisa Young, an astronomer at New Mexico Tech in Socorro. Radio telescopes are well suited to peering into the dense, cold clouds where molecules reside and sensing radiation emitted when they lose rotational energy. Today, the list of identified interstellar molecules includes many complex organics, including some thought to be precursors for life.

Radio telescopes also turned up objects previously unimagined. Quasars, the blazing cores of remote galaxies powered by behemoth black holes, first showed up in detailed radio maps from the late 1950s. Pulsars, the ultradense spinning cores of dead stars, made themselves known in 1967 when Jocelyn Bell Burnell noticed that the radio antenna array she helped build was picking up a steady beep … beep … beep from deep space every 1.3 seconds. (She was passed over when the 1974 Nobel Prize in physics honored this discovery — her adviser got the recognition. But an accolade came in 2018, when she was awarded a Special Breakthrough Prize in Fundamental Physics.)

Pulsars are “not only interesting for being a discovery in themselves,” Lockman says. They “are being used now to make tests of general relativity and detect gravitational waves.” That’s because anything that nudges a pulsar — say, a passing ripple in spacetime — alters when its ultraprecise radio beats arrive at Earth. In the early 1990s, such timing variations from one pulsar led to the first confirmed discovery of planets outside the solar system.

More recently, brief blasts of radio energy primarily from other galaxies have captured astronomers’ attention. Discovered in 2007, the causes of these “fast radio bursts” are still unknown. But they are already useful probes of the stuff between galaxies. The light from these eruptions encodes signatures of the atoms encountered while en route to Earth, allowing astronomers to track down lots of matter they thought should be out in the cosmos but hadn’t found yet. “That was the thing that allowed us to weigh the universe and understand where the missing matter is,” says Dan Werthimer, an astronomer at the University of California, Berkeley.

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And it was a radio antenna that, in 1964, gave the biggest boost to the then-fledgling Big Bang theory. Arno Penzias and Robert Wilson, engineers at Bell Labs, were stymied by a persistent hiss in the house-sized, horn-like antenna they were repurposing for radio astronomy. The culprit was radiation that permeates all of space, left behind from a time when the universe was much hotter and denser than it is today. This “cosmic microwave background,” named for the relatively high frequencies at which it is strongest, is still the clearest window that astronomers have into the very early universe.

Radio telescopes have another superpower. Multiple radio dishes linked together across continents can act as one enormous observatory, with the ability to see details much finer than any of those dishes acting alone. Building a radio eye as wide as the planet — the Event Horizon Telescope — led to the first picture of a black hole.

The Event Horizon Telescope, an international network of radio observatories, took this first-ever image of a black hole, at the center of galaxy M87.Event Horizon Telescope collaboration et al

“Not that anybody needed proof of the existence [of black holes],” Young says, “but there’s something so marvelous about actually being able to see it.”

The list of discoveries goes on: Galaxies from the early universe that are completely shrouded in dust and so emit no starlight still glow bright in radio images. Rings of gas and dust encircling young stars are providing details about planet formation. Intel on asteroids and planets in our solar system can be gleaned by bouncing radio waves off their surfaces.

And, of course, there’s the search for extraterrestrial intelligence, or SETI. “Radio is probably the most likely place where we will answer the question: ‘Are we alone?’” Werthimer says.

The ALMA radio telescope network in the Atacama Desert in Chile captured this image of what appears to be a planet-forming disk around the young star HL Tauri.ESO, NAOJ, NRAO

That sentiment goes back more than a century. In 1899, inventor Nikola Tesla picked up radio signals that he thought were coming from folks on another planet. And for 36 hours in August 1924, the United States ordered all radio transmitters silent for five minutes every hour to listen for transmissions from Mars as Earth lapped the Red Planet at a relatively close distance. The field got a more official kickoff in 1960 when astronomer Frank Drake pointed Green Bank’s original radio telescope at the stars Tau Ceti and Epsilon Eridani, just in case anyone there was broadcasting.

While SETI has had its ups and downs, “there’s kind of a renaissance,” Werthimer says. “There’s a lot of new, young people going into SETI … and there’s new money.” In 2015, entrepreneur Yuri Milner pledged $100 million over 10 years to the search for other residents of our universe.

Though the collapse of the giant Arecibo Observatory in 2020 — at 305 meters across, it was the largest single dish radio telescope for most of its lifetime — was tragic and unexpected, radio astronomers have new facilities in the works. The Square Kilometer Array, which will link up small radio dishes and antennas across Australia and South Africa when complete in the late 2020s, will probe the acceleration of the universe’s expansion, seek out signs of life and explore conditions from cosmic dawn. “We’ll see the signatures of the first structures in the universe forming the first galaxies and stars,” Werthimer says.

The Square Kilometer Array will connect radio dishes and antennas across South Africa and Australia, offering an unprecedented look at the universe.SKA Observatory

But if the history of radio astronomy is any guide, the most remarkable discoveries yet to come will be the things no one has thought to look for. So much about the field is marked by serendipity, Werthimer notes. Even radio astronomy as a field started serendipitously. “If you just build something to look at some place that nobody’s looked before,” he says, “you’ll make interesting discoveries.” More

When considering where to look for extraterrestrial life, astronomers have mostly stuck with what’s familiar. The best candidates for habitable planets are considered the ones most like Earth: small, rocky, with breathable atmospheres and a clement amount of warmth from their stars.

But as more planets outside the solar system have been discovered, astronomers have debated the usefulness of this definition (SN: 10/4/19). Some planets in the so-called habitable zone, where temperatures are right for liquid water, are probably not good for life at all. Others outside that designated area might be perfectly comfortable.

Now, two studies propose revising the concept of “habitable zone” to account for more of the planets that astronomers may encounter in the cosmos. One new definition brings more planets into the habitable fold; the other nudges some out.

“Both papers focus on questioning the classical idea of the habitable zone,” says astronomer Noah Tuchow of Penn State University. “We should extend the range of places that we look, so we don’t miss habitable planets.”

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Some overlooked planets could be much bigger than Earth, and potentially receive no starlight at all. Astrophysicist Nikku Madhusudhan of the University of Cambridge and colleagues propose a new category of possibly habitable planet that could be found at almost any distance from any kind of star.

These hypothetical planets have a global liquid-water ocean nestled under a thick hydrogen-rich atmosphere (SN: 5/4/20). Madhusudhan calls them “Hycean” planets, for “hydrogen” and “ocean.” They could be up to 2.6 times the size of Earth and up to 10 times as massive, Madhusudhan and colleagues report August 25 in the Astrophysical Journal. That thick atmosphere could keep temperatures right for liquid water even with minimal input from a star, while the ocean could protect anything living from crushing atmospheric pressure.

“We want to expand beyond our fixated paradigm so far on Earthlike planets,” Madhusudhan says. “Everything we’ve learned about exoplanets so far is extremely diverse. Why restrict ourselves when it comes to life?”

In the search for alien life, Hycean planets would have several advantages over rocky planets in the habitable zone, the team says. Though it’s difficult to tell which worlds definitely have oceans and hydrogen atmospheres, there are many more known exoplanets in the mass and temperature ranges of Hycean planets than there are Earthlike planets. So the odds are good, Madhusudhan says.

And because they are generally larger and have more extended atmospheres than rocky planets, Hycean planets are easier to probe for biosignatures, molecular signs of life. Detectable biosignatures on Hycean planets could include rare molecules associated with life on Earth like dimethyl sulfide and carbonyl sulfide. These tend to be too low in concentration to detect in thin Earthlike atmospheres, but thicker Hycean atmospheres would show them more readily.

Best of all, existing or planned telescopes could detect those molecules within a few years, if they’re there. Madhusudhan already has plans to use NASA’s James Webb Space Telescope, due to launch later this year, to observe the water-rich planet K2 18b (SN: 9/11/19).

That planet’s habitability was debated when it was reported in 2019. Madhusudhan says 20 hours of observations with JWST should solve the debate.

“Best-case scenario, we’ll detect life on K2 18b,” he says, though “I’m not holding my breath over it.”

Astronomer Laura Kreidberg of the Max Planck Institute for Astronomy in Heidelberg, Germany, thinks it probably won’t be that easy. Planets in the Hycean size range tend to have cloudy or hazy atmospheres, making biosignatures more difficult to pick up.

It’s also not clear if Hycean planets actually exist in nature. “It is a really fun idea,” she says. “But is it just a fun idea, or does it match up with reality? I think we absolutely don’t know yet.”

Rather than inventing a new way to bring exoplanets into the habitable family, Tuchow and fellow Penn State astronomer Jason Wright are kicking some apparently habitable planets out. The pair realized that the region of clement temperatures around a star changes as the star evolves and changes brightness.

Some planets are born in the habitable zone and stay there their entire lives. But some, possibly most, are born outside their star’s habitable zone and enter it later, as the star ages. In the August Research Notes of the American Astronomical Society, Tuchow and Wright suggest calling those worlds “belatedly habitable planets.”

When astronomers point their telescopes at a given star, the scientists are seeing only a snapshot of the star’s habitable zone, the pair say. “If you just look at a planet in the habitable zone in the present day, you have no idea how long it’s been there,” Tuchow says. It’s an open question whether planets that enter the habitable zone later in life can ever become habitable, he explains. If the planet started out too close to the star, it could have lost all its water to a greenhouse effect, like Venus did. Moving Venus to the position of Earth won’t give it its water back.

On the other end, a planet that was born farther from its star could be entirely covered in glaciers, which reflect sunlight. They may never melt, even when their stars brighten. Worse, their water could go straight from frozen to evaporated, a process known as sublimation. That scenario would leave the planet no time with even a cozy wet puddle for life to get started in.

These planets are “still in the habitable zone,” Tuchow says. “But it adds questions about whether or not being in the habitable zone actually means habitable.” 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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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