Skip to main content

From discovery to demotion: How a dwarf planet changed astronomy

Eris was discovered in the outer reaches of the Kuiper Belt in 2005 by Caltech astronomer Mike Brown. (Photo: W. M. Keck Observatory)

Astronomer Mike Brown awoke one morning in 2005 and once again checked his computer for updates, as he had done for the last seven years. He scanned through about 200 images, discarding most. Each spot was either too slow, too dim, or just a satellite or other object orbiting the Earth. One image of the night sky, however, caught his eye. A single light stood brighter than the others. The following pictures revealed a slight movement. "Oh, God," Brown thought. "We must have screwed up. It's too bright. It's too big. It's too far away."

Had he pointed the telescope at the wrong position in the sky, or accidentally seen an asteroid, or taken fast pictures? A minute or two of cross-checks verified the pictures were indeed accurate. The object glowed brighter than anything Brown and his team at Caltech's Palomar Observatory had ever discovered. The apparent slowness meant it was far from Earth. He ran a quick calculation: as big as Pluto, if not larger.

"It was obvious within the first 10 seconds that this changed everything," Brown said. He phoned his wife and told her the news: He had just discovered an object big enough to be the solar system's tenth planet.

Brown and his co-discoverers nicknamed their discovery Xena, after the '90s TV icon. But after realizing the impact it would have on the scientific community, the team officially named the object for the Greek goddess of discord and strife, Eris.

Meanwhile, a NASA mission called New Horizons was in the final stages before launch. In a few months it would send a spacecraft on a 10-year journey to Pluto, in a region never closely explored before.  The discovery of Eris would have ripple effects on the mission and its lead scientist, who suddenly found himself at the heart of an international controversy that rattled the foundations of one of mankind's oldest sciences. 

A solar system unraveling

At the time of the Eris discovery, our knowledge of the solar system's planets was fairly robust. Unmanned probes had visited the gas giants (Jupiter, Saturn, Uranus and Neptune) in the 1980s with subsequent missions to Jupiter and Saturn. Several missions surveyed the inner planets of Venus and Mars over the years. And while our innermost planet, Mercury, had only been partially explored, our outermost planet, Pluto, and the region beyond Neptune, had never been visited and remained mysterious.

It was the introduction of a class of highly sensitive detectors that would shed light on Pluto and revolutionize the field, ushering in a new era for astronomy. The technology applied charged coupled devices (CCDs), which are often found in digital cameras today, to a technique called drift scanning to track movement in the sky down to individual photons. This allowed astronomers to gaze beyond Neptune to a zone they soon realized was clustered with large, icy objects left over from the formation of the solar system. It became defined as the Kuiper Belt.

By 2001 several teams, such as the Deep Eliptical Survey, the Outer Solar System Survey, and the Lincoln Near-Earth Asteroid Research (LINEAR) project, were combing the Kuiper Belt for bright objects, with high success rates.

"We suddenly had the capabilities of surveying the whole sky. We found pretty much everything there was to find that was as bright as Eris," Brown said. "So it wasn't just Eris. It was the discovery of Eris and the other big dwarf planets, Quaoar (in 2002), Haumea (in 2004), Orcus (in 2004) and Makemake (in 2005).

Review our Q&A chat with astronomer Mike Brown and science writer Brad Hooker

"Each of these little objects holds a tiny bit of the claim to the history of the solar system. And each one is different. Each one had this story that we had to learn how to read," he said. "And we were starting to figure out these stories for the first time ever. No one had ever found these objects. No one knew anything about them. That's the part that I think, looking back, will be the highlight of my career."

If these contributions to astronomy were astounding, their impacts on the status of the ninth planet were profound. Brown, who studied Eris with his team for two years before announcing their findings to the world, knew the discovery would lead to one of two options: Either Eris would be classified as the tenth planet or Pluto would be downgraded to the dwarf planet subclass.

Two weeks after Eris first appeared and before Brown's team announced their findings to the world, a TV news channel covering the upcoming launch of the NASA flyby of Pluto caught his attention. The reporter described it as "a mission to the last unexplored planet." Brown thought: "Well, either way it goes, that statement's not going to be correct."


Eight months after the launch of the New Horizons spacecraft, the International Astronomical Union (IAU), the group governing the definition of planetary bodies, met to discuss Eris and its impact on the solar system. Due to Pluto's size and the fact that it exists within a zone of many similarly sized objects, the IAU reclassified Pluto as a dwarf planet and redefined this term to mean an object that is distinctly not a planet.

Almost immediately, outcry over Pluto's loss of planetary status flared across the world. American slang dictionaries gained a new phrase: to be "Plutoed," or unceremoniously downgraded. Three U.S. states denounced the IAU resolution. New Mexico declared a Pluto Planet Day. 

New Horizons' lead scientist Alan Stern found himself embroiled in the debate. Downgrading Pluto was "hugely damaging," said Stern, a AAAS fellow. In the mindset of the general public, he felt, the importance of the New Horizons mission was demoted as well.  Despite the controversy, the support of the scientific community was unshaken. The National Academy of Sciences voted New Horizons the nation's top priority for all scientific endeavors for that decade.

The mission's potential for new science was essentially limitless.

Stern knew well before Eris that not only was a new planet larger than Pluto likely to be found, but that the outer solar system was probably littered with Pluto-sized worlds. To describe them, he coined the term "dwarf planet" in 1990.

Even before the first spotting of Pluto in 1930, astronomers had only suspected a large planet lurked beyond Uranus and Neptune because of subtle gravitational disruptions in those planets. Though the disturbances were in fact never proven to exist, it wasn't until the late 1970s that scientists began questioning Pluto's size as a planet and its place in the traditional planetary system.

Pluto was the "harbinger," as Stern put it, "the brightest and the first discovered." Eventually, in 1992, scientists discovered a second trans-Neptunian body — as objects at this distance were called. With more and more of these dwarf planets, it became clear that something larger than Pluto would soon cross a telescope's vision.

"From my standpoint," said Stern, "the view most adults of today were taught of the solar system when they went to grade school was fundamentally an error, because we could not see deep enough in the solar system to see that the solar system did not consist of four rocky terrestrial planets, four gas giants and one oddball, Pluto."

Battling discord and strife

"The concept of a planetary system is historically one of the first scientific concepts that has been formulated in the entire history of science, not only of astronomy," said Giovanni Valsecchi, an Italian astronomer at the Institute for Space Astrophysics and Planetology.

"This concept has penetrated so much into the minds of people, of scientists," he said, "that when, at the beginning of the 20th century, a new mechanics was proposed to explain the behavior of the microscopic world, called quantum mechanics, the first version of it [was] basically a small planetary system within an atom, because this was practically a paradigm that seemed to be inescapable."

The late astronomer Brian Marsden, who at the time of the Eris discovery directed the Minor Planet Center under the IAU, had been seeing a growing problem. He was measuring the astrometry, or position and motion, of the trans-Neptunian bodies and, because Pluto was considered a planet, he had no clear data to compare it to the trans-Neptunians. His solution, as he proposed for several years to the IAU, was to cross-classify Pluto as a minor planet. But the scientific community chose not to raise the debate.

Ultimately, Brown's disclosure of Eris in 2005 obligated the IAU to take a position on the definition of planets. Giovanni Valsecchi helped organize and serve on the commission alongside Stern. "I proposed in particular that the definition of a planet was based mostly on the dynamics," Valsecchi said, "because the physical characteristics of bodies very far away from Earth are very difficult to establish unambiguously." Earlier, Stern had proposed labeling all of the new objects "planets" and classifying them in subsets according to size and behavior. Several other ideas were put forward.

Yet no one consensus prevailed.

The IAU formed a second committee, this time for a rushed vote without Stern, Valsecchi and 90 percent of the IAU members.

Here, the argument that dwarf planets are not fully-formed planets held the majority vote. This meant that Eris and Pluto would be lumped in with other dwarf planets, many of which were asteroids or unstable bodies that could be thrown out of the solar system at any time.

Planetary scientists, who, unlike the IAU, were in an entirely separate field from astronomy, contested the ruling. Stern likens this divide to the difference between divorce lawyers and tax lawyers. "They do very different things in law and they're not expert at one another's turf," he said. The way astronomers talk about planets at meetings isn't the same as planetary scientists. "Objects like Pluto are called planets hundreds of times per day at the microphone, because planetary scientists generally consider these planets," Stern explained. His colleagues refused to accept the IAU's ruling and circulated a petition that quickly gained more votes than the IAU had received in the recent commission. Though the petition never overturned the resolution, the heated IAU debate over the definition of a planet is still ongoing.

In a twist of irony, astronomers in 2011 suggested Eris might actually be slightly smaller than Pluto. But few outside astronomical circles seemed to notice.

In the seven years since the downgrading of Pluto, the IAU ruling has shown to have had little impact on the New Horizons mission, aside from the initial negative press, said Stern. The New Horizons team is instead looking to the future, anticipating an opportunity rarely seen in space exploration these days. The New Horizons spacecraft, which has been traveling at speeds exceeding 50COMMANUMBER000 miles per hour, will arrive in 2015 at a region of the universe never before observed in close detail.

"We've had many missions to Jupiter and the giant planets, many missions to Mars, many missions to asteroids and comets, but this is completely new," said Stern. "All of it will be headline discoveries, because we essentially don't know anything. We are going into terra incognita."

Representative Image Caption
Eris was discovered in the outer reaches of the Kuiper Belt in 2005 by Caltech astronomer Mike Brown. (Photo: W. M. Keck Observatory)
Blog Name


Brad Hooker

Related Scientific Disciplines