Why Icy Pluto Still Causes Heated Debates – 02/17/2023 – Science

Like many colleagues at Lowell Observatory, Kevin Schindler felt a shiver down his spine when the International Astronomical Union (IAU) announced in August 2006 that, in a nutshell, Pluto was no longer a planet.

The decision was particularly indigestible at the observatory based in Flagstaff, a city in the US state of Arizona. It was there that American astronomer Clyde W. Tombaugh discovered Pluto on February 18, 1930—a date still celebrated each year as International Pluto Day.

“Practically every major Pluto-related discovery has ties to Flagstaff,” Schindler, the observatory’s historian, tells the BBC.

“So we were a little apprehensive, to say the least, about what we consider to be a very bad decision by the IAU.”

Looks like the game has turned

But it looks like the icy planet has the last laugh: Over the last 17 years, Lowell Observatory has been so busy that it has had to build a new visitor center, which will be ready by 2024.

“By 2006, we were able to host around 60,000 visitors a year. In 2019, just before Covid-19, we were seeing nearly 100,000 people come and attend events like our ‘I Love Pluto’ festival. )”, says Schindler.

“We joke here that this controversy was so good for business that we wish it had happened sooner,” he adds.

“The IAU decision definitely seems to have made more people interested in astronomy.”

Pluto’s reclassification as a dwarf planet — a downgrade from the Solar System’s “first league” planets — still bothers some, and not just the people who work at the observatory.

Alan Stern, the NASA scientist who led the team behind the New Horizons probe that reached Pluto in 2015, has been a vocal critic of the IAU’s decision — taken by a vote of 424 delegates present at the organization’s August 2006 meeting. in Prague, Czech Republic.

“Voting is not how science works. We don’t vote for relativity theory or quantum physics,” Stern told the BBC.

“The IAU could have voted that the sky is green, which does not automatically make that statement true.”

So what is a planet?

At the heart of the Pluto controversy is what seems to be a basic question: what is a planet?

The word comes from the ancient Greeks: they noticed that Mercury, Venus, Mars, Jupiter and Saturn, the only planets visible to the naked eye, moved much more noticeably than the stars —which gave them the name planets (walkers).

But the definition of the term is another story. Although Pluto was quickly added to the Solar System’s roster of planets after its discovery in the 1930s, astronomers have speculated over the next few decades that the newcomer could simply be the first detected example of a series of small, icy bodies beyond Neptune’s orbit. a region known as the Kuiper Belt.

Speculation increased in 1992, when astronomers detected 1992 QBI, the first Kuiper Belt Object (KBO), measuring 160 km in diameter (significantly smaller than Pluto, which is 2,376 km).

But the 2000s began with three major discoveries of KBOs comparable to the size of Pluto.

One of these objects, Eris, discovered in 2005, was estimated to be even larger than Pluto, which led NASA to announce in July of that year that “scientists had discovered the 10th planet in the Solar System”.

These findings spurred the IAU to create a committee tasked with defining exactly what a planet is and submitting a final proposal to members. Ironically, one of the first reports not only kept Pluto as a planet, but also “promoted” Charon, the largest of its moons, Eris and Ceres — a celestial body between Mars and Jupiter that was briefly considered a planet after its discovery in 1801.

But in the end, IAU delegates voted for a definition with the following criteria: a planet must orbit a star; it must be sized to have enough gravity to take a spherical shape; and it must be large enough for its gravity to eliminate any objects of similar size close to its orbit around the sun – that is, it is an object of predominant size among those in neighboring orbits.

That last rule sealed Pluto’s fate, as it shares its orbital neighborhood with other icy KBOs. Therefore, it went to the category of “dwarf planet”.

But not everyone was convinced: experts like Stern have questioned the “orbital neighborhood cleanup” rule, arguing that Earth doesn’t meet the 2006 definition because it has more than 12,000 asteroids in its neighborhood.

Another controversy arose due to low voter turnout: less than 20% of the 2,700 delegates who attended the 10-day conference in Prague actually voted, as organizers left voting for the last day of the event.

“The IAU has made fun of itself because I keep seeing and hearing people and colleagues calling Pluto a planet,” says Stern.

The NASA scientist adds that researchers like him ignore the IAU rules and adopt the geophysical definition of a planet – it has enough gravity to be round and insufficient mass to undergo nuclear fusion in its interior. Under these rules, Pluto and all other “dwarfs” qualify as planets.

Stern, however, felt uplifted by discoveries from the New Horizons mission. Launched in January 2006, the probe reached Pluto nine years later and made several discoveries about the dwarf planet that forced scientists to revise their models. One of the findings was that Pluto may have “a vast ocean of liquid water moving under its surface”.

“We showed how much more complex Pluto was than we thought, and that made me even more convinced that the current definition of ‘planet’ is impractical.”

‘We still love Pluto’

The head of the IAU, American astrophysicist Debra Elmegreen, told the BBC by email that she is not surprised by the prolonged debate about Pluto.

“Pluto was discovered within the lifetime of the oldest of us, so it’s ingrained in our history,” wrote Elmegreen, who took office in 2018.

“Also, planets are the first thing most kids learn about space.”

But she doesn’t believe the IAU’s decision on Pluto will be changed any time soon, and diplomatically suggests it may be time to move on.

“We still love Pluto, but classification is fundamental to all science, because it’s how we recognize the similarities and differences between objects. It’s how we understand their underlying physics, chemistry or geology,” argues Elmegreen.

“Pluto will remain the prototype of this new classification (dwarf planet), but this does not change its importance in understanding the evolution of planetary systems.”

The head of the IAU recognizes that the debate really helps keep astronomy on people’s minds, so maybe she doesn’t mind a spoof Twitter account in the name of Clyde W. Tombaugh or a voting point created by Kevin Schindler in Flagstaff.

In recent years, visitors who want to make a donation to Lowell Observatory have been told to put their money in an object with different boxes — corresponding to different definitions for Pluto: planet, dwarf planet, other or ‘don’t care’. Visitors must decide which box to put the money in.

“I am happy to say that our customers are voting with their wallets”, jokes the historian.