Cosmic Space

5 years after flyby, the case for Pluto still holds up

It was exactly five years ago today that NASA’s New Horizons probe made a history-making flyby past Pluto — and since then, the mission’s scientific discoveries and newly raised mysteries continue to pile up.

“I think the solar system literally saved the best for last with Pluto,” New Horizons’ principal investigator, Alan Stern of the Southwest Research Institute, said in his anniversary blog post. “Of course, I’m a little biased — as we all are on New Horizons — but I can’t think of a more beautiful and scientifically richer way to have completed the first era of the reconnaissance of the planets.”

This year marks another, more personal anniversary: It’s been 10 years since the publication of “The Case for Pluto,” my book about the put-upon planet. Back then, the big question was whether Pluto deserved the planet label — and although I argued the case that it does, the clash over classification really isn’t that big of a deal anymore.

You can call Pluto a dwarf planet (my preferred term), a Kuiper Belt object or a “bloog.” (That last term is the one Caltech astronomer and self-described Pluto-killer Mike Brown came up with to make fun of the tiff over terminology.)

But in light of New Horizons’ discoveries, you can never call Pluto uninteresting.

It’s interesting to leaf through the pages of “The Case for Pluto” and size up how the speculation from 2010 matches up with the science as we know it in 2020. In honor of the fifth anniversary of the flyby and the 10th anniversary of the book, here are updates on five of the big questions about Pluto:

Is there liquid water on Pluto? Looks like it. New Horizons’ pictures of tectonic structures and mountains made of water ice, plus an analysis of the dwarf planet’s mass distribution, suggest that there are bodies of liquid water hidden beneath the surface layer of nitrogen ice. What’s more, shifts in the state of that water due to freezing may be what’s driving the creation of new faults in the surface ice.

“If Pluto is an active ocean world, that suggests that the Kuiper Belt may be filled with other ocean worlds among its dwarf planets, dramatically expanding the number of potentially habitable places in the solar system,” New Horizons team member James Tuttle Keane, a planetary scientist at NASA’s Jet Propulsion Laboratory, said today in a mission recap.

Does Pluto have plains of methane? Sort of. One of Pluto’s best-known features, a light-colored, heart-shaped zone that was named Tombaugh Regio in honor of the dwarf planet’s discoverer, is dominated by a plain known as Sputnik Planitia. The plain is made up of patchy nitrogen-ice glaciers, but New Horizons also detected the presence of frozen methane, carbon monoxide and water (in the form of icebergs).

A close analysis of the imagery led scientists to conclude that grains of methane sand have risen to the surface of Sputnik Planitia and are being blown around into icy dunes, probably by gentle winds in Pluto’s ultra-thin, nitrogen-rich atmosphere.

Will Pluto’s atmosphere freeze out? The latest evidence suggests a freezing trend. Pluto cycles through seasons in the course of its 248-Earth-year orbit, the dwarf planet’s elliptical orbit is currently taking it farther away from the sun. That means the already-chilly planet and its atmosphere will be getting even colder.

At the time of the New Horizons flyby, scientists saw signs that the atmosphere was still holding steady rather than freezing into flecks of ice, probably due to thermal inertia. But this year, a Japanese team reported that the pressure has apparently dropped by more than 20% since 2016. That’s a much more dramatic collapse than expected, and will need to be confirmed (or discounted) by follow-up observations.

Are there ice volcanoes? You bet … not only on Pluto but on its largest moon, Charon. The pictures from New Horizons suggest that slushy “cryolava” has blurped out onto the surfaces of the two worlds through fissures in the surface ice.

Although the flyby went by too quickly to see the actual blurping, scientists spotted large central pits on two Plutonian mountains known as Wright Mons and Piccard Mons that they believe serve as the mouths of ice volcanoes. And in a region on Charon called Vulcan Planitia, the New Horizons team saw signs of a huge flow of ammonia-rich water ice.

Is there another Planet X out there? Ask again later. Even when “The Case for Pluto” was written, there was plenty of speculation over whether an undetected planet much bigger than Pluto lurked on the solar system’s edge. Caltech’s Mike Brown and other researchers said anomalies in the orbits of objects in the Kuiper Belt hinted at the presence of what they called Planet Nine.

Years of searching through telescope data haven’t yet turned up hard evidence for Planet Nine’s presence, and some astronomers now speculate that the anomalies associated with the hypothetical planet are due instead to the gravitational influence of a grapefruit-sized black hole. Others suggest it’s just a glitch in the data.

Even if Planet X is crossed out. there’s much more to be discovered on the solar system’s last frontier.

The New Horizons team is still sorting through the data sent back from the last year’s follow-up flyby of a double-lobed Kuiper Belt object known as Arrokoth. A huge compendium of Pluto research, running to more than 1,000 pages in length, is being prepared for publication.

Stern and his colleagues are already working to identify a potential target for New Horizons’ third Kuiper Belt flyby. And they’re talking about sending out an orbiter with powerful sensors to conduct a longer-lasting survey of Pluto and the Kuiper Belt.

“By the time we mark the 10th anniversary of the Pluto flyby in July 2025, such a mission could even be under construction,” Stern wrote today.

Who knows? Maybe a 15th-anniversary edition of “The Case for Pluto” will be in the works as well.

By Alan Boyle

Mastermind of Cosmic Log, contributing editor at GeekWire, author of "The Case for Pluto: How a Little Planet Made a Big Difference," president of the Council for the Advancement of Science Writing. Check out "About Alan Boyle" for more fun facts.

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