NASA / JPL-Caltech

It turns out that Eris, shown in this artist's conception, may be Pluto's denser twin.

It's been almost a year since astronomers suggested that Eris, the icy world whose discovery prompted Pluto's controversial reclassification in 2006, wasn't as big as they originally thought. Now the official word has leaked out unofficially: Pluto just might be the largest dwarf planet after all — although Eris is still seen as more massive.

The latest measurements were reported last week in Nantes, France, at a joint meeting of the American Astronomical Society's Division of Planetary Sciences and the European Planetary Science Congress. But as the Planetary Society's Emily Lakdawalla explains, it took a while for the report to become public, due to worries about the journal Nature's rules on embargoes and confidentiality.

Here are the statistics: Based on measurements made last November during the dwarf planet's occultation of a faraway star, Eris' diameter is estimated at 2,326 kilometers (1,445 miles). A similar set of measurements, published in 2009. estimated that Pluto was at least 2,338 kilometers (1,453 miles). When you include the margin of error, Pluto is essentially Eris' equal in size.

"It could be smaller, it could be larger; basically, it is a twin," Lakdawalla quoted Paris Observatory astronomer Bruno Sicardy, the lead researcher for the Eris measurements, as saying at the conference.

Lakdawalla held back from reporting what Sicardy said because she was asked to. The research paper about the measurements is under consideration for publication in Nature, and Sicardy said the journal's editors told him he could discuss the results only if he instructed his audience not to report them publicly. The implication was that Sicardy's paper would be tossed out if his team's findings appeared in the press.

The audience was all abuzz about the findings, of course, but Lakdawalla said she wouldn't "break anything until somebody else breaks it."

She did, however, refer to the zipped-lip situation in a Twitter message to Embargo Watch's Ivan Oransky. Long story short, Oransky checked with Nature and was told that "researchers with papers in submission at a Nature journal can certainly present at a scientific meeting but shouldn't court the press." Oransky blogs about the back-and-forth today on Embargo Watch, but the bottom line is that Sicardy needn't have feared having his paper rejected, as long as he confined his public remarks to the presentation.

If Nature sticks to the reported publication plan, the paper will be published on Oct. 26. Today, a lot of the details came out not only on Lakdawalla's blog, but also on Scientific American's Observations blog — which is interesting, because Scientific American is part of the Nature Publishing Group. (SciAm's John Matson helpfully included a link to Sicardy's conference report.)

So what else do Sicardy and his colleagues say? Although Pluto and Eris are roughly the same size, Eris is more massive, which implies it's "mainly composed of rocky material, with a relatively thin ice mantle," the astronomers say. They suggest that Eris once had a thicker layer of ice, most of which was "blasted away" as the result of a catastrophic cosmic collision.

Sicardy and his colleagues also note that when you factor in Eris' distance, its observed brightness and its relatively small size, the dwarf planet stands out as one of the brightest bodies in the solar system, after the Saturnian moons Tethys and Enceladus. They suggest that the dwarf planet is so bright because it has a surface layer of nitrogen or methane frost, due to the freezing-out of its atmosphere.

A similar freeze-out might well happen on Pluto as it heads out to the farthest point of its orbit around the sun. Eris, meanwhile, is coming closer to the sun — and at some point the nitrogen or methane might thaw back into the atmosphere.

The two worlds seem destined to stand in the planetary pantheon as separated twins — in possession of moons, seasons, their own distinctive geologies and potentially some kind of cryovolcanic activity. Should they really be regarded as non-planets, or is it better to see them as a different class of planets? I argue for the latter in my book, "The Case for Pluto," but I'd love to hear what you think. Please feel free to add your comments below.

More about dwarfs and other planets:

• Pluto debate is about more than one little world
• Interactive: The new solar system
• Eris looks a lot like Pluto
• Eight decades of Pluto
• The Pluto files on msnbc.com

Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or adding me to your Google+ circle.

M. Showalter / SETI Inst. / NASA / ESA

Hubble imagery from June 28 and July 3 show the changing positions of Pluto's four known moons, including a newly discovered satellite temporarily designated P4.

Astronomers looking for rings around Pluto have instead made an unexpected find: a fourth moon circling the dwarf planet.

The object, temporarily designated P4, is probably the most dwarvish of Pluto's moons: It's estimated to be just 8 to 21 miles (13 to 34 kilometers) in diameter. In comparison, Pluto's diameter is about 1,400 miles, and its other three moons range in diameter from 648 miles (for Charon) to between 20 and 70 miles (for Nix and Hydra, discovered in 2005). The newfound moon orbits in a region between Nix and Hydra, and makes a complete circuit roughly every 31 Earth days.

P4 was detected in June, during a round of Hubble Space Telescope observations aimed at looking for rings or other potential hazards for NASA's New Horizons probe, which is due to zoom through the Pluto system in 2015. Alan Stern, a planetary scientist at the Southwest Regional Institute who heads the $700 million New Horizons mission, told me in an email that the discovery was a testament to the dwarf planet's continuing ability to surprise.

"Pluto's satellite system is truly knocking our socks off with surprises — it's magnificently complex, and getting more crowded all the time. I can't wait till we get there to see what other surprises this planet and its moons have in store for us!" he said.

The find is also a testament to Hubble's amazing vision. The object was spotted on June 28 using the space telescope's Wide Field Camera 3, and its existence was confirmed through follow-up observations this month as well as a search through archived imagery. The moon was not spotted in earlier imagery because the exposure times were shorter.

"I find it remarkable that Hubble's cameras enabled us to see such a tiny object so clearly from a distance of more than 3 billion miles (5 billion km)," Mark Showalter of the California-based SETI Institute, who led the Hubble observing program, said in today's announcement from the Space Telescope Science Institute.

P4 and Pluto's other moons are thought to be the result of a cosmic collision between the dwarf planet and another celestial body early in the solar system's history. Astronomers believe a similar smash-up gave rise to Earth's moon.

Pluto has gotten a bad rap in the past few years, due to its reclassification by the International Astronomical Union in 2006 as a dwarf planet rather than one of the solar system's major planets. Stern sees Pluto as just a different kind of planet rather than an also-ran, and I tend to agree with him. In any case, the fact that the world has a thin atmosphere, changing seasons and more known moons than Mercury, Venus, Earth and Mars combined demonstrates conclusively that you don't have to be one of the big planets to be fascinating. And there may be more to come as New Horizons closes in for its 2015 rendezvous.

"Pluto can retain moons out to almost 100 times the distance of Charon," Stern pointed out.

Update for 10:30 a.m. ET: Although having moons is certainly cool, that doesn't automatically qualify a celestial body to be a planet. A fair number of craggy asteroids possess a moon, or even two. The way the IAU sees it, a "planet" is a roundish celestial body that circles the sun and has "cleared the neighborhood of its orbit," which is widely seen as a deficient definition. A "dwarf planet" is a sun-orbiting celestial body that's big enough to crush itself into a roundish shape, but hasn't cleared out its neighborhood. The way I see it, dwarf planets are planets, too. But I realize a lot of smart folks see it differently. 

Connect with the Cosmic Log community by "liking" the log's Facebook page or following @b0yle on Twitter. You can also add me to your Google+ circle, and check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

P.A.S. Cruickshank

This artist's impression highlights Pluto's huge atmosphere of carbon monoxide. The source of the gas is erratic evaporation from the dwarf planet's mottled icy surface. The sun appears at the top, as seen in the ultraviolet radiation that is thought to force some of the dramatic atmospheric changes. Pluto's largest moon, Charon, is at lower right.

After nearly two decades of searching, astronomers have detected carbon monoxide in Pluto’s thin atmosphere, as they expected. But they didn’t expect to find so much of it. Pluto's dramatic seasonal changes serve as further evidence that the dwarf planet is one surprising little bugger.

"Everything about Pluto is surprising," Jane Greaves, an astronomer at the University of St. Andrews in Scotland, told me. Greaves presented the new results today at the Royal Astronomical Society's National Astronomy Meeting in Wales.

Five years ago, Pluto was at the center of a controversy over the definition of planethood — which resulted in the creation of the dwarf-planet category, a new class of celestial objects. More recent observations have pointed up still more peculiarities about Pluto. For example, scientists have found that the faraway world's surface features are changing, that its atmosphere contains clouds, and that it might even harbor a pool of liquid beneath its icy shell.

Pluto's thin atmosphere, which was previously known to contain nitrogen and methane, is thought to freeze out and rise up as the world traces its eccentric orbit around the sun. Traces of frozen carbon monoxide have been detected on Pluto's surface, which led astronomers to assume that carbon monoxide gas should be found in the atmosphere as well.

Greaves and her colleagues detected the presence of carbon monoxide in a big way. Previous observations suggested that Pluto's atmosphere extended out to a distance of more than 60 miles (100 kilometers). The new results, obtained by the 15-meter James Clerk Maxwell Telescope in Hawaii, indicate that the atmosphere goes out much farther: more than 1,875 miles (3,000 kilometers), or a quarter of the way out to Charon, the largest of Pluto's three moons.

"Carbon monoxide has been searched for but never before detected," Alan Stern, principal investigator for NASA's New Horizons mission to Pluto, told me in an email. "If confirmed, this is a significant technical achievement, and there will be some interesting scientific implications as well."

Why has the atmosphere grown that much? Greaves speculated that sunlight has been hitting bright patches of frozen material on Pluto's surface, warming up the ice and causing liberal amounts of carbon monoxide and other gases to boil off. "This is now for some reason boiling off more than it was 10 years ago," when scientists in Spain made an extensive study of Pluto's atmosphere, Greaves said.

That may seem counterintuitive, considering that Pluto was at its closest point to the sun back in 1989 and is now moving farther away. But Greaves suggested that Pluto was experiencing thermal inertia — the same phenomenon that explains why the hottest time of year comes in late summer rather than midsummer. Pluto is currently in the late summertime of its full orbital cycle, which lasts for 248 Earth years.

Mark Sykes, director of the Arizona-based Planetary Science Institute, said Pluto's larger atmosphere "is not necessarily a surprise."

"My thought is that we have the sun pushing for higher latitudes on Pluto, and that the net increase in gas production as those polar ices increasingly sublimate is dominating over the freeze-out on the dark pole," he told me in an email.

Carbon monoxide tends to act as a coolant — unlike carbon dioxide or methane. which are greenhouse gases linked to global warming on Earth. The balance between the trace amounts of carbon monoxide and methane is probably a critical factor controlling the ups and downs of Pluto's nitrogen-dominated atmosphere.

"Seeing such an example of extraterrestrial climate change is fascinating," Greaves said in a news release. "This cold simple atmosphere that is strongly driven by the heat from the sun could give us important clues to how some of the basic physics works, and act as a contrasting test bed to help us better understand the earth's atmosphere."

Greaves and other scientists will be tracking what happens to the carbon monoxide and other constituents of Pluto's atmosphere for years to come, but the biggest revelations are expected to come when the New Horizons probe flies past Pluto in 2015.

In the years since New Horizons was launched in 2006, Pluto has been removed from its niche as the "ninth planet" of the solar system — and is now seen instead as one of potentially scores or hundreds of dwarf planets. But Greaves said the icy world has retained its peculiar appeal, no matter what you call it.

"I don't think the name you classify it by is that big a deal," she told me.

More about Pluto:

• Pluto in the spotlight ... dead or alive
• Pluto's rival gets downsized
• Pluto in fact and fiction
• Eight decades of Pluto
• All about "The Case for Pluto"

In addition to Greaves, co-authors of the research paper titled "Discovery of Carbon Monoxide in the Upper Atmosphere of Pluto" include Christiane Helling and Per Friberg. The paper has been accepted for publication in the Monthly Notices of the Royal Astronomical Society Letters.

Join the Cosmic Log community by clicking the "like" button on our Facebook page or by following msnbc.com science editor Alan Boyle as b0yle on Twitter. To learn more about my book on Pluto and the search for planets, check out the website for "The Case for Pluto." 

Virgin

The Virgin Group declares that its founder, Richard Branson, has bought Pluto and is reinstating it as a planet.

Which of these headlines from today are April Fools' jokes? British billionaire buys Pluto, reinstates it as planet ... Quest to find Northwest Passage stymied by imaginary mountains ... One Mercury probe rediscovers another ... Spaceship guru hangs it up, is moving to lake resort ... Arsenic life found in sea monkeys ...

Tales of discovery are tailor-made for scientific foolery, because scientific advances and exploration almost always take place outside the sphere of everyday life.

You can't instantly verify whether gorillas at the Port Lympne Zoo are being issued iPads — but who knows what weird behavior primatologists might be studying? So if The Sun has a big spread about the "Planet of the Apps," there's just enough plausibility to keep you going. (And in fact, zoo is already famous for honest-to-goodness shots of a gorilla that walks like a man, a phenomenon that's the subject of serious study.) 

Without further ado, here's a roundup of today's scientific foolery and non-foolery:

TODAY'S FOOLERY:

Richard Branson buys Pluto, reinstates it as a planet: After setting up what's likely to be the first private-sector space tourism venture, this is a natural for the British billionaire. Here's what Virgin says in its announcement: "As a firm supporter of small businesses, Sir Richard is hoping to hoping to set an example for struggling entrepreneurs facing setbacks by having Pluto reinstated as an official planet, after its declassification by the International Astronomical Union in 1996 [actually, it was 2006]. Already at the forefront of space travel with Virgin Galactic, Sir Richard is having a special deep space vehicle built that will help bulk up Pluto to its required planetary mass." Virgin says the mission is due for launch on April 1, 2012.

JHUAPL

The Messenger mission team distributed this April 1 photo of the ancient Mariner 10 probe at Mercury.

Encounter with the ancient Mariner: The team behind NASA's Messenger mission to Mercury released this photo of NASA's Mariner 10 probe, which flew past the planet in 1974 and 1975 but then faded into oblivion ... or did it? Navigation team members speculated that solar neutrinos or outgassing may have caused changes in the ancient Mariner's trajectory. One engineer described the sight as follows: "A speck, a mist, a shape, I wist!"

Unicorn at Tower of London? It's raven mad: Britain's Metro reports on mysterious remains discovered by archaeologists.

First-ever poker tournament in space: CardsChat says it's working with Virgin Galactic to get "poker gladiators" onto the International Space Station for a $10 million duel. Next up: Lunar Poker Tour 2012. (True story: Virgin was working on an online game with a space ride as the ultimate payoff, but that project fizzled out.)

Herschel to be refilled with helium: The team behind the Europe's Herschel Space Observatory suggest that the probe could be brought in for docking at the International Space Station for a mission-extending coolant fill-up.

Animal Planet announces TV deal with Bronx Zoo cobra: The latest celebrity gets its own reality-TV show, according to USA Today.

Physicists arrested after supercollider break-in: They really did take a peek at the abandoned Superconducting Super-Collider site in Texas, but Physics Buzz kicks it up a notch with an April 1 arrest report.

ThinkGeek.com

ThinkGeek features Arsenic-Based Sea Monkeys as one of its April 1 featured items.

Arsenic-based Sea Monkeys for sale: This offering from ThinkGeek capitalizes on findings claiming that microbes from a California lake could be switched from using phosphorus to using arsenic. ThinkGeek has a whole lineup of April 1 products, including "Star Wars" lightsaber popsicles and De-3D glasses. (Those might help when you're checking out the XK3D online comic.)

In a surprise find, it's Hugs, not Higgs: CERN reports that a 16-year-old student has found the elusive but charming Hugs boson.

NON-FOOLERY:

Imaginary mountains thwart expedition: Britain's Royal Society has put a series of historical manuscripts online as part of its "Turning the Pages" project — including Edward Sabine's account of an 1818 voyage in search of a Northwest Passage through the waters of the Canadian Arctic. The expedition's commander, John Ross, ordered the ships to turn back because the way was blocked by what he called "Croker's Mountains" — a mountain range that turns out not to have existed. Why did Ross think they were there? Who knows?

Spaceship guru retires, will leave Mojave: The Los Angeles Times reports that Scaled Composites' Burt Rutan, designer of the first private-sector craft to go into outer space (SpaceShipOne) as well as Virgin Galactic's suborbital spaceship (SpaceShipTwo), is retiring after today and will move from Mojave, Calif., to the resort area of Coeur d'Alene, Idaho. The story makes it sound as if Rutan, who has been dealing with health problems, will have to pass up the opportunity to fly into space.

Belly button biodiversity under study: New Scientist highlights the Belly Button Biodiversity project, which sounds like an April Fools' joke but apparently is not.

COOL EXTRAS ... NO FOOLING:

• Technolog: All the April Fools' news on the Internets
• In-Game: April Fools' Day roundup, video game edition
• Slate: April Fools' Day Defense Kit
• Discovery News: Five historic hoaxes
• LiveScience: Five fake scientific breakthroughs
• YouTube: History's most famous April Fools' joke
• 2010: Monster bug? It's no joke
• 2009: Foolery goes high-tech
• 2008: The origin of fools
• 2004: Scientific foolery 
• 2003: Fooling the enemy

Join the Cosmic Log community by clicking the "like" button on our Facebook page or by following msnbc.com science editor Alan Boyle as b0yle on Twitter. To learn more about my book on Pluto and the search for planets, check out the website for "The Case for Pluto." And if you're listening, Sir Richard ... sign me up for that Pluto-plumping mission.

NASA

An artist's conception shows NASA's New Horizons probe at Pluto.

Few folks involved in the space effort span as wide a spectrum as Alan Stern: The 53-year-old planetary scientist is working to get himself and other researchers onto suborbital space planes to do science — and he's also heading up the science team for one of NASA's farthest-flung space efforts, the New Horizons mission to Pluto.

New Horizons is due to reach the dwarf planet by 2015, putting Pluto back in the international spotlight. And by that time, Stern may well have taken a trip to outer space himself — perhaps in XCOR Aerospace's Lynx rocket plane or Virgin Galactic's VSS Enterprise. We'll talk about Pluto as well as the pioneering efforts to commercialize space travel during "Virtually Speaking Science," at 10 p.m. ET Sunday on BlogTalkRadio and in the Second Life virtual world.

It's unusually fitting that Sunday's show features a chat with Stern, who is a former NASA associate administrator and currently serves as associate vice president for research and development for the Southwest Research Institute's Space Science and Engineering Division. Sunday happens to mark the 81st anniversary of the Lowell Observatory's announcement of Pluto's discovery.

Some folks have gone so far as to celebrate March 13 as "Planet Pluto Day." This year, the Greenwood Space Travel Co. in Seattle will get a jump on the holiday by holding its annual pro-Pluto rally at 2 p.m. PT Saturday, one day before the anniversary. I'll be there, of course, talking about the state of the planet search and about my book, "The Case for Pluto."

Maybe I'll see you at the march protesting the International Astronomical Union's putdown of Pluto and other dwarf planets. "It's not a long march — just down the street and back," Justin Allan, store manager at Greenwood Space Travel, told me.

Even if you're a Pluto-hater, you'll be welcome. The beauty of the Pluto protest is that it doesn't take itself too seriously. I wish the same could be said of everyone who's been involved in the planethood debate.

If you can't make it to Seattle on Saturday, please tune in for Sunday's show. To get ready for the program, I e-mailed Stern a few questions about his twin interests, Pluto and private spaceflight. Here's an edited version of the quick Q&A:

Cosmic Log: You're involved in so many different angles of the space frontier ... is there a unifying principle that ties all of them together?

Alan Stern: Well, I work on what I am interested in and where I think I can make a difference.

Q: What's the status of the New Horizons mission? It sounds as if the spacecraft is back in hibernation, but I'm sure that doesn't mean that the science team is hibernating as well....

A: New Horizons is doing very well — we're just now crossing the orbit of Uranus and in great shape from every perspective. We're even planning the details of the Pluto system encounter already.

Q: How has the scientific perspective on Pluto changed in the past year, or in the past five years? Some folks, such as Caltech astronomer Mike Brown [who discovered the dwarf planet that led to Pluto's downfall], say only a very few scientists are still arguing to have Pluto put in the planet category. I'm guessing that's not your perception.

A: Mike knows that's not so.

Q: Do you think developments in the Dawn mission (for example, the Vesta observations expected this year) will have any effect on the scientific discussions about small bodies in the solar system? Any dramatic results that may come to light?

A: Hard to tell. Maybe, but Dawn is visiting much smaller words than Pluto — Vesta and Ceres could together fit inside Pluto about 15 times!

Q: On commercial space, how do you think the recent suborbital research conference changed the landscape for spaceflight? How do you see the next year shaping up on the commercial front, for suborbital as well as orbital ventures?

A: The next year will be very important, because many of the suborbital companies will be testing their vehicles in flight and even making missions to space.

Q: What do you think will come out of the current deliberations involving NASA and the would-be providers of commercial space taxis for the International Space Station?

A: Several viable commercial crew capsules will be the most likely development, along with launchers than can be man-rated for these capsules to fly on.

Q: What are your thoughts on the decadal report for planetary science. You're a planetary scientist who had to struggle to get your mission to Pluto funded, as well as a former NASA official who had to struggle with planetary mission cutbacks ... what strategy would you suggest for moving forward with the missions on the table for the next decade? Do you think the decadal survey came up with the right strategy?

A: It was a great effort by the planetary community. Unfortunately, owing to the increasing costs of missions and overrun pressures, the next decade looks bleaker than the past one.

How do your opinions line up with Stern's? Tune in via the Web on Sunday to chat with Stern, co-host Robin Snelson of the Space Studies Institute and yours truly. And, oh, by the way ... Happy Pluto Day!

Join the Cosmic Log community by clicking the "like" button on our Facebook page or by following msnbc.com science editor Alan Boyle as b0yle on Twitter.

ESO

An artist's impression shows Pluto with its largest moon, Charon, facing a distant sun.

Who killed Pluto? Who said it was dead? The dwarf planet is still kicking, thanks to a new book by its "killer" as well as new rounds of research that reference the icy world.

The Pluto-killer, of course, is Caltech astronomer Mike Brown -- who along with his colleagues found a world on the solar system's icy frontier that outweighed Pluto. The discovery of that bigger world, now known as Eris, set off an international debate that led to Pluto's removal from the International Astronomical Union's official list of planets in 2006.

Brown (whose Twitter handle is @plutokiller) tells the story of the planet quest and Pluto's setbacks in a book titled "How I Killed Pluto and Why It Had It Coming." But the book is as much about his own life as a scientist and a father. Brown's daughter, Lilah, was born during the very time that his biggest scientific discoveries were coming to light.  

Brown told me it would have been "impossible" to tell the scientific story without including the story of Lilah's birth and babyhood. "They are so intertwined in my life that I can't help but mix them," he said. "Lilah's birth and the mere fact that she was a week earlier than I anticipated changed the way some of the astronomy happened. I couldn't tell either story without mixing the two together."

Even as the book was being written, Lilah was drawn into the Pluto drama.

"When she started being conscious of the book and the title of the book, she didn't really think much about whether Pluto should be a planet or not, but she was pretty sure that killing was bad," Brown said. "And so maybe six months ago, she became angry at me for having killed Pluto. She would tell me that killing is bad, and I shouldn't do it, and I should make Pluto come back. I should unkill Pluto."

Bob Paz

Caltech astronomer Mike Brown is the author of "How I Killed Pluto and Why It Had It Coming."

Brown said Lilah eventually reconciled herself to her father's ways. She told him to keep looking for a new planet -- and if he ever found one, would he please name it Pluto? Now that the book has finally come out, she's positively proud of her planet-killing pop. "She's no longer mad at me," Brown said. "Now she goes around and tells everybody that her daddy killed Pluto."

The 45-year-old professor said his 5-year-old daughter has done a good job of winning over her peers. "The kindergartners all think it's great," Brown said. "The parents are a little less sure what to think."

I can't help but imagine Lilah as a rebellious teenager, wearing a "Save Pluto" T-shirt just to spite her father. That might liven things up at the Brown household: Mike Brown said he feels a continuing obligation to explain why it makes the most sense to think of the solar system as having eight planets, plus smaller non-planets that just happen to include Pluto and the half-dozen or so worlds he has had a hand in discovering. After all, that's why he wrote the book.

"I would be very glad to be done with it," he said of his Pluto-killing role. "I do actually think that these issues and these questions and these conversations are profound for our solar system, so I think they're worth having. I wouldn't argue, like some people, that they don't matter ... that they're just semantics. I do think they matter profoundly. So as long as the discussion is continuing, then I think I will feel the need to be part of it."

You might think Brown would be more interested in boosting the status of the worlds he found. In the book, he tells how his wife, Diane, tried to keep him from dissing his own discoveries. And in fact, for a while he was OK with seeing Eris as "the 10th planet." But as the debate continued, Brown came around to the opinion that the solar system's list of planets had to stop at eight.

Divisions in the solar system
It's not so much that Brown defends the IAU's controversial definition of planethood. Brown said the definition was "pretty crummily written" but nevertheless ended up expressing the right concept. For Brown, the bottom line is that the eight largest things that go around the sun are in a special category for which the name "planet" should be reserved.

"Right now in the solar system, we are perhaps lucky, or perhaps it's a matter of physics ... but we have a solar system that draws a very strong line between the eight largest objects, which are in circular orbits and dominate the solar system; and everything else, the next biggest thing being Pluto or Eris, flip a coin. That division is easy to see and to make," he said.

"The funny thing that will happen is if there is something out there that someone finds that breaks that very clear division," Brown continued. "Something bigger than Mercury that is in a non-circular orbit and kicked around by the giant planets. It seems almost inevitable that something like that is out there. ... And when that object is found, it won't be a question about Pluto at that point, it will be a question of, 'OK, what is this?' It is going to be a difficult argument to say that something that's bigger than Mercury shouldn't be a planet. I'll make it, but I'm not sure I'm going to win that one."

Brown said he's still on the hunt for just that type of planet, somewhere on the very fringe of the solar system, "because there's nothing more fun than making astronomers argue all over again." He's involved in the SkyMapper project to survey the Southern Hemisphere's skies from Australia -- a celestial frontier that's not been as thoroughly explored as northern skies.

Pluto in the press
Of course, Brown doesn't have to wait until something bigger than Mercury shows up to have an argument, or at least a discussion. For some reason, Pluto and its little pals just keep coming up as topics for astronomers to talk about:

• A month ago, astronomers reported that they took a fresh set of measurements for Eris' size, and came to the conclusion that it was about the same size as Pluto or perhaps even a bit smaller. Brown noted that the comparative size "doesn't matter at all" when it comes to the dwarf planets' status in the solar system. What's more, Eris is known to be about 25 percent more massive than Pluto -- which would suggest that the two worlds have different compositions.
• In contrast, the similarities between Pluto and Eris are highlighted in an article appearing in Nature this week. The article was written by Alan Stern, a planetary scientist at the Southwest Research Institute who heads up the science team for NASA's New Horizons mission to Pluto. Stern observes that the surface composition of the two dwarf planets are "surprisingly similiar," with the abundance of nitrogen at 90 percent or more, and methane at 10 percent or less. The similarities extend as well to Triton, a moon of Neptune that is thought to have originated in the same zone that gave rise to Pluto and Eris. Stern said recent reports suggest that New Horizons' findings "will be of relevance to a broader suite of small planets common to the outer solar system."
• Just today, researchers reported in the journal Science that some of Earth's precious metals must have been left behind by a collision with a Pluto-sized celestial body 4.5 billion years ago. The lead researcher, Bill Bottke, is a colleague of Stern's at the Southwest Research Institute. "The populations that were hitting Earth, the moon and Mars were pretty top-heavy," Bottke told Space.com. "Most of the mass was in the big guys." Big guys? He's talking about Pluto-sized objects, right?

Such references demonstrate that Pluto isn't dead yet. Lilah Brown needn't have worried so much about her father's murderous ways: Pluto is still out there, secure in its orbit. Scientists (including Brown) are still fascinated by dwarf planets and seeking to learn their secrets. Regular folks are fascinated by the story of Pluto's ups and downs -- which is why people keep writing about it. That goes for Brown's book as well as my own, "The Case for Pluto," which takes up the other side of the argument.

The interest in Pluto and its kin is likely to rise even higher in 2015 -- when New Horizons is due to fly by Pluto while NASA's Dawn probe settles into orbit around another dwarf planet, Ceres in the asteroid belt. (Dawn's rendezvous with the asteroid Vesta is likely to be one of next year's astronomical highlights.)

Brown's book provides yet another opportunity to read about, think about, and talk about how we see the cosmos around us -- and whether you think Pluto is dead or alive, that's a good thing.

More about 'How I Killed Pluto and Why It Had It Coming':

• Bad Astronomy: How to settle the 'What's a Planet' debate
• On Point: The man who killed Pluto (and other space odysseys)
• Universe Today: Q&A with Mike Brown, Pluto killer
• Daily Kos: How I killed Pluto
• New York Times: When a heavenly body got the boot
• The Atlantic: Read an excerpt from 'How I Killed Pluto'

More about 'The Case for Pluto: How a Little Planet Made a Big Difference':

• 80 years of Pluto
• A Pluto pilgrimage
• Pluto maps raise new questions
• Interactive: The new solar system
• More Cosmic Log coverage of Pluto
• Read an excerpt from 'The Case for Pluto'

Correction for 2 p.m. ET Dec. 10: I think of the Southwest Research Institute so much as SwRI that I mistakenly wrote Southwest "Regional" Institute. Thanks to Brent Markus for pointing out the error.

Connect with the Cosmic Log community by "liking" the log's Facebook page or following @b0yle on Twitter.

A. Schaller / STScI

New observations suggest that the dwarf planet Eris, shown in this artist's conception, isn't as big as scientists thought it was.

Everyone knows that the solar system is no longer seen as a nine-planet set. When you count the dwarfs, there could be scores or hundreds of planets out there -- as I write in my book, "The Case for Pluto." But a case could be made that Pluto is once again the ninth-largest planet orbiting the sun, based on observations reported over the weekend.

Pluto was ousted from the No. 9 spot five years ago, after the discovery of another dwarf planet on the solar system's icy frontier, known today as Eris. It was Eris' apparent status as an object slightly bigger than Pluto that brought the controversy over the definition of a planet to a head. If Pluto was an honest-to-goodness planet, shouldn't Eris be one as well?

In 2006, the International Astronomical Union approved a definition that established a new class of objects, called dwarf planets, which were big enough to be basically round but not gravitationally dominant enough to "clear out the neighborhood of their orbit." What's more, the IAU ruled that dwarf planets were not really planets.

My book delves into the questions raised about that definition, particularly in light of what we've been learning about planetary systems since then. Now there's a new question: Is Eris bigger than Pluto after all? Based on observations of Eris' occultation of a faraway star in the constellation Cetus, the answer could well be no.

Sky & Telescope's Kelly Beatty reports that the latest observations suggest Eris is actually slightly smaller than Pluto. He quotes the Paris Observatory's Bruno Sicardy as saying Eris is "almost certainly" no wider than 1,454 miles (2,340 kilometers), compared with Pluto's estimated width of 1,456.5 miles, plus or minus 6.5 miles (2,344 kilometers, plus or minus 10 kilometers).

"If the early results hold up, this time it's the dwarf planet Eris' turn to be demoted, and Pluto might have just regained its status as the largest object in the Kuiper Belt," Beatty writes. The Kuiper Belt is the broad zone of icy objects that lie beyond Neptune's orbit.

Gathering the data for the measurements was a grand astronomical feat: Three teams of scientists watched the distant star disappear when Eris crossed in front of it. By analyzing how long the star was covered over, as seen from three vantage points in Chile, the astronomers could calculate how wide Eris' round disk was. Previous estimates were based on indirect data, such as Eris' brightness.

Further observations will be required to reduce the uncertainties surrounding the two worlds' widths. The current estimates are so close that Caltech astronomer Mike Brown, whose team discovered Eris and two other dwarf planets, can justifiably say Eris and Pluto are "more or less the same size." And when you rank the two by mass rather than size, Eris clearly comes out on top. That implies that Eris' interior is denser and thus rockier than Pluto's.

"How could Eris and Pluto look so similar in size and exterior composition yet be totally unalike on the inside?" Brown writes. "As of today I have absolutely no idea. ... Something is going on in the outer solar system, and I don’t know what."

Whether Pluto is bigger or smaller than Eris really doesn't affect its status as a dwarf planet. But it does illustrate that small celestial objects can deliver some big scientific surprises -- and that it's a huge mistake to write off the little guys of the solar system.

Correction for 6:50 a.m. ET Nov. 8: I originally wrote that Pluto might be the ninth-widest object in the solar system, neglecting to take into account that some moons are wider than Pluto (and in fact wider than Mercury). Thanks to Stevesliva for pointing that out.

More on dwarf planets:

• Pluto debate is about more than one little world
• Interactive: Guide to the new solar system
• Pluto maps raise new questions
• 80 years of Pluto

Connect with the Cosmic Log community by "liking" the log's Facebook page or following @b0yle on Twitter. You can also check out "The Case for Pluto," Alan's book about the controversial dwarf planet and the search for new worlds.

Pluto and its discoverer play key roles in "Percival's Planet," a novel that weaves a fictional tale around historical and scientific facts from 80 years ago.

That's not the way Michael Byers expected things to go when he began thinking about the book. The University of Michigan creative-writing professor intended to write about his grandparents and their turbulent relationship in the 1920s and 1930s. But once he started telling the story of his grandfather's Harvard law-school education, his courtship and marriage, he realized that the tale was "very boring."

"I couldn't quite find the interest in the material that I felt was needed," Byers told me, "so I actually put the whole project down." Instead, he wrote a novel about something else entirely, titled "Long for This World."

By the time he returned to the family saga, another angle popped into his head: Byers recalled that while his grandfather was in law school, other folks with Harvard connections were engaged in a more exciting project: looking for the mysterious Planet X that had been predicted by Percival Lowell, a Boston brahmin-turned-astronomer.

"I decided to turn my grandfather from a lawyer into an astronomer," Byers said. And not just any astronomer: In Byers' story, the fictional character is a close colleague of Clyde Tombaugh, the real-life astronomer who found Pluto in 1930 at the Lowell Observatory in Flagstaff, Ariz. Byers' character, named Alan Barber (Alan B. - hey, I like that!), is given the task of checking Tombaugh's math to make sure what he found was a real planet.

Henry Holt and Co. via AP

"Percival's Planet" uses the search for Planet X as its starting point.

The resulting novel blends fictional characters (such as Jean and Sarah, Tombaugh's supposed daughter and granddaughter) with real ones (such as Percival Lowell's meddlesome widow, Constance, as well as Tombaugh's wife, Patsy, who is very much alive today at the age of 97). The reviews have been mixed: The Seattle Times' Misha Berson calls "Percival's Planet" an "absorbing, fascinating new novel," while The Associated Press' Patrick Condon says it's a "strangely earthbound book."

I wasn't about to let Condon's discouraging words keep me away from the novel, in large part because of my own work of nonfiction about the dwarf planet's discovery, "The Case for Pluto." Sure, I like a good romance as much as the next guy, but I was primarily interested in how Byers handled the story of Pluto's discovery. It turns out that was a subject of interest for Byers as well.

"I really did want to know more about what they thought they had," Byers told me. "It seemed to me that they were a little cagey about what they came up with, and they were cautious. ... It seemed to me that one of their difficulties was in telling the story to themselves of what they had done."

That squares with my impression: Veteran astronomer Brian Marsden, who supported Pluto's controversial reclassification as a dwarf planet almost exactly four years ago, once said that the Lowell Observatory's astronomers "bamboozled" the world into accepting the smaller-than-expected world as one of the solar system's major planets. But the historical record shows that the Lowell scientists were unusually circumspect about describing what they found - until the press anointed Pluto as the "ninth planet."

The astronomers waited for a month before announcing what they found - and when the announcement was finally issued, they noted only that the object behaved like a "Trans-Neptunian body at approximate distance [Lowell] assigned." Tombaugh was reportedly so baffled by Pluto's dimness that he wondered whether he had merely found the moon of a planet yet to be discovered.

"Clyde is figuring out that they're a little unsure of what they found," Byers said. "Basically, he asks 'What is it?' ... and he gets told, 'Well, it's Planet X, kid, get used to it.'"

Once the rest of the world accepted Pluto as a planet, so did the astronomers. One of the difficulties was that Pluto was the first object of its kind to be discovered. The next object in the icy belt of material beyond Neptune would not be discovered until 1992.

"When they found Pluto in 1930, they didn't have a better word for it," Byers observed. "It was a planet, and that was the best applicable term to come up with. They could have invented a term, like dwarf planet, I suppose. But it wasn't there. ... I think they acted in fairly good faith, let's say. They weren't trying to fool anybody, but they didn't stand in anybody's way."

Because Byers' story ends in 1990, there's just a tiny whiff of the debate over Pluto that was to come. "As the author, I ackowledge that there's a controversy over its status," he told me. "The story goes on, but I choose to leave the story back in 1990 as a tip of the hat to Clyde himself, just to honor him and his discovery. I don't have a horse in this race. I leave that to the pros to figure out."

Byers isn't so interested in asking whether Pluto is or isn't a planet (and it's a type of planet, by the way). He's much more interested in asking why people care so much about the answer - and here again, Byers' view squares with my own.

"It matters to us what these facts are," he said. "It matters to us deeply. And we don't notice how much it matters until those facts change. And the most moving aspect of the controversy surrounding the planet's reclassification has been that recognition in people: that it matters to them how the solar system is laid out. It's probably something no one would have given a second thought to, had Pluto not been forced to suffer its demotion. I like that."

If you like that, too, give Byers' book a look - and take a look at the other works from the Pluto Authors' Fraternity, including my own "Case for Pluto," Neil deGrasse Tyson's "The Pluto Files," Laurence Marschall and Stephen Maran's "Pluto Confidential," Alan Stern and Jacqueline Milton's "Pluto and Charon," Paul Sutherland's "Where Did Pluto Go?" ... and the true story of Clyde Tombaugh, David Levy's "Clyde Tombaugh: Discoverer of Planet Pluto."

Update for 5 p.m. ET: Still more authors are due to be inducted into the fraternity. Caltech astronomer Michael Brown is coming out with "How I Killed Pluto and Why It Had It Coming," and longtime Plutophile Laurel Kornfeld is working on a book as well, titled "The Little Planet That Would Not Die: Pluto's Story." Keep an eye out for "Pluto: Sentinel of the Outer Solar System," written by British astronomer Barrie Jones. The book gets into the nitty-gritty of Plutonian planetary science, including actual math and data diagrams. Jones also presents a sensible view on Pluto's current status:

"I dislike the creation of the class 'dwarf planet' separate from the distinct class 'planet.' I would much prefer a class 'planet' with a sub-class 'dwarf planet,' and another sub-class, possibly named 'large planet.' ...

"So, for now, we have to make do with a flawed, incomplete classification system for planets.

"With things as they are, is the solar system 'stuck' with just having eight planets? Not necessarily. Who knows what lurks in the outer depths of the solar system?"

Who indeed?

More about the planet search:

• Hunt for new worlds goes into overdrive
• Pluto debate is about more than one little world
• Interactive: The new solar system
• Alien planets locked in close embrace

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