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Five years ago, the world paused for a moment to admire the view of a distant world called Pluto. Its discoverer, Clyde Tombaugh wasn’t around to see the spectacular New Horizons views of the planet he discovered in 1930. But, in another sense, he WAS there with the rest of us.

Pluto's colorful surface. Pluto was discovered by Clyde Tombaugh in 1930 and explored by New Horizons in 2015. — Pluto’s colorful surface units show different compositions and terrains. Courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

A few grams of Clyde’s ashes fly on the New Horizons spacecraft that flew by his planet. So, for a collective moment, we explored Pluto with its discoverer and the team of scientists he inspired.

The late Clyde Tombaugh, discoverer of Pluto. courtesy NMSU. — The late Clyde Tombaugh in 1989. He died in 1997 at age 90,. Courtesy NMSU.

I’ve often wondered what Clyde would think of Pluto, up-close, and personal. I only met him a few times. He was a man with a great sense of humor and humility about his accomplishment. So, I imagine that he’d love it all. It would have been cool to have him on the science team as New Horizons flew by. His family members attended the flyby festivities in mid-July 2015 and celebrated the mission.

A Virtual Pluto Celebration

Today, we are in a very different world, one where a pandemic has closed us off to social gatherings like the one we all experienced at “mission control” at Johns Hopkins Applied Physics lab half a decade ago. Instead of meeting to talk over old times at Pluto, now we have to do it virtually.

So, instead, let’s imagine a circle of scientists sitting here with us. They’re reminiscing about the amazing mission to Clyde’s world. I asked a collection of New Horizons researchers to do exactly that. What did they learn about Pluto? If they could take Clyde on a guided tour, what would they show and tell him about Pluto?

Show and Tell for Clyde

Obviously, the first thing to show him would be heart-shaped Tombaugh Regio area of Pluto. It became THE ‘iconic’ view of the planet from the beginning of the flyby. That image changed forever how we saw that distant planet. Its icy terrain and lobe-shaped Sputnik Planitia section show glaciers butting up against mountains of water ice. Those features would likely excite Clyde. Those who knew him would be expecting him to crack a few puns to celebrate the occasion.

Mountains on Pluto — This image shows the inset in context next to a larger view that covers most of Pluto’s encounter hemisphere. The inset was obtained by the Multispectral Visible Imaging Camera (MVIC) instrument on New Horizons. North is up; illumination is from the top-left of the image. The image resolution is about 1050 feet (320 meters) per pixel. The image measures a little over 300 miles (almost 500 kilometers) long and about 210 miles (340 kilometers) wide. It was obtained at a range of approximately 9,950 miles (16,000 kilometers) from Pluto, about 12 minutes before New Horizons’ closest approach to Pluto on July 14, 2015. Courtesy NASA/JHU-APL, SWRI/New Horizons mission.

But, he might also be touched by other views. The most telling and effective image of Pluto is a view of its atmosphere and surface together. It encapsulated the planet, its atmosphere, and a feeling.

Pluto's mountains, plains, and layered hazy atmosphere.

For Anne Verbiscer, a member of the New Horizons team, that’s the view she would share with him. “I’d show him that image taken at twilight,” she said. “I was reduced to tears when I saw it. I’ve had the same reaction to only one other planetary image returned by a spacecraft…it was the first image of Enceladus taken by Cassini for which I had done the “commanding”. Enceladus was in eclipse (behind Saturn), so I had very little information to use to set the exposure time correctly. I was sure that the image would either be black or over-exposed. When that image arrived on the ground, I saw that it was perfect, and yes, I sobbed, just like I did when I saw Clyde’s amazing world in all its glory on that September weekend in 2015.”

That view reminds all of us of the strange beauty of planetary exploration. There’s always something new to be seen, and when the exploration ends, there’s much to learn from the data collected. “I would show Clyde that image of Pluto at twilight (the one with the bottom of Sputnik Planitia in the foreground, the mountains rising above the limb, and the hazes above the horizon)….. and just drop the mic,” said Anne.

Clyde’s Surprising World

Principal investigator Alan Stern, not often at a loss for words, finds it hard to say what surprised him most about Pluto. “Before the flyby I might have not been surprised to find one of these: an interior ocean, or blue skies, or bladed mountains, or vast glaciers, or cryovolcanoes, or a paleolake,” he said. “But we found them all! And that is the most surprising aspect to me. That is, just how incredibly complex and diverse and frankly amazing as a scientific wonderland that Pluto turned out to be.”

To give you an idea of what amazed Alan and the others, take a look back in time. Clyde’s view in 1930 was of a small dot. That’s about all any of us can see, even with the best ground-based telescopes Even a high-resolution Hubble Space Telescope view showed a bland surface with a few bright and dark areas. No details of what was to come.

A Surprising World

New Horizons showed that every region on Pluto has something different on it, with some features unique to that world alone. Five years later, after much study and analysis, all that geology tells us something we didn’t know about the planet before the spacecraft went by: it’s not a dead world. “It shows the action of a wide range of planetary processes,” said Hal Weaver, another team member and long time planetary scientist. “That includes some that happened long ago, and others indicating ongoing activity into the current epoch.”

A closeup of the highlands on the edge of Sputnik Planum. There are also ice pits and other features indicating some kind of geologic activity in the past and possibly in the present. Courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

In other words, Pluto is alive. That’s a surprise, even today. And, that’s what Hal would tell Clyde first. If he could go to Pluto on a guided tour, the first place Hal would land would be Sputnik Planitia. “It’s responsible for Pluto’s “mojo”,” he says. “Especially its periphery. And, it’s part of the region named for Clyde! And, I’d bring along his extended family, too!”

Pluto and the followup mission to Arrokoth have accomplished another amazing feat. Those flybys prove the outer solar system isn’t what people expected it to be. “Pluto and Arrokoth have showed us that the Kuiper Belt is so full of surprises and revelations about the formation and evolution of the solar system,” said Paul Schenk, of the Lunar and Planetary Institute in Houston. “Going back to the Belt should be a high priority, to look at small, medium, and large-sized bodies and find out what happened there in the early days of our solar neighborhood.”

Beyond Clyde’s World

Nowadays, New Horizons is far beyond the Plutonian system. A few years ago, it explored Arrokoth, which is another Kuiper Belt object. It has other tasks ahead of it, including possible visits to new worlds. Its story is really one of opening the exploration of the third regime of the solar system. We’ve explored the inner solar system, the “middle” solar system of the gas and ice giants, and now the distant regime is open to our view.

Indeed, the exploration of the outer solar system really began with this mission. It is the first spacecraft to give us a true “up close and personal” look at the Kuiper Belt neighborhood. It’s only fitting that another spacecraft should make its way out there, to follow up on what New Horizons started.

Team member Kelsi Singer pointed out that there would be much to explore. “If we could go back with a spacecraft that orbits Pluto [instead of flying by], we could learn if Pluto has an ocean beneath its icy shell,” she said. “We could also further investigate how all its unique features formed. Of course, a lander or rover would be fabulous, but we have much to learn from an orbiter, first.”

Celebrating a Milestone with Clyde

Pluto and its exploration five years ago sparked our imaginations. It opened our eyes to an alien world. And, it shows us that preconceptions about planets are meant to be shattered. If New Horizons does nothing else, it will have shown us to expect the unexpected at every turn.

Alan Stern, who waited many long years for that exploration, working with some of the best scientists in the solar system, would just as soon be out there in an orbiter right now. He and a continuing team of scientists want to follow the trail that their spacecraft blazed.

And, if he could take Clyde along with him on that trip, he would. “I’d tell Clyde, who I knew, that he discovered an amazing world that has literally revolutionized our knowledge of planetary science in multiple important ways, and that in my view, we now know that in Pluto, the solar system saved the best for last!”

The backlit view of Pluto as New Horizons ‘waved’ a last good bye. NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Remember hearing about Planet 9 a while back? That’s astronomer Mike Brown’s designation for a planet that he thinks is out there in the outer solar system. So far, it remains elusive, despite everybody’s best observational efforts. (Of course, we already have a planet 9, called Pluto, so to avoid confusion, it sometimes gets referred to as Planet X.)

So, why does Mike thinks he’s got gravitational evidence of a massive world somewhere out in the Kuiper Belt? Essentially, he has observed the changes in orbital motions of bodies “out there”, and imputes those changes to the gravitational effect of a planet hiding somewhere. If it exists as a planet (and not, say, a loosely-bound clump of large planetoids or something), then eventually someone should be able to catch a glimpse of it as it reflects light, or emits a little heat (since it would be warmer than surrounding space). But, so far, nobody’s seen this possible planet.

Black Holes in the Outer Solar System

What if it’s not a planet? What if it’s something massive, but not another world? Say, like a black hole? That’s the conjecture behind a new method that two astronomers at Harvard have devised. They suggest that a small, planet-mass black hole could also exert a gravitational tug on objects in the outer solar system, perturbing their orbits. The method, developed by Dr. Avi Loeb and graduate student Amir Siraj, suggests searching for emissions as the black hole swallows up material.

Artist's conception of accretion flares resulting from the encounter of an Oort-cloud comet and a hypothesized black hole in the outer solar system.
This might explain the Planet 9 that planetary scientists have tried to find.
Credit: M. Weiss — Artist’s conception of accretion flares resulting from the encounter of an Oort-cloud comet and a hypothesized black hole in the outer solar system. This might explain the Planet 9 that planetary scientists have tried to find. Credit: M. Weiss

How would that work? Well, if there’s really a black hole out there, it’s probably continually accreting matter. Its “food” would come in the form of gas and dust from interplanetary space, and the breakup of bodies in the Oort Cloud or Kuiper Belt. That accretion action creates higher temperatures in the region around the black hole. Any frozen bodies, such as cometary nuclei, would start to melt as they passed by. Eventually, they’d be too weak to keep their structural integrity, and they’d break apart. Once that happens, their material swirls into the black hole. That process emits radiation.

Look for the Hungry Black Hole.

So, the idea is to search for flares of this radiation when “stuff” near the black hole is superheated in the accretion disk. It’s about the only way that a black hole gets “lit up”. Otherwise, it’s invisible to the eye. But, the radiation emitted from the accretion disk is detectable. And, if a hungry black hole is gobbling up comet nuclei bits and pieces, or material from rocky bodies that collide “out there”, then the flares from that feast would show up in our detectors.

So, Why Go After Planet 9?

So, why pick Planet 9 as a possible study subject for this method? For one thing, it’s relatively close by, as such things go. That makes it a good subject to try out the method before using it on more distant objects. Whatever it is has a measurable gravitational effect on bodies beyond the orbit of Neptune (that’s how Mike discovered it).

Interestingly, a black hole at the same distance would do the same thing. And, right now, nobody has detected any light or other radiation from a possible planetary body in the supposed “regime” where the suggested Planet 9 exists. But, Brown laid some groundwork for testing the idea, since he’s found “something” that remains unknown. Loeb has suggested that a grapefruit-sized black hole with a mass five or ten times that of Earth could have the same effect as a massive planetary body (or dwarf planet body) like the proposed Planet 9.

Modeling a Black Hole Instead of Planet 9

So far, this is all a theoretical model. And, it’s not the first time that astronomers have suggested the idea of Planet 9 as a black hole. There are a lot of riddles to solve.

For one thing, planet-mass black holes are a relatively new concept. Yet, astronomers study stellar-mass and supermassive black holes, as well as intermediate-mass ones. So, they can extrapolate some of what they know about the big black holes to understand planet-mass ones. As far as I know, none have actually been discovered, so that’s another problem to solve.

But, the idea of a black hole with a few times the mass of Earth affecting orbits of distant solar system objects is intriguing. And, it can be studied. The LSST telescope at the Vera Rubin Observatory in Chile covers large areas of the sky very quickly. The idea is to point it at a broad region where the supposed Planet 9/Black Hole candidate may exist. The large field of view of the VRO telescope would be very useful since nobody knows exactly where the object lies. it’s kind of like aiming at the broad side of a bar to look for a nail in one board.

The Outer Solar System as a Treasury

We all know that the Kuiper Belt and Oort Cloud regions teem with remnants of solar system formation. There’s a storehouse out there of the ices that existed in our protosolar nebula. Now, they exist safe in the coldest regions of the planetary system, well away from the Sun. That makes those objects treasuries of material that predate the Sun and planets.

I think it would be pretty cool if those regions also harbored black holes. The smallest black holes (the micros and the minis) are largely thought to be theoretical. Finding a planet-mass black hole in our backyard would actually be a major discovery. It would be even cooler than a theoretical Pluto-killer planet that, so far, hasn’t been seen.

Want to know more? Check out this story from Harvard Center for Astrophysics and the Black Hole Institute for more details.

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What Would Clyde Think?