Dr. Will Grundy, an astronomer and planetary scientist with the Lowell Observatory and a member of the New Horizons team, has been studying Pluto his entire professional life.
Guest | Dr. Will Grundy, Astronomer, Lowell Observatory
Host | Matthew S Williams
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Episode Notes
Dr. Will Grundy, an astronomer and planetary scientist with the Lowell Observatory and a member of the New Horizons team, has been studying Pluto his entire professional life. Together, we sat down to discuss matters of planetary science, the Great Planet Debate, and the importance of icy bodies in the outer Solar System.
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Resources
Lowell Observatory: https://lowell.edu/
I Heart Pluto Festival 2025: https://iheartpluto.org/
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For more podcast Stories from Space with Matthew S Williams, visit: https://itspmagazine.com/stories-from-space-podcast
Pluto and the Great Planet Debate | A Conversation with Dr. Will Grundy | Stories From Space Podcast With Matthew S Williams
Matt: [00:00:00] The authors acknowledge that this podcast was recorded on
the traditional unceded lands of the Lekwungen peoples. Hello, and welcome
back to another episode of Stories from Space. I'm your host, Matt Williams,
and joining me today is a very special guest, Dr. Will Grundy, an astronomer
and planetary scientist, and a member of the New Horizons Mission science
team.
And we will be discussing Pluto, its discovery, what we've learned about it to
date, what mysteries we hope to learn more about in the future, and, of course,
the great planet debate. Where does Pluto fit in, in terms of classification into
our solar system? Is it a dwarf planet? Is it a planet planet? And what
implications does that have for planetary researchers and scientists?
Currently involved in the study of large icy objects in the outer solar system. Dr.
Grundy, thank you for coming aboard. [00:01:00]
Dr. Grundy: Happy to be here.
Matt: So the planetary debate, this is an issue and it is a subject of ongoing
controversy. It has been ever since 2006 with the IAU's official decision to
reclassify Pluto as a dwarf planet.
But before we get into all that, Well, I'd like to hear a little bit about yourself
and how you came to be at the Lowell Observatory and why this is such a
significant institution. And also, yes, your own work with Pluto.
Dr. Grundy: So I've been interested in Pluto since, uh, probably since I was a
kid. Um, but, uh, I, I did a number of research projects related to Pluto when I
was a grad student and, uh, and my first postdoc, which was in France, um,
doing laboratory studies of the materials that are on Pluto's surface.
Um, and Lowell Observatory happened to [00:02:00] have. Uh, one of the few,
uh, infrared spectrometers at the time that, uh, was suitable for observing Pluto.
And I figured, yeah, I'd like to get my hands on that. So I brought my next
postdoc, my second postdoc, uh, to the observatory and they couldn't. Chase me
away after that.Matt: Excellent. And as my listeners have been informed, Lowell Observatory
founded by Percival Lowell, was where Clyde Tombaugh originally discovered
Pluto.
Dr. Grundy: Yep. That's, that's right. And, uh, that historical fact did not
escape me when I was thinking about where I would like to, uh, do my work.
Although all of those characters had left the scene long before I showed up.
Um, in fact, I'm sitting in the building where Clyde blinked the plates and made
the discovery. And, uh, so that's kind of cool.
Yeah. So can you tell my listeners a little bit about Tombaugh's discovery?
Like, [00:03:00] what were some of the key moments and really inspiring stuff
that you learned about as a kid that made you want to study Pluto?
Well, to be honest, I grew up with the exploration of the planets by spacecraft.
And, you know, I was a kid when the Voyager spacecraft flew past, uh, Jupiter
and Saturn. And, uh, I was, uh, Uh, an undergrad when it flew past the earnest
system, and, um, and I had just started grad school when it flew through the
Neptune system.
And every time that happened, it was just fascinating to see those worlds. Uh,
you know, all of the small IC satellites and, and, uh, the rings and all of these
things in such amazing detail for the first time ever. And that seemed like, uh,
uh, a worthy career path if I could just get my foot in the door. And of course,
back then there was only really one more opportunity.
[00:04:00] So it was like, hmm, maybe I ought to, maybe I ought to look at that
one. Um, now since then, there's been a ton more. Uh, objects like Pluto
discovered. And so, uh, there's rich pickings for generations to come. Uh, it
does take a long time to get out there with a spacecraft.
Matt: In fact, looking back, were you ever disappointed that the Voyager
probes never did a flyby Pluto?
Dr. Grundy: They could have . Uh, it was an option that was built in to the
program, and they, uh, opted instead for a close flyby of, uh, Titan, which is
also a spectacularly fascinating planet in its own right, um, with all kinds of
interesting processes and interactions going on between its surface and its
atmosphere and so on.Um, so, cool. It was a decision that was made by the smartest minds at the time,
and in hindsight, maybe it wasn't the right decision, but, uh, but it, you know, I,
I, I can't second guess them, but if I were in their shoes, I probably would've
done the same [00:05:00] thing.
Matt: Uh, now did you say Titan or Triton?
Dr. Grundy: Titan. Titan. Okay. So, so basically the, the, the large moon of, of
Saturn. And as a result of that, Voyager won. Uh, basically got deflected out of
the plane of the solar system, and, uh, couldn't visit any more planets, and that's
why only Voyager 2 went on to do the further exploration, and after hitting
Uranus and Neptune, it could not have made it to Pluto, but the Voyager 1 could
have if it had not done that, that, uh, close flyby of Titan.
Matt: You yourself were involved in the New Horizons mission and the science
team that was the first spacecraft in history to fly by Pluto in 2015, if I
remember correctly. So, what exactly, uh, was your position with the team and
how exciting was it?
Dr. Grundy: Well, it, it, it was very exciting. Um, I mean, it was a slow burn
because it took us nine and a half years of, are we there yet, uh, to get there.
[00:06:00] Um, and, and quite a few years before that to do the engineering and
build the spacecraft and get launch approval and, and so on. So really, uh, we
started working on this in the. Very beginning of the 2000s.
Well, even even earlier than that. Um, so it's been been a very long project. And
of course, you know, 9. 5 years to get the Pluto and that was in 2015. And now
we're in 2025 so coming up on the 10th anniversary of the fly by and the
spacecraft is traveled another 9. 5 years beyond Pluto. So it's it's getting to be
quite a ways out in the solar system.
Now, my role on the team was The, uh, lead of the, uh, surface composition, uh,
team team or science team team, um, we had divided the team up into, uh, into,
um, a geology and geophysics team, uh, surface composition team and
atmospheres team and particles and plasmas team, [00:07:00] uh, just to work
on different, um, uh, aspects, scientific aspects of the, of the system and, yeah.
Uh, an advantage of that was that each of us could draw on any of the
instruments on the spacecraft. So it wasn't these little walled territories of, of,
uh, you know, instrument teams that were not. Willing to share their data with
the other instrument team. So it was much more synergistic and, and, uh,
Collaborative, I think, than the style of some of the larger missions where,where the individual instrument teams are very jealous of what science can be
done with their data.
And it makes it harder to combine the data from the different teams. Um, but
the, the, uh, the excitement of, of seeing a new world for the first time is really
fabulous and I'm, I'm very happy to have been part of that. Um, I mean, really,
that's the high point of my career and the other thing that I, I think, uh, you
know, I saw it [00:08:00] when I was growing up in the, in the 70s and the 80s,
um, but a whole new generation hadn't seen that and this was their first and only
one of these.
So, uh, yeah. You know, we, we've managed to bring a whole new generation
into this sort of, uh, discovery style of science and, and I'm excited for the future
because there's so many more of these small icy bodies out in the Kuiper belt
that, um, I don't know how often we'll manage to, um, drum up the money to
send a spacecraft to them, but, uh, there's plenty more exploring to be done.
Matt: And in fact, it was the discovery of these. Many other bodies there. Um,
correct me if I'm wrong, the first was Iris that was discovered in the outer solar
system.
Dr. Grundy: I don't remember what order they were discovered in, but yeah,
Eris and Maki Maki are sort of the two, maybe most Pluto like in terms of their
compositions.
And then there's Haumea, [00:09:00] which is more similar to Charon in terms
of its surface composition.
Matt: And just a quick note there. Yeah. Charon being Pluto's largest satellite.
And in fact, the. The system itself, Pluto and Charon are, their, their size of
mass distribution is such that many people think of it as a, uh, that they're co
orbiting each other rather than Charon orbiting Pluto.
So, was there any discoveries that came out of this mission that were really sort
of myth busting or, you know, we didn't expect to see that?
Dr. Grundy: Almost too many to list, but yes, uh, so, certainly, you'll hear this
repeated quite a bit, that the expectation was that Pluto would be geologically
dead, and the, the reasons why people might think that were that it's just very
small and very far away from the sun, so what's, what's there to power it?But, that, that [00:10:00] wasn't really. Really true, because the existence of
methane ice on its surface, which was detected through Earth based telescopes,
you know, decades ago, basically tells you that it had to be active at some level,
because methane ice very quickly deteriorates into, you know, tar like gunk, and
it's a one way street.
So, either there's new methane coming out from the interior, or the methane is
continually turning over, or You know, there's seasonal patterns of
redistribution or something is going on. So, really, we knew that there was some
activity. But what we didn't have was the imagination of what that could be
when it's written out on the scale of a planetary surface.
You know, I've played with methane ice in the laboratory, and, you know, it
looks like, kind of looks like water ice. It's transparent, clear, and, you know, it
just Turns into gas at very low temperatures, but methane ice in conjunction
with nitrogen ice and carbon monoxide ice, which are the three main [00:11:00]
volatile materials on Pluto's surface, do all kinds of stuff.
Uh, you know, when the sun shines on them, they sublimate and they migrate
around to the other side of the planet, but where they pile up, they can form
glaciers and, and the glaciers flow downhill and can erode the geology and, and
the glaciers are not much like the, Glaciers on earth in the sense that on earth
the rocks underneath are dense and the water ice is not so dense So it stays on
top and it erodes the rocks, but it stays on top of them
What nitrogen ice is actually more dense than water ice slightly and the glaciers
when they're nitrogen rich can penetrate into the water ice because The densities
are, well, because it's more dense than the water ice, so, you know, what
happens then when you've got glaciers diving into the ground, you know, when
they get deep enough, I suppose they encounter warmth, and then they
sublimate away and come back up to the surface, and we have no idea what that
all entails.
And then there's this giant basin, Sputnik Planitia, that's full of this kind
[00:12:00] of ice mixture, so much so that the warmth And there's not a whole
lot of warmth because Pluto is so small, uh, but the warmth from radioactive
decay from rocks in Pluto's core is sufficient to drive a slow simmer, you know,
like a Boiling pot of oatmeal, you know, it takes 100, 100, 000 years to turn
over, but it's continually sitting there and churning and erasing any impact
craters that hit it.So it's basically a crater free surface that that one region, you know, who, who
would have imagined that something like that would be. Going on. Uh, not me.
Anyway, I'm not that creative.
Matt: Well, I know what you mean there. I remember learning of that. That, in
fact, Pluto is likely to be an ocean world is now the Like the common, uh, uh,
term for it and that.
Dr. Grundy: Yes. Yeah. I mean, it's really changed our view of where
habitability can be because, you know, the, the traditional [00:13:00] idea of
only where liquid water is stable on the surface, basically confines you to earth
and maybe Mars and maybe Venus once upon a time before things went wrong.
But this extends it all the way out through the Kuiper belt.
You know, if, if Pluto has a warm, wet interior, it could be habitable, I mean,
decades, if not centuries before we would actually confirm that, but, um, you
know, the same could be true for Eris, which is comparable in size to Pluto, but
has more rocks, so more heat.
Matt: Thanks so much. And to circle back a little bit there, um, I just had to ask
about the exciting discoveries and unexpected stuff.
But it was the discovery of additional bodies in the outer solar system that were
comparable in size and mass to Pluto. This is what led to the great planet
debate. As I recall, and the IAU's decision to formally sort of nail down, pin
down the definition [00:14:00] of planet. Now, were you personally privy to any
of that?
Dr. Grundy: No, I didn't go to that meeting, and, and to be honest, um, a lot of
the planetary science community just shrugs that off. Because it's really not
scientifically useful. It's really the IAU panicked because they were worried. Oh
my goodness. What if we start getting new planets every year? We're, you
know, school kids are going to have to memorize these things.
It's like, that would be horrible. You know, it's more, more planets is better as
far as I'm concerned, because I'm a planetary scientist. I study these things. And
if you have more than one that are similar to each other, basically it's nature
running an experiment for you. You know, you don't get to make your own
planets.
Uh, but if, if Nature made something like Earth and put it where Earth is, and
they made something very similar to it and put it a little closer to the Sun. Wehave a natural experiment, and nature did that with Earth and Venus. And so
you can do the comparisons between them to learn much, much [00:15:00]
more about them than you could if you just had one of them.
And so we've got, you know, five terrestrial planets. I would count the moon as
one. We have the four giant planets. And okay, maybe Uranus and Neptune are
ice giants, and the other two are gas giants. Um, but now it looks like we have
half a dozen or more of these small icy bodies, too. So that's fabulous. We can
do comparative planetology with them, too.
And they have just as rich Uh, planetary phenomena going on as the other
classes of planets do. You know, just as an example, I talked a little bit about
nitrogen, methane, and carbon monoxide ice. On Earth, we have water. That's
the one volatile that can, you know, condense onto the ground and also go up
into the atmosphere.
And look at all the stuff that water does for us. You know, it basically creates
glaciers. It creates, you know, the polar ice caps. It makes clouds. It makes rain
and snow and all of that stuff. Pluto has three of these [00:16:00] species that
can all sublimate up from the surface into the atmosphere and condense back
onto the surface or make clouds or whatever, and they do it at different
temperatures because they have different volatilities and so they're sometimes
getting mixed together and sometimes getting separated apart, uh, basically
through distillation.
Um, you know, so the complexities are, you know, sort of mind boggling.
Matt: To, to list all the volatiles there. So you mentioned carbon monoxide, that
is one, methane, and the third being nitrogen, because, yeah, nitrogen ice. I
remember seeing, reading a lot about that on, on Pluto's surface. And so, I, I
know for a fact that, uh, several people who are on the science team for New
Horizons and planetary scientists, Yes, they, they have maintained that Pluto
should still be treated as a planet, and were even a little miffed at the fact that it
was, uh, that the nomenclature seemed to be [00:17:00] catching on with
scientific publications and whatnot, the term dwarf planet now being used.
Dr. Grundy: Well, I mean, uh, so Adjectival qualifications of planets are fine,
you know, there are lots of different categories of planets, you know, I talked
about the giant planets and the ice giants and the terrestrial planets, and I'm fine
to call these ones icy dwarf planets or what, what have you, but my general
impression in the scientific community was that this, this was not seen as
Particularly scientifically relevant or meaningful and so most of the literatureyou'll see people talking about Europa as a planet You'll see people talking
about you know Titan as a planet and you go to scientific meetings, then you'll
hear that routinely and Really?
It's because these are planetary bodies. They're they're too small to be stars.
They're big enough to be round. They have surfaces Often atmospheres,
sometimes moons of their [00:18:00] own, they have geology, they have, you
know, basically what do you want in a planet? Um, so, so there is a, a definition
of planet that's called the geophysical definition, which is tended, tends to get
more used by, by planetary scientists who are actually interested in these things,
where the, uh, IAU definition is maybe more often used by textbook authors
where they're trying to limit themselves, you know, so if you, if you're not
interested in these little guys, Okay, fine.
You've cut yourself off, but you know, you're not doing yourself any favors.
Matt: Well, yeah, and I sensed as much when you said titans, a very interesting
planet, because I have seen, uh, papers and I have spoken to, uh, uh, another,
uh, planetary scientists like yourself who said, Yeah, we're using the, um, a
geophysical definition there.
Anything big enough to be a sphere should be called a planet. And, yeah, that
honestly convinced me. I thought, for certain. And, uh, thank you [00:19:00]
for, for mentioning, you know, what the, what was behind the thinking at the
IAU, because I honestly, uh, that has made more sense than any other
Dr. Grundy: I mean, yeah, it's so the funny thing is that the IU argues that it's a
scientific, uh, classification that they did, but what they really did is it's
basically the astrological definition of a planet, which is that it has to be big and
influential, right?
It has to influence human affairs. Uh, so that's kind of what they did was they
said, okay, well, okay, only if they're big enough to be influential, do we count
them as planets?
Matt: I, I, I believe astrologists have something to say about that too. Uh, and
again, yeah, they, they, they've rolled with the times, haven't they?
Uh, it just, it ended at, uh, Saturn once upon a time, and I think then at Neptune,
they incorporated. So, yeah, it's like, you're astronomers, come on, roll with it.
And yeah, so, so we're coming up on a rather major milestone here, an[00:20:00] anniversary of Pluto's discovery, the 95th. Is there any plans for that?
In fact, we're, what, 14 days out from the 95th anniversary?
Dr. Grundy: Yeah, so, so here in Flagstaff, we have been doing, um, uh, I
Heart Pluto Festival every February for, I guess, six years now. Um, and all
kinds of events going on, and speakers coming to town, and, you know, it's a
small town thing. Flagstaff's not a huge, uh, metropolis, but, uh, But that's
coming up next week. And, uh, you know, one of the local breweries is making
a special edition beer for it.
And, you know, there's a big event in the, in the big theater downtown, uh, with,
you know, guest speakers from out of town and Adam Nimoy, uh, you know,
Leonard Nimoy's son, and, um, Alan Stern, who's the PI of the, uh, the New
Horizons mission, and, [00:21:00] uh, David Levy, who discovered, uh, co
discoverer of the Shoemaker Levy 9 comet that crashed into Jupiter a while ago,
are gonna be on, uh, have a discussion, moderated by David Eicher, who's the,
uh, editor in chief of Astronomy Magazine, so, um, that'll be fun.
There's a, uh, pub crawl and, uh, you know, all kinds of, all kinds of events.
Matt: Oh, that sounds like a lot of fun. And when you mentioned the specialty
beer that's being brewed, uh, yeah, well, I really hope the whole, you know,
tariff situation doesn't happen because yes, American beer was, uh, one of the
products that we were told, uh, don't expect on the shelves.
And I thought, no, for the most part, it's not going to affect my life, but I do.
Um, uh, I'm a big beer lover and of craft breweries. And yeah, if I can't get that
from the States, I'm going to be sad.
Dr. Grundy: Yeah. So I'm not at liberty to, to, to spill the beans on what
[00:22:00] this year's is going to be, but, uh, in the past we did a Pluto Porter,
uh, we did a Lowell logger.
Um, what else was there? Uh, Oh, there was a still air, um, uh, beer that was in
honor of, uh, Clyde Tombaugh's telescope making efforts where he did it in a
root salad to have air still enough that he could do the, the optical testing on his
mirrors, his homemade telescope mirrors. Um, and that had like a hint of
passion fruit in it.
That was, that was a really good one. Um, yeah, you'd like, you'd like these,
these, these guys are good.Matt: Yes, please. No tariffs. Okay. Is there. This is a bit of an oddball question
there, but is there any, uh, examples, say, in science fiction, for example, hard
science fiction, though, not, uh, not your more fantasy oriented, where Pluto
was depicted in a way that you thought was, [00:23:00] well, excellent, if not
100 percent accurate?
Dr. Grundy: You know, I haven't seen one yet, but a lot of this stuff is pretty
new, so I wouldn't be If people are off writing it right now, um, you know, the,
the, the new results are still coming out. I mean, basically a generation of grad
students started grad school, you know, in the years immediately after the flyby
and all the data is public.
So they could download it and do new projects with it and bring their creative
ideas to the table. And they're all now entering into the professional science
process. You know, faculty positions at universities and stuff like that. And so
the ideas that came from New Horizons are rippling outward still.
And for instance, there's a big scientific conference being organized for the
anniversary this [00:24:00] summer at APL where the encounter took place.
And I'm sure we're going to see all kinds of new ideas come out of that, you
know, because people have stuff up their sleeve that they're going to want to
show off to the community.
Matt: Well, I'd love to contribute if I can, you know, so to do some of that on
the side, but, uh. Yes, writing Pluto into fiction as it actually is, I think is
something that, uh, yeah, any, any purveyor of hard science fiction, they should
really get on because yeah, the discoveries of the other bodies out there too.
Yeah. That's going to keep coming. Right.
Dr. Grundy: Yeah. Well, I think so.
Matt: Excellent. I had one very quick idea I wanted to run by you. Um, a while
back I thought, would it be possible, or would it be practical, I guess, a
classification scheme of planets where, yeah, any, any large spherical bodies of
planets, But you've, you've got to divide it up based on sort of [00:25:00] their
position in the solar system.
So, um, rocky planets, gas giants, icy planets. And I thought, yeah, just based
on, you know, the, the nomenclature and so forth. And, and, you know, how the,Dr. Grundy: yeah. So an interesting thing is back in Galileo's time. They
almost did something like that, where they had primary planets, and those were
the ones that were going around the sun, and secondary planets, and those were
the ones that were going around another planet.
And that's pretty close to what you're describing. I think they, they didn't know
about their compositions or anything back then. So they didn't differentiate
between Rocky and gas giants and so on. But, but, you know, when Galileo
discovered the moons going around Jupiter, and of course we knew about the
moon going around our own earth and realize that this was the same kind of
thing.
The first reaction of the scientific community was, okay, this is planets, but
they're in a different type of orbit.
Matt: Yeah. Oh, that's interesting. So yes. So the debate is ongoing [00:26:00]
and it's, it's, it's going to, it's going to go places and interesting stuff could come.
Yes. Well, thank you so much, Dr. Grundy for coming on the show.
And, uh, Also for fitting us into your very busy schedule, and I am very sure the
listeners will appreciate it and best of luck in your future research.
Dr. Grundy: Thank you very much. It was a pleasure to be on.
Matt: And folks, be sure to check out the website for the Lowell Observatory
and to learn how you might be able to enjoy their anniversary celebrations if
you live in the area.
And to find similar events that could be taking place in your area. And be sure
to tune in next time, where we will be taking a look at what the James Webb
Space Telescope has discovered so far, and where we will be speaking with Dr.
Mariba Jha, a MacArthur Genius Fellow, a TED Fellow, a professor of
astrophysics, and the director of the Jha Decision Intelligence Group at the
University of Texas at Austin.
And the co [00:27:00] founder and chief scientist of both Gaiaverse Ltd. and
Privateer Space, where we will be talking about humanity's future and the need
for sustainability in space. In the meantime, thank you for listening. I'm Matt
Williams, and this has been Stories from Space.