Is Pluto a mega-comet?

Short answer: Well sort of, but it’s more useful to think of it as being built from the same stuff as comets. Like an Uncle of the comets. Read on for an explanation.

Earlier this week my wife mentioned a headline she saw.  “Pluto is a giant comet?  Whoa!”

It was all over the place:

And the less fantastically-titled Scientists introduce cosmochemical model for Pluto formation.  The paper this is based on (written by Christopher Glein and Hunter Waite) is called Primordial N2 provides a cosmochemical explanation for the existence of Sputnik Planitia, Pluto (downloadable here).

Which leads me to this post. Let’s unpack what’s going on.

Here we go.

First, what do you think of when someone says “Pluto”?

Many of you will immediately think of Pluto as the ninth planet because that’s what you learned in school (and so did I). A lot of people are angry about Pluto’s reclassification as a dwarf planet. I won’t jump into the debate about whether Pluto should be called a planet now (although in my mind it’s clearly not a planet but rather one of the biggest members of the Kuiper belt, a population of icy stuff on the Solar System’s outskirts).

NASA’s New Horizons mission zoomed past Pluto in 2015.  It took some incredible images like this one:


New Horizons’ view of Pluto, with a zoom on Sputnik Planum. From this article on Credit: NASA/JHUAPL/SwRI

New Horizons found Pluto to be an active, diverse world with a mix of heavily cratered terrain and surprisingly smooth regions (including a giant heart-shaped one, a candidate for most romantic spot in the Solar System).  Sputnik Planum (flyover animation here) is a smooth icy plain on which no craters were detected, implying that it must be very young, perhaps less than one million years old (which is super young for the 4.6-billion-year-old Solar System).  It appears to be made of Nitrogen-rich ices.

New Horizons made all sorts of other discoveries related to Pluto’s geology and atmosphere, its big moon Charon and other small moons (for example, see here).  And it’s not done: New Horizons will fly by another outer Solar System object in January 2019.

Second, what do you think of when someone says “comet”?

Personally, I think of something like this:


Comet Hale-Bopp, photographed passing close to the Sun in 1997. Credit: Dan SchechterSide note: I was in college when this comet was a big deal and I got up in the middle of the night several times to check it out in the sky.  I got pretty cold, and I now subconsciously associate comets with me feeling cold, which is not that unreasonable since they’re big ice balls.

Wikipedia defines a comet as…

…an icy small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process called outgassing.

The outgassing produces the coma, the big fuzzy patch that we often think of as the “comet”.  The coma is often as big as the Earth and in some cases as big as the Sun!  The thing that’s doing the outgassing is much smaller, typically a few kilometers across (Hale-Bopp was a big one, about 60 km across). The tails are gas and dust that is blown back from the comet by the Sun.

ESA’s Rosetta mission visited — and dropped a lander down on — comet 67P/Churyumov–Gerasimenko.  Here is what it looked like from close-up:


Comet 67P as seen by the Rosetta spacecraft.  Notice the jets coming off the surface — those are the source of the comet’s coma and tails.  Credit: ESA/Rosetta/NAVCAM.

Rosetta was able to measure comet 67P’s chemistry.  It measured the chemical signature of water (a bad match to Earth’s water) and Xenon (a good match to Earth’s atmospheric Xenon).  It also measured Nitrogen on the comet, which confirmed that comet 67P was born in a very cold place and whose chemical composition can be linked with the Nitrogen ices found on Pluto.

Now, the main course: the new study

In simple terms, the new study found a match between the nitrogen in comet 67P and on Pluto.  From their press release:

We found an intriguing consistency between the estimated amount of nitrogen inside the glacier and the amount that would be expected if Pluto was formed by the agglomeration of roughly a billion comets or other Kuiper Belt objects similar in chemical composition to 67P, the comet explored by Rosetta.

That’s the main point, the one that was picked up by all those articles.  That’s pretty interesting: there is a link between the measured composition of a comet and of Pluto.

Cool? Absolutely.  Surprising?  Not really.

Comets are ice balls.  They come from the outer Solar System. Pluto lives in the outer Solar System. It’s no big surprise that they’re made of the same stuff.  It would be puzzling if they weren’t.  It’s like coming home from a trip to Tokyo and saying “OMG, the sushi in Japan that tastes just like ours!”  As my son likes to say — well, duh!

Here is a zoom-in on the Solar System to give you a sense of scale:


A zoomed-out view of the Solar System, from the inner Solar System (top left) to the Oort cloud of comets (bottom left). Credit: NASA/JPL-Caltech/R. Hurt.

There are two main reservoirs of comets: the Kuiper belt and the Oort cloud.   The Oort cloud is a gigantic cloud that stretches to the edge of the Solar System, more than a hundred thousand AU from the Sun.  The Kuiper belt lies past the orbit of Neptune. We can tell from a comet’s orbit where it came from.

Now, is it right to say that Pluto is a “giant comet”?

We think all comets formed in the same region of the Solar System exterior to the planets’ orbits.  Just like asteroids are leftovers of planet formation in the inner Solar System, comets are leftovers of planet formation in the outer Solar System. Once the giant planets grew large cores, small bodies that weren’t incorporated into the giant planets were flung all over the place — some were deposited into the asteroid belt and some may have delivered Earth’s water (see here). A lot of comets were launched into interstellar space (like ‘Oumuamua, and we’ll get to that story in a sec). The comets we have today are the leftovers that survived the Solar System’s early craziness, and now live in either the Kuiper belt or Oort cloud.

Pluto is puny compared with the planets but it’s still way bigger than a comet.  It’s about 1200 km in radius, more than 5 times smaller than Earth and about two-thirds the size of the Moon.  Its mass is 0.2% of Earth’s (about 1/6th of the Moon’s). Compare that with a typical comet, 100 meters up to 100 km in size.

But we think Pluto grew in the same place as comets out past the planets.  You can think of Pluto as a planetary seed whose growth was prematurely stunted (sort of like Mars).  Whereas Mars’ growth was stopped abruptly (perhaps by Jupiter’s migration or an early giant planet instability), Pluto’s growth probably was just slowed down by growing up in the low-density outskirts of the Solar System.

Pluto is made from the same stuff as comets. But it’s much bigger and still out in the outer Solar System.  It’s like an uncle from the old country and comets are its brothers and sisters that moved away.  They share the same bloodline but they move in very different circles.  And when the brothers and sisters arrived in their new home, they prospered (as in, they heated up and got comas and tails) whereas the uncle who stayed put stayed poor.

I would hesitate to call Pluto a mega-comet.  Maybe a comet uncle?  Or an icy dwarf with a comet-like composition?

Even the definition of a comet is kind of ambiguous.  Wikipedia’s definition requires a “comet” to make a coma and tails when it passes close to the Sun.  But there is a class of Solar System objects called the Damocloids that have comet-like orbits but no comas or tails.  They are thought to be comets that lost their surface ices after many passages close to the Sun and became extinct. Models suggest that after a few hundred passages close to the Sun, comets become extinct.  At that point these objects look like comets but no longer behave like comets. They don’t make comas or tails anymore, so does that mean that they are no longer comets?

This same apparent contradiction applies to the interstellar object ‘Oumuamua, which also looks like a comet but didn’t produce any coma or tails when it passed close to the Sun.  It might actually be a piece of an extinct exo-comet, that is, a comet that formed around another star. Check it out:


For details, see this article or the video below.

I don’t want to get bogged down in semantics, so maybe just saying “Pluto and comets formed in the same part of the Solar System and are made from the same stuff” is good enough.

There is an interesting side note in the new article (inferred from the relative carbon monoxide abundance).  From the press release:

Our research suggests that Pluto’s initial chemical makeup, inherited from cometary building blocks, was chemically modified by liquid water, perhaps even in a subsurface ocean

A liquid ocean underneath Sputnik Planum?  That sounds awesome!  The idea has been out there for a couple years but still, awesome.  And there’s even a new article speculating about whether Pluto’s ocean could harbor life. I am super intrigued by the subsurface oceans like the ones under the icy surface of Jupiter’s moons Europa and Ganymede.

To wrap up, I think this new study is super interesting. It’s a nice combination of data from two different space missions.  And I don’t think Pluto cares whether we call it a planet, a dwarf planet, or a giant comet.

Questions?  Comments?  Words of wisdom?




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