In 2005, astronomers discovered the Solar System’s tenth planet, and they named it Xena.

The International Astronomy Union promptly stepped in and renamed it Eris…

… and reclassified Eris as a dwarf planet.

Eris has a rather wonky orbit. At closest approach to the Sun, Eris travels just inside the orbit of Pluto. Then it journeys far off into space, to a distance almost three times as far away before looping back again.

Combine this high eccentricity with a high inclination. Eris’s orbit is tilted by almost 45º relative to the rest of the Solar System. As a result, while Eris is sometimes called the largest most massive object in the Kuiper belt, it really isn’t a Kuiper belt object at all. It’s more like a Kuiper belt visitor.

That seems really strange, but if there’s one thing I’ve learned during this 2015 Mission to the Solar System, it’s that what seems strange at first turns out to be quite normal.

Many astronomers would classify Eris as part of the scattered disk. The scattered disk is a collection of objects that are… well… scattered. And there are lots of these scattered objects in wildly eccentric and/or inclined orbits. It’s sort of like our neat and orderly Solar System is surrounded by a swarm of bees.

When I was a kid, the Solar System was easy. Just memorize these nine planets and remember there’s an asteroid belt between Mars and Jupiter. The discovery of Eris marked the beginning of a whole new understanding of the Solar System.

Now we have eight planets, an asteroid belt, the Kuiper belt, the Oort cloud (maybe), and at least two “detached objects” (coming soon to Sciency Words). The Solar System has become crazy complicated, and each new discovery only seems to make things more complicated yet—which is why Eris’s official name is oddly appropriate. In Greek mythology, Eris was known as the goddess of discord.

P.S.: Today’s post is the final post for the 2015 Mission to the Solar System. I have traveled (metaphorically at least) from the Sun all the way out to the Kuiper belt, and I’ve shared some of the fruits of my research here on my blog. I’ve had a lot of fun on this adventure, and I hope you have to. Everyone have a safe and happy New Year, and I’ll see you all in 2016.

When I was a kid, something really bothered me about the Solar System. If Pluto’s orbit crosses the orbit of Neptune, why don’t the two planets (Pluto was still considered a planet back then) collide with each other?

According to one of my science teachers, Pluto has just been lucky so far. But sooner or later, my teacher said, a collision will happen.

THIS IS FALSE!

Regular Planet Pailly readers will know that I sometimes complain about the science education I received as a child. I’ve had to unlearn a lot of the things I’d been told, and this is another example of that.

Odds are there used to be other dwarf planets that crossed Neptune’s orbit. Probably lots of them. They’re gone now, either because they collided with Neptune or (more probably) they got hurled out of the Solar System by Neptune’s gravity.

At least one former dwarf planet was yanked out of its original orbit and became Neptune’s largest moon. That would be our old friend Triton. The one that looks like a cantaloupe.

Pluto survived only because it happened to have an orbital resonance with Neptune. Pluto completes exactly two orbits for every three orbits of Neptune. The math works out such that Pluto and Neptune have never met. They miss each other every single time one crosses the other’s orbit.

So I guess my old science teacher was right about one thing. Pluto is lucky. Lucky enough that it has its Neptune dodging orbit.

P.S.: Pluto isn’t the only lucky one. Several other objects have 2:3 orbital resonances with Neptune, allowing them to safely cross Neptune’s orbital path. The most noteworthy is Orcus, which is under consideration to be classified as yet another dwarf planet.

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Today’s post is part of Pluto/Kuiper belt month for the 2015 Mission to the Solar System. Click here to learn more about this series.

This is Makemake.

Makemake is a pretty typical dwarf planet. It has a nice, spherical shape, unlike Haumea. Its nearly circular orbit keeps is well inside the Kuiper belt at all times, unlike Pluto or Eris; however, Makemake’s orbit is inclined 29º relative to the plane of the Solar System.

Kuiper belt objects with such extreme inclinations are collectively known as the “hot population.” Why are they hot? Because scientists are bad at naming things. The hot population is just as cold as the rest of the Kuiper belt.

Based on current estimates, Makemake is either the third or fourth largest Kuiper belt object, after Pluto, Eris, and possibly Haumea. Makemake and Haumea are similar enough in size that astronomers can’t be sure which is bigger. The current best guess seems to be that Makemake is slightly larger.

The weirdest thing about Makemake is that it appears to have no moons. This is truly surprisingly. Most dwarf planets and indeed many known Kuiper belt objects have managed to pick up a moon or two. With so much rocky and icy debris floating around, it’s hard not to.

Yet somehow, Makemake has managed to stay single, making this “hot population” dwarf planet the Kuiper belt’s most eligible bachelor.

Recent headlines have been saying that we’ve discovered volcanoes on Pluto. But these aren’t “normal” volcanoes. They’re cryovolcanoes: volcanoes that erupt with ice rather than lava.

You may have also heard or read somewhere that cryovolcanoes are something new, something never before seen elsewhere in the Solar System.

Okay, cryovolcanism is nothing new. It was first observed on Triton, Neptune’s largest moon, back in 1989. Similar cryovolcanic activity has been studied in detail on Enceladus, one of Saturn’s moons, and there’s compelling evidence that Jupiter’s moon Europa has it too.

When cryovolcanoes erupt, they spew a mix of icy cold materials, often water with other chemicals like nitrogen or ammonia. This icy mix may originate from subsurface reservoirs of liquid water or, in the case of Europa and Enceladus, vast subsurface oceans that may (or may not) be capable of supporting alien life.

But Pluto’s cryovolcanoes are still kind of special. Previously known cryovolcanoes are basically cracks or fissures amidst an otherwise relatively flat landscape. Pluto’s are tall mountains with openings on top. They look like volcanoes.

So what we’re seeing on Pluto is new to us, but it’s not entirely new. In fact, we should really stop thinking of cryovolcanism as rare or strange or exceptional. Given how many places in the outer Solar System either have or are suspected to have cryovolcanic activity, it’s actually Earth’s “normal” red-hot lava volcanoes that are weird.

Links

Volcanoes on Pluto Look a Lot Like Those on Earth and Mars from Ars Technica.

Pluto Revealed with Cathy Olkin from TEDx Detroit.

Don’t feel too bad for Pluto. Pluto has a lot of new dwarf planet friends out in the Kuiper belt. Today, we’re meeting the dwarf planet known as:

HAUMEA

Haumea is a funny looking object. It’s sort of bulgy at the middle. Just look at this totally legit Hubble image I found.

Okay, that’s actually not a Hubble image. Haumea is so small and so far away from Earth that we can’t get a clear image of it, not even with Hubble. So how do we know what Haumea looks like?

In our telescopes, Haumea appears as just a tiny point of light, but its brightness fluctuates at regular intervals. The best explanation for this is that Haumea must be misshapen. As the dwarf planet rotates, the wider sides reflect more sunlight than the narrower sides.

The changing brightness also tells us Haumea’s rate of rotation. As it turns out, a Haumean day is less than four hours.

Gravity tries its best to pull Haumea into a spherical shape, but planets (and dwarf planets) always tend to bulge a little at the equator due to their own rotation. Since Haumea rotates extra fast, it’s extra bulgy, and also somewhat elongated along one axis. Hence the peculiar, oval shape in the fake Hubble image above and also in many other artistic renderings of what Haumea might look like.

So even though we’ve never actually photographed Haumea, we know it’s a funny looking object. The question I really want to know is how many other Haumea-like planets and dwarf planets might be out there in our universe?

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Today’s post is part of Pluto/Kuiper belt month for the 2015 Mission to the Solar System. I can’t believe this mission is almost over! Click here to find out more about this series.

The 2015 Mission to the Solar System has finally brought us to Pluto, so for today’s edition of Sciency Words, let’s talk about the scientific term:

PLANET

In 2006, the International Astronomy Union established the first ever official definition of a planet. To be a planet, an object must:

• Orbit the Sun and not orbit anything else.
• Be roughly spherical.
• Have cleared most other objects or debris from its orbital path.

Pluto failed this last test. We now know that Pluto is just one of many small, icy objects in a sort of second asteroid belt known as the Kuiper belt.

Rather than demote Pluto to the ignoble status of “large asteroid,” the I.A.U. created a new category of astronomical objects called “dwarf planets.” The name comes by loose analogy with a category of small stars called “dwarf stars.” It’s worth mentioning that while dwarf stars are still a type of star, dwarf planets are not considered a type of planet.

To qualify as a dwarf planet, an object must:

• Orbit the Sun and not orbit anything else.
• Be roughly spherical.
• Have NOT cleared most other objects or debris from its orbital path.

So did the I.A.U. get it right? Do these definitions of planet and dwarf planet make sense? Some astronomers would say no (don’t get too excited, Pluto fans; it’s not for the reason you think).

How are we supposed to apply these definitions to exoplanets, planets orbiting other stars? Most exoplanets cannot be observed directly. The few that are visible in our telescopes appear as just a few pixels. So how can we see if they’re spherical? How can we tell if there’s debris in their orbital paths or not?

The I.A.U. is now considering defining planets in a more mathematical way, using the following factors:

• The planet’s mass (we can usually estimate that without directly observing the planet).
• The planet’s orbital period (which can also be estimated without direct observation).
• The mass of the host star (or stars).

If the mathematics work out such that an object should be able to clear its orbital path of debris within a predetermined timeframe (10% of the host star’s lifespan, perhaps), then it qualifies as a planet.

We can safely assume that any object with enough mass to clear its orbit would be also spherical, due to its own gravity. So astronomers wouldn’t have to visually confirm that an exoplanet is round.

Right now, this idea is only a proposal. But if the I.A.U. adopts this new definition of planet, it can be applied equally well to objects in our own Solar System and objects orbiting distant stars.

P.S.: Under the new definition, Pluto still isn’t a planet.

Links

A Quantitative Criterion for Defining Planets by Jean-Luc Margot.

A New “Mathematical” Definition Proposed for What Constitutes a Planet from Universe Today.

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Today’s post is part of Pluto/Kuiper belt month for the 2015 Mission to the Solar System. Click here to learn more about this series.