I'm always trying to find new ways to wrap my head around everything that's out there in our solar system. We've got planets, dwarf planets, asteroids, moons, and a whole bunch of other things. It's enough that it can be hard to keep track of everything that's out there, and it can be even harder to become familiar with individual objects. This post is a list of top tens in the solar system. Most of these top tens deal with size—the ten biggest things orbiting the sun, the ten biggest dwarf planets, asteroids, etc. I made these lists for myself, so that I could get to know our solar system a bit better. Hopefully it will be interesting to someone else, too.

#1 Top ten biggest things around the sun

The sun is by far the largest object in the solar system (except for that one time when a comet's halo actually grew bigger than the sun for few days back in 2007). It's about ten times bigger than Jupiter (the biggest planet) and over 100 times bigger than the Earth. For the sake of this post, I'm going to be talking about everything except the sun.

Rank	Name	Diameter^?	Size vs. Earth
1	Jupiter	139,822	11x Earth
2	Saturn	120,536	9.5x Earth
3	Uranus	51,118	4x Earth
4	Neptune	49,528	3.9x Earth
5	Earth	12,742	1x Earth
6	Venus	12,104	95% Earth
7	Mars	6,792	53% Earth
8	Ganymede	5,268	41% Earth
9	Titan	5,152	40% Earth
10	Mercury	4,879	38% Earth

The four confirmed giant planets (Jupiter, saturn, Uranus, and Neptune) are just that—giant. It turns out Earth is actually a good size to compare to, since it falls right in the middle of the list. Earth is a mid-sized planet, but still the biggest of the non-giants. Apart from the planets, you may have noticed a few less-familiar names in the list. Ganymede and Titan are both moons (of Jupiter and Saturn, respectively), but even though they are “just” moons, they are both bigger than the planet Mercury. The next runner up for the top ten list is actually another one of Jupiter's moons, Callisto, which is basically the same size as Mercury—literally 99% Mercury's size. If we don't include moons in our list, then Mercury comes in at number eight and Pluto and Eris are the ninth and tenth objects, at 19% and 18% the size of Earth. With moons included, Pluto and Eris rank as numbers sixteen and seventeen.

If Planet Nine exists, it probably fits in somewhere between Earth and the ice giants Uranus and Neptune, at around two to four times the size of Earth.

#2 Top ten moons

As we've already seen, some of the solar system's moons are actually pretty big. Many pictures of the solar system include Earth's moon and no others, even though it's only the fifth largest moon in the solar system. Two moons (Ganymede and Titan) are both bigger than the planet Mercury, and a total of seven moons (Ganymede, Titan, Callisto, Io, “Luna”, Europa, and Triton) are all bigger than the dwarf planet Pluto. There are a total of eighteen moons in our solar system that are large enough to be round. Here are the top ten moons in the solar system:

Rank	Name	Diameter^?	Size vs. Luna	Size vs. Mercury	Size vs. Pluto
1	Ganymede¹	5,268	1.5x Luna	8% > Mercury	2.2x Pluto
2	Titan²	5,152	1.5x Luna	6% > Mercury	2.2x Pluto
3	Callisto¹	4,821	1.4x Luna	99% Mercury	2.0x Pluto
4	Io¹	3,630	1.1x Luna	74% Mercury	1.5x Pluto
5	“Luna”³	3,474	1x Luna	71% Mercury	1.5x Pluto
6	Europa¹	3,138	90% Luna	64% Mercury	1.3x Pluto
7	Triton⁴	2,707	78% Luna	56% Mercury	1.1x Pluto
8	Titania⁵	1,578	45% Luna	32% Mercury	67% Pluto
9	Rhea²	1,529	44% Luna	31% Mercury	64% Pluto
10	Oberon⁵	1,523	44% Luna	31% Mercury	64% Pluto

Runners up are Saturn's moon Iapetus, at 62% the size of Pluto, and Pluto's own orbital companion Charon, at 51% the size of Pluto. (In fact, Charon is so big compared to Pluto that the two objects actually orbit each other, and are considered by some to be a double planet.) We often learn about the major planets and completely ignore their moons, but the reality is that the moons are much more like planets than many people think. Apart from many of the moons listed being large enough that they could have been planets, Titan has a full atmosphere with clouds, rain, rivers and seas, Europa and Enceladus both have subsurface oceans that are currently the most likely places we know of to look for life, and a number of the moons are also good candidates for human colonization.

#3 Top ten asteroids

When the first asteroids were discovered in the early 1800s, they were considered planets for around half a century and even given their own planetary symbols before being reclassified as asteroids. Although they're no longer considered planets, they're still interesting places to study and explore. Ceres, the largest asteroid, is the only round asteroid, and is currently classified as a dwarf planet. It was visited by the Dawn spacecraft in 2015.

Rank	Name	Diameter^?	Size comparison
1	Ceres¹	946	1.3% Luna; 7.2% Pluto
2	Vesta	525	56% Ceres
3	Pallas	514	54% Ceres
4	Hygiea	435	46% Ceres
5	Interamnia	324	34% Ceres
6	Europa	313	33% Ceres
7	Davida	293	31% Ceres
8	Sylvia	292	31% Ceres
9	Cybele	267	28% Ceres
10	Eunomia	266	28% Ceres

The asteroids in this list are quite large, compared to the hundreds of thousands of other asteroids out there. Even so, there are a number of other larger objects—trojans, centaurs, and trans-Neptunian objects—that are sometimes considered asteroids. I've only included main-belt asteroid in this list, but I've separated the other large non-planets out into their own lists below.

#4 Top ten TNOs

TNOs (trans-Neptunian objects) are icy objects that orbit out beyond Neptune. Because ice isn't as hard as rock, many TNOs have been pulled into a round shape by their own gravity, making them dwarf planets. (Not all TNOs are dwarf planets, but the biggest ones—the ones in this list—are, although only five have officially been classified as such.) The most famous TNO is beloved old Pluto, but there are other worlds out there that are just as interesting. Because most of the TNOs we know about have been discovered during this century (i.e., after the year 2000), there is still a lot that we don't yet know about them. As such, there's a good chance this top ten list won't look the same even a few years from now.

Rank	Name	Diameter^?	Size vs. Pluto	Size vs. Ceres
1	Pluto*	2,374	1x Pluto¹	2.5x Ceres
2	Eris*	2,328	98% Pluto²	2.5x Ceres
3	OR₁₀	1,535	65% Pluto	1.6x Ceres
4	Makemake*	1,447	61% Pluto	1.5x Ceres
5	Haumea*	1,403	59% Pluto	1.5x Ceres
6	Charon	1,212	51% Pluto	1.3x Ceres
7	Quaoar	1,046	44% Pluto	1.1x Ceres
8	Sedna	1,032	44% Pluto	1.1x Ceres
9	Salacia	883	37% Pluto	93% Ceres
10	MS₄	865	36% Pluto	91% Ceres

As I mentioned before, Pluto's orbital companion Charon is large enough to be a dwarf planet in its own right; some preliminary versions of the definition of “dwarf planet” actually included it as one. Runners up for the top ten are Orcus (35% the size of Pluto—88% Ceres') and 2013 FY_2_7 (34% Pluto's size—85% Ceres'). It's worth noting that of the top ten TNOs, all but Pluto and Charon were discovered after the year 2000, cementing the fact that this is a new era of solar system discovery, with plenty of new objects yet to be found.

Other things in the solar system

Most people know about planets, moons, and asteroids, and even people who don't know what TNOs are have at least heard about dwarf planets. But there are a number of other types of objects out there in the solar system that we hear less about. These include trojans—small bodies that share a major planet's orbit, centaurs—small bodies that orbit between the orbits of the outer planets, and near-Earth objects—small bodies that orbit (partially or completely) within the orbit of Mars.

#5 Top ten trojans

Rank	Name	Diameter^?	Size comparison
1	Hektor¹	242	26% Ceres' size
2	2010 EN₆₅ ²	200	82.5% Hektor's size
3	Agamemnon¹	166	69% Hektor
4	Diomedes¹	164	68% Hektor
5	Äneas¹	143	59% Hektor
6	Patroclus¹	142	59% Hektor
7	2006 RJ₁₀₃ ²	138	58% Hektor
8	Achilles¹	136	56% Hektor
9	2011 HM₁₀₂ ²	135	56% Hektor
10	2014 QO₄₄₁ ²	130	54% Hektor

Although there are thousands of other trojans—some of which orbit Earth, Mars, and Uranus—there are only eighteen others known that have diameters of 100 or larger.

#6 Top ten centaurs

Orbiting out between the giant planets, centaurs display some properties of regular asteroids and some properties of comets, with a few centaurs officially classified as both. On top of that, two of the largest centaurs, Chariklo and Chiron, both have rings, making them the only known objects other than the giant planets to have them. They are called centaurs due to their hybrid nature, after the half-man-half-horse creatures of Greek mythology.

Rank	Name	Diameter^?	Size comparison
1	Chariklo	259	27% Ceres' size
2	Bienor	204	79% Chariklo's size
3	Chiron	188	73% Chariklo
4	Pholus	157	61% Chariklo
5	Amycus	90	35% Chariklo
6	Asbolus	75	29% Chariklo
7	Hylonome	70	27% Chariklo
8	Cyllarus	65	25% Chariklo
9	Crantor	60	23% Chariklo
10	Nessus	60	23% Chariklo

The three largest centaurs—Chariklo, Bienor, and Chiron—are listed as probable dwarf planets by Mike Brown. He also lists Pholus and Amycus as possible dwarf planets.

#7 Top ten NEOs

NEOs (near-Earth objects) are mostly very small objcts that orbit at least partially within the orbit of Mars. These are of particular interest to astronomers, both because they are close enough that we have plenty of opportunities to study them and because we want to make sure none of them are going to hit us any time soon. While NEOs may be small compared to other things out in space, some of those listed here are still much bigger than even the biggest mountains on Earth. I've included Mount Everest for size comparison, which is 8.848 high at its highest point.

Rank	Name	Diameter^?	Size comparison
1	Ganymed	33	3.7x Mount Everest
2	Eros	18.4	2.1x Everest
3	Don Quixote	18.4	2.1x Everest
4	Eric	10.8	1.2x Everest
5	1998 QE₂	10	1.1x Everest
6	Sisyphus	8.5	96% Everest
7	1990 TR	4.3	49% Everest
8	Geographos	3.5	39% Everest
9	Toutatis	3	34% Everest
10	1993 UC	2.7	31% Everest

#8 Top ten (or fourteen) SSOs with the most (known) moons

Earth has one moon, Venus and Mercury don't have any, Pluto has five (depending on how you count), and the giant planets each form their own little solar systems with dozens of moons each and probably more we haven't even discovered yet. It might surprise some people to learn that even a few otherwise-unremarkable asteroids have more than one moon.

Rank	Name	Moons
1	Jupiter	67
2	Saturn	62
3	Uranus	27
4	Neptune	14
5	Pluto¹	5
6	Mars	2
7	Haumea	2
8	Sylvia	2
9	Eugenia	2
10	Elektra	2
11	Minerva	2
12	Kleopatra	2
13	Balam	2
14	Litva	2

With nine solar-system objects that each have two moons, I can't include some in the list and not others, so this list ends up being a Top-14 list. For comparison, there are around 45 known objects in the solar system with a single moon and a few more that are harder to categorize (like two same-sized object that orbit each other and have an additional object orbiting both of them). The two-mooned objects in this list are listed from largest to smallest.

#9 Top ten furthest known objects

The question of which solar-system objects are the furthest from the sun is actually a bit complicated. Because orbits are not perfectly circular, some objects may be very distant from the sun at one time, but relatively close at another. For example, one object, 2014 FE₇₂, goes all the way out to over 3000 AU (1 AU is the distance from the sun to the Earth), but also swings in to less than 40 AU (around the average orbit of Pluto).

Some lists of the furthest objects in the solar system use the furthest distance an object goes, even if at other times it is much closer; others look only at objects that never get closer than a certain distance; and still others look at the semi-major axis—a sort of average distance. Because there are comets that go so far out we don't actually know their farthest distance, it seems that making the list based on farthest distance isn't the way to go, since we can't actually make any definitive judgments there. The semi-major axis seems like a good compromise, but if an object comes in so close that it spends part of its orbit in the main asteroid belt, it doesn't really make sense to call it one of the farthest objects in the solar system. Because of this, I've chosen to make my list based on where objects actually are right now. This means that there are objects that may go out further, but for the moment, these are currently the farthest known objects.

Rank	Name	Distance
1	v774104	103
2	Eris	96.2
3	“DeeDee”	91.6
4	OR₁₀	87.6
5	2013 FS₂₈	86.2
6	Sedna	85.6
7	2014 FC₆₉	84.4
8	2006 QH₁₈₁	83.6
9	“Biden”	83.3
10	2013 FY₂₇	80.2

Because these regions of the solar system are so far away, there are likely many more objects out there that we haven't even found yet—we were only able to discover objects like Sedna and Biden because they happen to be at the closest part of their orbits! As such, there's a good chance that this list will change substantially within the next few years as our ability to peer farther out into space improves.

#10 Top ten places to look for life

For many, the most important reason to keep exploring space is to find out if there's life anywhere else out there. While other stars have planets that may be similar to Earth, the best place to start looking is right here in our own cosmic backyard. And although most people probably think first of Mars when they think of looking for life in the solar system, there are actually a few other places that may have even better chances of life.

Rank	Name
1	Europa
2	Titan
3	Enceladus
4	Mars
5	Venus
6	Io
7	Ceres
8	Callisto
9	Ganymede
10	Small bodies

While this is a top ten list like the rest, everything past the first four is a bit of a stretch. Europa, Titan, Enceladus, and Mars are all considered good places to look for life by a number of scientists. The subsurface oceans on Europa and Enceladus offer excellent possibilities for life, the surface oceans on Titan do as well, and Mars has always been a favorite choice due to its similarity to Earth and presence of small amounts of liquid water. The other objects listed here are sometimes listed as possible—though not probable—locations for life. And of course, while there are equations to predict the likelihood of different kinds of life in the universe, this top-ten list is clearly the least numerically-founded list I've included.

Final thoughts

As I said at the outset, I compiled these lists mostly for myself, because I wanted to get to know more than just the major planets and the official dwarf planets. By learning the names and a few properties of nearly 100 solar-system objects (10 lists × 10 objects, minus some repeats), I've expanded my understanding of what's out there, but the really interesting thing for me has been taking these lists and learning a bit more about each item individually.

With the now-tired debate about Pluto's planetary status still raging in just about every solar-system-related comment section on the internet, we sometimes lose sight of the fact that whatever they're classified as—planets, dwarf planets, asteroids, or just “small solar system bodies”—each object out there is an entire world, just waiting for us to learn more.

Gwistix [ˈɡwɪs.tɪks]

21 March 2017

Solar System Top Tens