Our solar system has always seemed a vast place, even to the most ancient astronomers who knew little of the universe other than what the sun, moon, and stars could reveal to the naked eye.
As our view of the universe got bigger over the millennia, the view of our solar system has gotten smaller and smaller, but in many ways, we actually know less about the solar system as we know it today than our ancestors did of their more limited view of the stars and planets.
We've obviously learned a lot since the days of ancient Greek, Babylonian, and Chinese astronomy, though, which only inspires modern astronomers to learn even more about our home among the stars.
What is the solar system?
The solar system is a region of space, and all of the objects contained within which are gravitationally bound to the sun.
It formed about 4.6 billion years ago out of a massive pocket of gas, dust, and other debris known as a molecular cloud in a manner that is still debated.
Something, possibly a supernova, disrupted the cloud enough that a density imbalance formed, which then created a center of gravity strong enough that it accreted more of the cloud into itself, eventually forming the sun.
The sun is estimated to contain about 99% of the material from that original molecular cloud, with the remaining 1% accounting for all of the planets, asteroids, and everything else.
How many planets are there in the solar system?
As of right now, there are officially eight planets in the solar system, with an unknown number of dwarf planets contained almost exclusively in an outer belt of rocky material known as the Kuiper Belt.
The planets, in order of distance from the sun, are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
A planet is defined as a massive body that has a regular primary orbit around the sun, has gravity strong enough that it overcomes rigid body forces to form into a spheroid shape, and (crucially) must have cleared the neighborhood around its orbit.
Dwarf planets are those bodies that have the first two of the three required characteristics but have not yet cleared their orbits of material.
What happened to Pluto?
When it was discovered in 1930, Pluto was considered the ninth planet in our solar system, a status it held until 2006 when it was reclassified as a dwarf planet.
A lot of people who grew up learning that Pluto was a planet have clung to the idea in the nearly two decades since, making Pluto's status as a planet a kind of cultural touchstone for many people.
But once you understand the reasoning behind the demotion, it makes a lot of sense.
How many dwarf planets are there in the solar system?
There are currently six recognized dwarf planets: Pluto, Ceres, Eris, Makemake, Haumea, and 2015 RR245 (which does not have an official "name" yet, as of 2022).
If we were just talking about six additional dwarf planets in the solar system, you could make the case that there were really 14 planets in total, rather than eight, but the six known dwarf planets aren't alone, not by a long shot.
There are more than 20 dwarf planet candidates that we know of and there are likely many more that we still haven't spotted yet, making dwarf planets much more common than traditional planets, and also something rather different than the planets as we know them.
Dwarf planets are somewhere between a protoplanet and a full-fledged planet like Mercury and are more like the eight major planets as they were in the very early days of planetary formation after the birth of the solar system.
What is Planet Nine?
Planet Nine—sometimes called Hypothetical Planet X—is not a dwarf planet (we think), but could be an object with a highly elongated orbit far beyond the orbit of even Pluto.
Its existence is suggested by mathematical models that explain the unusual movements of smaller objects in the Kuiper Belt, but no direct observation has ever been made, so there's no confirmation that such a planet exists.
"The possibility of a new planet is certainly an exciting one for me as a planetary scientist and for all of us," Jim Green, director of NASA's Planetary Science Division, says.
"This is not, however, the detection or discovery of a new planet. It's too early to say with certainty there's a so-called Planet X. What we're seeing is an early prediction based on modeling from limited observations. It's the start of a process that could lead to an exciting result."
If such a planet exists, it would have about 10 times the mass of Earth and be similar in size to Uranus and Neptune. It would also orbit about 20 times farther away from the sun than Neptune does, meaning it could have an orbital period between 10,000 and 20,000 years.
What is the smallest planet?
The smallest official planet is Mercury, with a mean radius of about 2,439 kilometers and a mass of 3.3011×1023 kilograms. This puts it at about 5% the weight of Earth, and about 38% the size of Earth.
This makes Mercury only slightly larger than the moon (at least relative to everything else in the solar system) since the moon is about 27% the size of Earth and about 2% as heavy as the planet it orbits.
What is the largest planet?
By far, the largest planet in the solar system is the gas giant Jupiter.
While Jupiter is about 1000 times less massive than the sun, this is still large enough that it has a noticeable gravitational effect on the sun, so that it doesn't properly orbit the sun like the rest of the planets, but the sun and Jupiter both orbit the same barycenter.
This barycenter is a distance about 1.07 times the radius of the sun from the sun's center. We don't notice this "wobble" in the sun's position because Earth's mass is so insignificant to the sun that our barycenter with the sun is still effectively the center of the sun, so the sun never moves from our perspective.
What is the coldest planet?
Neptune is the coldest planet in the solar system, with an average temperature of -353°F (-214°C), which is just about 60 degrees Kelvin warmer than absolute zero (0 Kelvin), the temperature where all molecular motion stops.
That's just Neptune's average temperature, though. The temperature of some parts of Neptune's atmosphere has been seen to dip as low as -392°F (-235°C, 38°K), which is actually colder than the mean temperature of Pluto.
What is the hottest planet?
Given its proximity to the sun, you'd assume Mercury was the hottest planet in the solar system, but you would be wrong. The hottest planet is actually Venus, and it's not even close.
The mean temperature of Mercury is 333°F (167°C), which is hot enough to melt Indium (melting point 315°F (157°C)), but just short of melting Lithium (356°F (180°C)).
Venus, meanwhile, has a mean temperature of 867°F (464°C), which not only melts Lithium, but also Tin (450°F (232°C)), Lead (621°F (327°C)), and even Zinc (787°F (419.5°C)).
This is due to Venus' thick carbon dioxide atmosphere which creates a runaway greenhouse gas condition where the sun's heat can pass through the atmosphere, but the heat from the planet cannot escape, raising the planet's surface-level temperature to nearly three times Mercury's.
Mercury, meanwhile, has almost no atmosphere to speak of, so there is nothing to hold in the heat from the sun.
Which planets can support life?
Of all the planets in the solar system, only Earth is able to support life.
The next closest planet would be Mars, but its atmosphere is too thin to keep the planet warm enough for life as we know it to survive, and the atmosphere is too oxygen-poor for animals to breathe.
There's no liquid water on the surface, so there's nothing for animals and plants to drink either.
There is the possibility that some microbial life might exist beneath the planet's surface where it is warmer and unseen liquid water aquifers could exist, but this is purely speculative.
Of all the bodies in the solar system, there are a few moons that are known to have a liquid ocean beneath their icy surfaces, so life as we know it might have been able to develop within them, but there is no clear evidence that this is the case.
How big is the solar system?
Defining the boundary of the solar system isn't the easiest task, but officially it extends as far as its gravitational influence is stronger than that of the stars around us.
The line of this influence is pretty fuzzy, but it's known as the heliopause, and that reaches roughly 120 astronomical units (AU), with 1 AU being the distance between Earth and the sun, or about 93 million miles.
This gives the solar system a diameter of about 240 AU, which translates to just under 34 billion miles (about 55 billion kilometers) wide.
Have humans ever left the solar system?
Only a few human beings have even made it to the moon, much less other planets or out beyond the solar system, but a part of us has; two parts, to be exact.
The NASA Voyager 1 and Voyager 2 probes were launched in 1977 and have been traveling at about 3.5 AU a year.
In August 2012, Voyager 1 became the first human-made object to exit the solar system. Voyager 2 followed suit a few years later, in November 2018.
No other artificial probe has traveled anywhere close to that far, though New Horizons is well on its way. Unfortunately, its power source is likely to die long before it reaches the heliopause, so we're not likely to ever know when the New Horizons probe leaves the solar system.
How many solar systems are there?
Technically, there is only one solar system: our own.
As for the rest of the galaxy, we don't call those solar systems, but rather star systems or stellar systems, to distinguish our sun and its planets from other stars and their exoplanets.
In our galaxy, there are as many as 400 billion stars, each existing as its own star system, with the number of exoplanets probably numbering in the several trillion ranges.
Where is the solar system in the galaxy?
The solar system resides on the inner edge of an outer arm of the Milky Way known as the Orion-Cygnus Arm, which is sometimes called the Local Arm, and was previously known as the Orion Spur.
Our location in the Orion Arm puts us about halfway between the center of the Milky Way and its edge, a position about 27,000 light-years from the galactic center.
The Orion Arm is no slouch either. While it is classified as a spur, which means it is a collection of gas and dust between major arms of the galaxy, recent research has shown that the Orion Arm is much bigger than previously believed.
"Our study reveals that the Local Arm is not only a tiny spur of the Milky Way," said Ye Xu, an astronomer at the Chinese Academy of Sciences, who led a research team mapping out the Orion Arm's features. "It includes a prominent major arm nearly extending to the Perseus Arm and a long spur branching between the Local and Sagittarius Arms."
Xu told Space that the features of the Orion Arm the researchers discovered "are comparable to those of the Galaxy's major spiral arms such as Sagittarius and Perseus."
Doing this kind of work is definitely a challenge, especially because we are trying to map out the floorplan of a house based on what we can see while standing in the living room.
"Determining the structure of the Milky Way has been a long-standing problem for astronomers because we are inside of it," Xu said. "While astronomers agree that our galaxy has a spiral structure, there are disagreements on how many arms it has and on their specific location."
How scientists figured out our place in this complex swirl of dust and gas is a long story, but let's just say it took a lot of observation and a whole lot of math.
Our solar system's place in the universe
There are a few ways we can look at our solar system's place in the universe.
For one, we can position ourselves in our own galaxy, then identify our neighboring galaxies in our Local Group, and extend this even further out to superclusters and beyond to create a map of the universe that we can put a pin in labeled "Home".
Once the James Webb Space Telescope comes online, that will certainly get easier to do and we'll definitely be able to come up with a better map and our place in it.
But if we want to take a step back and look at the big picture, as far as we know, our solar system is unique among the stars, because on a small planet inside it, there are life forms that are able to look out into the universe and draw those maps and name the things we see.
It's likely that we aren't alone in the universe, but for now, we are the only creatures who've been able to look out and admire the beauty of the universe at all, which makes our little corner of the universe pretty special.