Units in Space

The Spacing Out Blog is back! After a hiatus to update our website, it’s time to start nerding out about space again, and I figured a question from one of our readers was a good place to begin.

We received a question via the survey for our Spacing Out newsletter having to do with measuring distances in the universe using things like the size of our solar system or the size of our Sun as the units. It caught my attention and made me think about all the various way we measure things in space, some of them quite logical and others begging the question of who thought that was a good idea.

So I thank you, Dear Question Asker, for asking about distances and units of measurements in space. Let’s play with numbers.

Solar System Distances

Let’s start with one of those units that makes most of the world scratch their head in confusion: miles. I’ve used miles my whole life and I’m the first to admit they don’t make sense. It’s supposed to represent the distance a Roman would cover taking a thousand steps with each foot, right, left, right, left. It took a long while for that measurement to get standardized, and a lot of the world didn’t bother.

Kilometers are much more sensible, being based on something that doesn’t change from person to person—the Earth itself. A meter was originally defined as one ten-millionth the distance from Earth’s pole to the equator (on a line that runs through Paris, to be quite specific). A kilometer is a thousand meters, so one ten-thousandth the distance from the pole to the equator.

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These are the distances we like to use on Earth. For space though, their use is very limited beyond, say, the orbit of the Moon. Even for a distance as small as Earth’s distance to the Sun we’re starting to get into stupidly large numbers: 93 million miles or 150 million km. So let’s get rid of ‘em and use a far more sensible unit for such a distance: the astronomical unit, or AU. 

One AU is the average distance between Earth and the Sun. This is a handy unit for measuring distances within the solar system, because it gives you Earth, and that sweet habitable zone orbit it hangs out in, as a reference. How close is Mercury to the Sun? 0.4 AU. You might not know exactly how far that is, but you can guess it must be toasty territory. How far is Neptune? About 30 AU, the frigid part of the solar system.

And how far to the edge of the solar system, the point where the Sun’s gravity is no longer controlling things? The exact distance isn’t known, but estimates put it around 100,000 AU away from the Sun. 100,000 isn’t that big a number (at least not when you’re dealing with astronomy), but it’s going to get annoyingly big irritatingly quickly if we keep moving out, so let’s shift it to another, cooler unit: the light year.

It’s hard to say whether this unit counts as sensible or not, but it’s awfully fun to use. A light year is the distance you will travel in a year if you move at the speed of light, the fastest speed in the universe (note that similar units such as light hours or light minutes also exist). One light year is roughly six trillion miles, or about 63,240 AU. This puts the edge of our solar system roughly 1.6 light years away from the Sun. So if we want to make up our own unit of measure as suggested by our Question Asker, 1 Solar System, then measured from one edge to the other it would be about 3.2 light years, 200,000 AU, or about 19,000,000,000,000 miles.

Outside the Solar System

This brings up a new unit, one of the dumb ones: parsec. Star Wars fans know the Millennium Falcon can make the Kessel Run in twelve of them, which itself doesn’t make a ton of sense since this is a unit of distance. The word is a combination of the words “parallax” and “arcsecond”. 

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Parallax is the way a star appears to change its position in the sky because the Earth is changing its position around the Sun. You can witness the parallax effect yourself if you hold something, say a pencil, out in your hand and look at it with first one eye shut and then the other. The distance that the pencil’s position appears to change when you do so is its parallax. An arcsecond is one-sixtieth of a degree across the sky, which spans 360 degrees total, so an arcsecond is 1/1,296,000th of the width of the sky. One parsec is the distance at which a star’s position will appear to change by one arcsecond over six months, as the Earth moves from one side of its orbit to the other. I told you it was dumb.

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It’s a distance whose use has never become instinctual for me, the way light years have. Unfortunately (for me) it's a very popular unit of measurement amongst astronomers. One parsec also happens to be very close to 1 Solar System, our imaginary unit, in distance, a total coincidence since one is based on the extent of the Sun’s gravitational influence and the other on Earth’s orbital movement around the Sun. One parsec is 3.26 light years, or 1.02 Solar Systems.

Let’s look at some distances in space. The nearest star to the Sun is the wee red dwarf Proxima Centauri. 

Even though it’s closest, it’s so faint that you can’t see it in the sky with your eyes. It’s about 25 trillion miles away (and that’s the last time I’m going to use miles), about 269,000 AU (aaaand also the last time I’ll use AU), 4.25 light years, 1.3 parsecs, and 1.33 Solar Systems.

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The center of the Milky Way is about 30,000 light years, 9,200 parsecs, or 9,380 Solar Systems. The Andromeda Galaxy, our nearest large galactic neighbor, sits about 2.5 million light years away. That’s 781,250 Solar Systems and 766,500 parsecs. We’re starting to get into huge numbers again, so astronomers will often refer to distances in megaparsecs, units of a million parsecs. You could say the distance to the edge of the visible universe (once you take the expansion of the universe into account) is 46.5 billion light years, 14.3 billion parsecs, or 14,300 megaparsecs. That’s also about 14.5 billion Solar Systems. 

There’s just no way to measure the distance to the universe’s edge and make the number small.

Mass

Another unit astronomers tend to reference is how much stuff something is made of, its mass. This often gets conflated with weight, but weight is how gravity pulls on mass. If you’re in the void of space, you may be weightless, but you still have mass. On Earth we use pounds (a dumb unit of obscure origin) or kilogram (nice and sensible, originating from the mass of one liter of water) for both mass and weight, so it can get a little confusing.

As with miles and kilometers, these units aren’t much good in space. Our Earth, for instance, contains about 13,000,000,000,000,000,000,000,000 pounds of mass (about 6,000,000,000,000,000,000,000,000 kg)

Obviously these numbers are ridiculous to try and deal with, so we just call this 1 Earth mass, which even has its own symbol, M⊕.(also called MEarth). Similar to the AU, this allows us to easily measure masses in our solar system. For instance, you can easily say that Jupiter is about 318 M⊕ (or one Jupiter mass, M_J) or that the Sun is 333,000 M⊕. This, in turn, is 1 solar mass, or M_☉. 

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When we measure things outside of our solar system, we often use units like these. Found a new exoplanet? If it’s a small one, you’re likely to describe its mass in terms of M⊕. A large rocky world, containing twice Earth’s mass, for instance, would be described as being 2M⊕. A much more massive exoplanet might be described using Jupiter masses. The most massive known exoplanet weighs in somewhere in the vicinity of 30 M_J. Meanwhile, the star to which these theoretical exoplanets belong will be described using the Sun. Small stars have only a fraction of the Sun’s mass, with Proxima Centauri being a mere 0.12 M_☉. The very largest stars will have a mass somewhere in the vicinity of 200 M_☉.

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I’ll note here that the size of exoplanets is also often given in terms of comparisons to our solar system, generally in Earth radii, R⊕, Jupiter radii, R_J, or solar radii, R_☉. We’ll also describe the planets’ distances from their star in AU, just as we do in our own system. It’s just easier to picture an alien solar system if we compare them to our familiar surroundings by using similar units of measurement.

Once you’re talking about things star-sized and above, solar masses is actually the favored unit of mass measurement for astronomers, so we don’t have to make up units this time to appease our Dear Question Asker, like we did with our Solar System unit. The supermassive black hole at the center of our Milky Way? Somewhere in the neighborhood of 4.3 million M_☉. The mass of the Milky Way itself? Up for debate, but a recent study puts it at around 200 billion M_☉.

Yes, those numbers are once again starting to get stupidly big, but so far nobody has bothered to come up with a bigger standard unit of mass than the mass of the Sun. Feels like an oversight if you ask me (but then nobody did).

Measuring Space

There are a lot of things in the universe that are frustratingly difficult for us to measure. The real reason one Solar System isn’t a standard unit of measurement for things in space is that we don’t actually know exactly how far out our solar system goes—the exact edge is too faint for us to see. We can’t use the mass of the Milky Way as a bigger standard mass unit than solar masses because it’s so hard for us to measure it accurately from our position inside of it.

That’s why we lean so heavily on things we can measure to great accuracy—the distance from the Earth to the Sun, the mass of Jupiter, the distance light will travel in a year—to help us understand our universe. After all, it’s a huge, wild, whacky universe out there, and talking about supermassive black holes and distant galaxies and alien solar systems in terms like AU and solar masses not only make the numbers we’re working with more practical to deal with—they can also make that strange universe seem a little more familiar. And feeling a little more at home in the vastness of space will always be a good thing in my book.