The Size of the Milky Way

For centuries, everything we thought about the universe was contained in a tiny package that centered on the Earth, and then the Sun after the heliocentric model was accepted. We only knew about what we could see and assumed that since it was observable, it had to be relatively close. If only we knew.

But first, we should see how the evolution of determining the size of the Milky Way galaxy itself took place.

The first actual attempt to determine the size of the Milky Way galaxy was done by William Herschel. He did a simple thing: he just decided to count all the stars he could see, and assuming that the Sun was the center of the galaxy (galactocentricism - yes, there is a word for it), was able to predict, correctly, that our galaxy was a disk. Of course, he was wrong about where the Sun is located with respect to the disk of the Milky Way.

The shape of our Galaxy as deduced from star counts by William Herschel in 1785; the solar system was assumed near center. (NOTE: The image shown is flipped 180 degrees on the horizontal axis from the original, as first published in the Philosophical Transactions of the Royal Society in 1785; the bifurcated arms of the illustration should be on the left.)

"Herschel-Galaxy" by Caroline Herschel. Licensed under Public Domain via Wikimedia Commons

It wasn't until the 1900s that a more extensive survey of the Milky Way took place to determine the size of the Milky Way. Jacobus Kapteyn (cap-tine) deduced that the Milky Way was similar to Herschel's drawing in that the Milky Way is disk-like, heliocentric, and about 20,000 parsecs in diameter.

Kapteyn's Model of a Heliocentric galaxy

Harlow Shapley immediately published a rebuttal stating that the Sun not at the center, but about 2/3 of the way out into the disk and the Milky Way was about 120,000 parsecs in size. Shapley used the distribution of globular clusters and found them more concentrated around the center of the disk of the Milky Way, instead of near where the Sun is located.

Shapley's Model to Determine the Size of the Milky Way

Who was correct? Actually, both were correct, but also, both were wrong. Kapteyn was around the correct size, but Shapley had the Sun in the correct relative location. This actually led to a debate between Harlow Shapley and Heber Curtis, referred to by astronomers as the Great Debate (or the Shapley-Curtis debate).

Planets as Dark Matter

We know planets exist. We see the evidence right here in our own solar system. We have also found over 1000 exoplanets outside our solar system. So could planets also be a component of dark matter?


Yes and no. Yes, in that planets do not emit radiation in visible light (they can emit infrared if they are warmed by a parent star. They also reflect light that is incident on them if they have a large enough albedo. But if the planet is a rogue planet, i.e. does not orbit a star, could it be an answer to our dark matter question?


No, in that planets cannot make up a significant amount of dark matter. Just look at our solar system. Yes, Jupiter is a large planet, but even so, it is 1047 times less massive than the Sun. In our solar system, the Sun accounts for over 99.8% of all the mass including all the planets, dwarf planets, asteroid, comets, satellites, everything. Now imagine that all planetary systems are similar in this nature. Even if all planetary system had 10 planets, remember that those planets would only account for 0.2% of the mass of that system. That is not a lot of mass, especially considering what the curve looks like.

Could rogue planets then be the answer? Rogue planets had to form around a star. That is the definition of a planet - it formed from the stellar cloud that a star formed in. It become rogue only when the star supernovae or possibly a passing body (star, brown dwarf) pulled on the planet to eject it from the system. Rogue planets are out there, however the total mass they contain is not enough to give a significant dent to the dark matter issue.