Are we in danger from a rogue planet?

Contributed by
May 19, 2011
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Yesterday, I wrote about a new study that indicates that free-floating planets in the Milky Way may outnumber planets orbiting stars, and even be more numerous than stars themselves. It's an amazing result! The most likely scenario is that these planets formed in solar systems similar to ours, but got ejected due to gravitational interactions with other planets in the system. These planets get literally tossed out into space, wandering the galaxy forever*.

This made me wonder: if these numbers are correct, how likely is it that such a rogue planet might actually be close by on a cosmic scale? And given the kind of topic I like to write about, are we in any danger from a close encounter with one of these galactic nomads?

These wandering planets are so dark and distant they are currently essentially impossible to detect using regular techniques, so we don't know if any are in our galactic neighborhood or not. The only way to get a grip on how close one might be is to look at it in a statistical sense: on average in the galaxy, how many of these planets are there per cubic light year of space? Then we can fiddle with the number a bit to see how far away one of these planets could be.

Let me be clear up front about something. No doubt there will be people who may want to claim these rogue planets might explain Nibiru or Planet X or the Mayan apocalypse. These people are wrong (again, and as usual). As you'll see, the math absolutely does not support such a claim at all. So if you hear someone talking doomsday, send 'em here.

And I might as well address the TL;DR crowd: the conclusions I draw here are that a) on average, a rogue planet may be closer than I would've initially guessed, but 2) not nearly close enough to be a concern in any way.

OK then, got it? Onward to the math!


Crank up the volume

Basically, all we need to do is take the number of rogue planets in the galaxy and divide it by the volume of the galaxy, and that gives us the density of these planets in space: how many there are in a cube a light year on a side. If the answer is, say, 1 then we expect to have one rogue planet inside a one-light-year-wide cube centered on the Sun. So let's see what the math tells us.

First, there are a lot of rogue planets. In the study, they say there are very roughly as many of them as there are stars in the Milky Way. Let's call it 200 billion.

spitzermilkyway_sunscalebarSecond, the volume of the galaxy isn't hard to estimate. I've done it before, and the details are there if you want them. Let me cut to the chase: the Milky Way has a volume of roughly 2 x 1013 cubic light years: that's 20 trillion cubic light years!

That's a lot, too. Dividing them to find the density, we get:

2 x 1011 planets / 2 x 1013 cubic light years = 0.01 planets per cubic light year

In other words, We'd expect to find one of these wandering planets in a volume of space encompassing 100 cubic light years. That's a cube about 4.6 light years on a side (or, if you prefer, a sphere about 3 light years in radius).

Hey, wow, wait a second: The nearest star, the Alpha Centauri triple star, is about 4.3 light years away. That means there's a pretty good chance that, statistically speaking, there may be one of these rogue planets closer to us than the nearest star!

That's actually quite shocking to me. Seriously: wow. I've often wondered if we'd ever find a brown dwarf -- a faint, "failed" star -- closer than Alpha Cen, but it never occurred to me there might be a planet closer by! That's pretty cool.


Stranger planet danger

OK then. Are we in any danger from these puppies? Could one pass close enough to us to cause earthquakes, say?

No! And I mean categorically, no. 100 cubic light years is a vast, mind-numbing volume of space: about 1041 cubic kilometers! That's a huge amount of real estate to tool around in. Even if a planet got as close as a light year away -- ten trillion kilometers, or 6 trillion miles -- the effect on us would be essentially nothing.

Remember, we're talking about planets with about the same mass as Jupiter. Our Jupiter gets as close as about 600 million kilometers from Earth. The Moon itself has no substantial effect on earthquakes, and at most Jupiter's effect is a tiny fraction of that. A planet ten thousand times farther away than Jupiter may as well not exist as far as gravitational effects are concerned.


Oh comets, where Oort thou?

So we're safe from direct problems. What about indirect ones?

Out way past Neptune is a population of icy bodies that, when they fall toward the Sun, turn into comets. There may be a trillion of these guys within a light year or so from the Sun, in a region called the Oort cloud. Could a rogue planet dislodge a bunch of these and drop them toward us, triggering impacts and a mass extinction?

In a word, no. Again, the volume of space we're talking about here is staggering. Even a trillion comets spread out over that amount of space makes things pretty thin out there; on average those objects are a billion kilometers apart. The odds of a planet getting close enough to dislodge even a single Oort cloud object is pretty small. And even if it did, it's only one comet! The odds of it hitting the Earth are even teenier. We're a pretty small target in a whole lot of solar system.

And let's have a sanity check: if this were a real danger, we'd see evidence of it in the fossil record. A planet-wide bombardment of giant comets -- even from a single big comet -- in recent geological history would be hard to miss. We don't see it, therefore this isn't a danger.

I'm not saying asteroid and/or comet impacts aren't a danger at all, just that ones triggered by a rogue planet whizzing past us is incredibly small. We need to take impacts seriously in general, no matter what the cause. But in this case, interstellar planets doing the deed specifically aren't a worry.


You can breathe easy These results both surprise and don't surprise me. I'm very much surprised that one of these interstellar nomads could be closer than even the nearest star; that's amazing. But I'm not surprised they pose no real danger. The solar system and the Earth are terribly old, and there's been lots of time for disasters to occur. If these planets were a real and immediate threat it seems clear we'd have known about it long before now (as we know about, say, asteroid impacts). The very fact that life has been around for billions of years, and complex life for hundreds of millions, means rogue planets don't create cosmic calamities often enough to be a worry.

In a nutshell: I'm not concerned about it.

That is, in an "Oh my FSM we're all gonna die!" kind of concern. As a scientist, I find these objects totally fascinating. If there is one within a couple of light years, and it's still retained enough heat from its formation to glow in the infrared (as I discussed in the post yesterday), it may be possible to detect one directly in the next few years. It would be too far away to send a space probe (let alone visit), but with sensitive telescopes it's not crazy to think we might actually be able to actually see one.

And that would be truly cool.


* For those prone to worry, this is not going to happen here in our solar system. The planets ejected likely suffered their indignity when their systems were very young, and super-Jupiter-sized planets still migrated in toward their parent star. Our solar system should be pretty stable over the next few billion years.


Related posts:

- The galaxy may swarm with billions of wandering planets
- How many habitable planets are there in the galaxy?
- Is there another planet in the solar system?
- No, there's no proof of a giant planet in the outer solar system (this may seem to contradict the link above, but it's a different topic!)