Thank you to Trade Coffee for  supporting this episode of SciShow.

You can go to drinktrade.com/scishow to get a free bag of coffee with  any subscription purchase. Close your eyes and picture the  most typical planet in the galaxy.

Is it small and rocky? Giant and  gassy? Maybe it’s somewhere in between.

Well whatever planet is  swirling around in your mind, it’s probably basking in  the light of a nearby star. And that’s where your picture is wrong. It turns out, most planets don’t orbit stars.

They’re incredibly difficult to  find, but over the past decade, astronomers are learning just how  abundant these free-floating planets… also known as rogue planets… are. And in doing so, they’re shifting  not just how we think about planets, but how we ponder our place in the universe. [♪ INTRO] So how do you make a rogue planet? Astronomers have identified  a few ways it can happen, and some might require stretching our  current definition of what a “planet” is.

First, you’ve got your standard  planetary formation story, with what might seem like a downer ending. As a baby star forms, it’s surrounded by  a big disk of gas, dust, rock, and ice. Thanks to gravity, and a series of dramatic  collisions, some of that debris ends up condensing into bodies large enough  for astronomers to deem them planets.

But gravity is also responsible for  making some of these new worlds go rogue. It could be a planetary sibling, or a  random star just minding its own business on its trip through interstellar space. But if something big gets a bit too close,  it could gravitationally tug on a planet in just the wrong way and  knock it out of its system.

Next, you have what’s called core collapse. This is when a big blob of dust and gas  floating in the middle of cosmic nowhere has enough mass that gravity just  squishes it down to form a planet. And if you’re thinking, “Hey, that sounds  like how stars form!” You’re right.

It’s the same idea, just  with way less mass involved. But speaking of how stars form, you  could also get a free-floating planet if a star is trying to form, but  something interrupts the process. Maybe a bunch of bodies in a stellar  nursery do their own gravitational jostling and kick one out before it grows big enough.

Or maybe the energetic stellar wind  shooting out of one star blows away a bunch of gas that another had  worked really hard to accumulate. Either way, there isn’t  enough mass in the picture, so you don’t get a star, you get  yourself a very gassy planet. While we know rogues can form  under any of these conditions, we don’t actually know which  is the most or least common.

Maybe most rogue planets did  start out having a parent star, or maybe their origins are more diverse. One of the things keeping us  from answering that question is just how hard it is to find  rogue planets in the first place. It’s hard enough to find bound  planets…the ones zipping around stars.

And the most common planet-finding techniques  kind of require there to be a star. First off, we have the transit method, where scientists track a  star’s brightness over time. And if that brightness dips by the  same amount over and over again at regular intervals, researchers  know they’ve found an orbiting planet.

The radial velocity method also  involves watching starlight. But it focuses on the color of  the light, not its brightness. Basically, as a planet orbits a star, its  own gravity makes the star wiggle around.

While the planet pulls the star away  from us, the star’s light looks redder. And while the planet pulls the star  toward us, the light looks bluer. If that redder-to-bluer shift  happens on a regular schedule, you’ve got yourself a planet.

Astrometry is also wiggle-based,  but it’s looking for the actual movement of the star in space. And finally, we’ve got direct  imaging… just looking at the planet. But for now, the technology we have isn’t great for just scanning the skies  and seeing what’s out there.

It works best on pre-set targets…like  stars that might have planets around them. Now, even for bound planets,  these methods aren’t foolproof. For one thing, they’re biased towards  finding big, hot planets that are either super close to their stars, or in the  case of direct imaging, super far away.

But rogue planets can’t really be  found using these methods at all. They’re tiny dark blobs floating  amidst a similarly dark cosmic sea. Luckily for astronomers, you don’t have  to see a planet to know that it’s there.

Gravitational microlensing allows them  to find rogue planets by observing how an otherwise invisible clump of  matter distorts light they can see. You’ve probably seen the effect of gravitational  lensing on massive galaxy clusters. This one is my favorite,  because it makes a smiley face.

What’s happening here is that  the mass in a galaxy cluster distorts the fabric of spacetime itself. It acts like a lens, and any  light coming from a galaxy behind that cluster gets bent all out of shape. The light also gets magnified and  astronomers often take advantage of this quirk of physics to study things  that are dim and far away.

That technique is called gravitational lensing. Meanwhile, in gravitational microlensing, you use the warped light to study the lens itself. And yeah, that lens can be as small as a planet.

When a rogue planet briefly passes  between us and a random background star, it will cause a microlensing event. A combination of how much  brighter the starlight appears, and how long the event lasts, tells  astronomers how much mass the lens has. Now just like all our other  planet-hunting methods, this works best on larger objects,  like gas giants or even more massive brown dwarfs, which are this sort of  intermediary object between planets and stars.

The first planet ever discovered using  microlensing was spotted in 2003, and it was over twice the mass of Jupiter. The first microlensing survey of  rogue planets was published in 2011. And based on the results, the authors  estimated that for every star in the galaxy, there are roughly two rogue  planets about the mass of Jupiter.

But that estimate was based  on a very small dataset, and it didn’t really take lower-mass  rocky planets into account. Over the years, newer studies  would propose their own. And the latest one, released  in 2023, is a bit of a doozy.   It comes from the Microlensing  Observations in Astrophysics collaboration, or MOA, and claims that there are  anywhere from 8 to 44 rogue planets for every star, compared to an  estimated 3.2 to 4.3 bound planets.

Which means there could be as many as 10 rogue worlds out there for every bound one. While the study hasn’t officially  completed the peer-review process, experts think it’s a solid estimate. The whole MOA survey is built off of  over 3500 potential microlensing events, and each potential incident  passed a hefty qualification criteria before it was added to the final dataset.

After gathering that microlensing data, the team worked to construct a mass function  for our galaxy’s free-floating planets. A mass function is basically a  model that charts how many bodies you’ve got with different masses, whether  it’s a cluster of stars that all formed from the same stellar nursery, or all  the rogue planets in the Milky Way. Here’s one of the mass  functions the MOA team produced.

You can see that there are a bunch  more planets with smaller masses. These would be rocky bodies similar in mass to, or even less massive, than the Earth. So it might be easier to  find Jupiter-sized worlds, but that doesn’t mean that’s  what’s mostly out there.

Based on their mass function,  the team could compare the collective mass of all the rogue  planets out there to that of all stars. Our Sun is above average in mass, but astronomers use it as  a reference point, anyway. And for every solar mass  of star, there looks to be between 88 and 368 Earths-worth of rogue planets.

To translate that into a number of  planets, and not the total mass, that would depend on the  mass range they considered. Like, if they only looked at gas  giants, it’d be a lot smaller because you have to pack a lot more of  that mass into each individual planet. But the team didn’t just look at gas giants.

They considered planets with masses all  the way from just one-third of an Earth, to over 20 times the mass of Jupiter. And that’s how they got 8 to  44 rogue planets per star. Now, you might be looking at  that estimate and thinking, “Well that’s a very large margin  of error,” and you’re right.

But it’s to be expected. For one thing, astronomers need to  hedge their bets because microlensing can’t always tell the difference  between a rogue planet and a bound planet that has a very wide orbit. We’ll have to wait for better  equipment to narrow those error bars.

But lucky for us, we won’t have to wait too long. The Nancy Grace Roman Space Telescope is on track to launch around 2026 or 2027. If all goes to plan, it’s going  to be as much of a big deal for exoplanet research as the  Webb is for infrared research.

Roman comes equipped with a Wide Field Instrument that’s just as sensitive as Hubble’s, but  the field of view is 100 times bigger! Astronomers expect Roman to provide  a rogue planet count that’s at least 10 times more precise than our current one. Eventually, we may learn that the number of rogues is closer to 8 per star than a whopping 44.

But whatever it ends up being, experts  believe they’ll outnumber bound worlds. And one day, they might actually be able to answer why rogues are more abundant. But we don’t have to wait  for that answer to ponder an even more important question:  what does this mean for us?

From a scientific perspective, the  revelation that your average planet isn’t orbiting one of the oodles and  oodles of stars out there suggests that stellar system formation  might be even more melodramatic and complicated than astronomers expected. And it poses some great follow-up  questions too, like “Did Earth lose a whole bunch of siblings that we  could find if we looked hard enough?” “Or is there some way that these rogue worlds could be habitable for some  kind of alien critter?” Or “If it turns out most of the  planet-sized balls in our galaxy never orbited a star in the first  place, do we have to sit through yet another round of astronomers  re-defining what a “planet” is?” But maybe it’s more interesting to think of what this shift in our understanding means to humanity? While we’ll never go back to  being the center of the universe, scientists have become more and more  convinced that our solar system is odd.

And now, not only is our  Solar System a little funky, but stellar systems might be  a little funky themselves. So keep that in mind the next time you’re  pondering your place in the universe. While Earth revolves around the Sun,  some of our worlds revolve around coffee.

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It’s an aromatic dark roast and  it’s just a classic cup of coffee. And it came in an easily resealable bag  which I, personally, am a huge fan of. If you want to try it for yourself,  you can go to drinktrade.com/scishow to get a free bag of coffee  with any subscription purchase.

Thank you to Trade Coffee for  supporting this episode of SciShow. [♪ OUTRO]