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Just three decades ago, the only planets we knew of in the universe were the eight in our solar system. Since then, we've found over 5,000 circling other stars in the galaxy.

That's an incredible number that shows just how far we've come. But these 5,000 discoveries also highlight how far we have to go because the exoplanets we've found don't represent all the planets our galaxy contains.

They're just the ones that were easiest to find, and the quest to understand just what's out there and find other Earth-like worlds largely depends on us correcting those biases.

Now, one factor that makes exoplanet discoveries so tricky is that most of the time when astronomers talk about discovering exoplanets they're not actually seeing the planet itself.

Planets are just too faint compared to their host stars, so in direct images they usually get washed out by starlight. So astronomers typically have to study exoplanets indirectly, and they have a few ways of doing that.

The first exoplanet discoveries were made using what's called the Wobble Method. We often think of a star like a pin that holds steady while planets orbit around it, but in reality, a star and a planet both orbit a shared center of gravity.

Basically, if you put the star and the planet on either end of a seesaw, the center of gravity is the spot where you'd have to put the fulcrum to balance out the two sides.

Since stars are many times more massive than their planets, that center of gravity is typically located inside of the star. That's why it looks like only the planet orbits while the star stays still.

But the star actually does make a super tiny motion around the center of gravity. Astronomers can't usually detect that motion itself, but as the star moves toward us and then away, its light gets slightly Doppler-shifted.

In other words, it appears slightly more blue, and then slightly more red. That shift is often detectable, and it's a tell-tale sign that a star is moving around a central point, possibly because of a hidden planet.

In the early 2000s, scientists also began using another method to identify exoplanets known as the Transit Method. In this method, astronomers track the brightness of a star over time, trying to detect mini eclipses that happen when a planets passes in front of its star in what's known as a transit.

A planet is way too small to blot out more than a fraction of its star's light, but it does cause a small dip in brightness. So when scientists see dimming at regular intervals, it can be a sign of an invisible world.

These two methods have been used to identify a majority of the exoplanets we know about today, but the problem is that they're not good at detecting all kinds of planets.

They both favor large planets that orbit close to their stars. These planets both create more of a wobble and eclipse more of their star's light, compared to smaller planets. Plus, planets with tighter orbits circle their stars more often than distant planets, so their patterns are easier to pick up over relatively short periods of time.

Like, if you were watching our solar system from somewhere else in the galaxy, you'd have to wait over 165 years just to see Neptune transit twice.

All this means that using the tools we have today, it's not easy to detect small planets like Earth or distant planets like Neptune in other star systems.

Which is not to say it doesn't happen. Like, astronomers found seven Earth-like planets around the red dwarf TRAPPIST-1. But also, all of these planets orbit super close to their star, closer than Mercury orbits our sun.

So to really get a realistic picture of what's out there, we have a couple of options. For one, we can learn from less common ways of discovering exoplanets such as gravitational micro-lensing.

This technique takes advantage of the fact that, in space, a massive object can act like a magnifying glass for a star or galaxy behind it. That's because as light travels toward Earth from the more distant object and passes through another object's gravitational field, it bends, just like light passing through a lens, and when it does that, it appears brighter to us on Earth.

Every now and then, a star in our galaxy will cross in front of a more distant star and create this lensing effect.  And if the closer star has a planet that will cause a little extra brightening.  It's a minuscule effect, but it's actually detectable by powerful telescopes like Hubble, which can pick up even tiny planets.

This technique is harder to use because it depends on rare, chance alignments of stars.  But, it's one way to get a sense of what else is out there.

We can also acknowledge the biases of each discovery method and try to use statistics to estimate the true distribution of exoplanets based on what's been found and what we know about each method's biases.

With time, as scientists develop new tools, it will become easier to spot a wider range of planets. But, for the time being, the vast majority of planets in our galaxy remain undetected. So it's important to keep in mind that what we find is not the same as what's out there.

We live in a universe where only a fraction of all stuff is even visible to us, and what's hidden is just as important, complex, and fascinating.

Thanks for watching this episode of SciShow Space, and thanks as always to the wonderful patrons who made it happen!

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