In 1584, Italian monk and philosopher Giordano Bruno theorized that there were "countless suns and countless Earths all rotating around their suns."

Bruno was convicted of heresy for his ideas about the universe, but countless sky-watchers since his time have dreamed about those other worlds, too.

And, I’m happy to report that they are no longer being convicted of heresy, and the science of astronomy has proven them all right.

Today, we know that there are planets out there orbiting distant stars -- we call them exoplanets, and we're finding more of them every day, almost literally.

The first exoplanet was discovered in 1995, when Swiss astronomers Michel Mayor and Didier Queloz found a planet orbiting the distant star 51 Pegasi.

This marked the beginning of a whole new era of astronomy, and a flood of discoveries followed.

Dozens of exoplanets were detected by the year 2000, and today the count of known exoplanets is around 2,800.

But we have a lot of work ahead of us:

A study in 2012 predicted that each star in the universe may have an average of 1.6 planets orbiting it. Considering there are probably like 10 to the 24th stars in the cosmos, that’s a lot of worlds to catalog.

Thankfully, we’re developing technology sensitive enough to find more of them than ever, and we’re perfecting a couple of crafty techniques to detect planets that would otherwise remain totally invisible to us.

And, for the first time, both of these methods will be put to use in the first U.S. observatory specifically devoted to exoplanet research.

It’s called Minerva, and it’s being built on a California mountain top, with plans to go online by late summer 2014.

You might say that it’s going to open up whole new worlds to us.

(Intro)

It took generations of daring thinkers to take us from Giordano Bruno to Edwin Hubble, who in 1924 looked through California's Hooker Telescope and realized that those cloudy-looking patches in the sky were actually distant galaxies -- clusters of hundreds of billions of stars far from our own.

Suddenly there were whole new celestial neighborhoods for us to explore. But stars are one thing; we can see those.

Planets don't make their own light. They’re millions of times dimmer; they get lost in the light of the stars they orbit.

So astronomers had to learn how to detect the effects that nearly invisible planets have on their stars, in order to know that they’re there. And that took a long time.

It’s only been in the last 20 years that we've arrived at a couple of good, reliable techniques for finding, and sometimes, directly observing these planets.

These methods are still so new that Minerva will be the first observatory to combine both of them, to become a kind of planet-hunting machine.

Up until now, the biggest superstar in exoplanetary science has been NASA's Kepler Space Telescope, which launched in 2009.

It was THE go-to device that astronomers used for exoplanet research, and in its just-over two years of service, it has found more than 960 alien worlds and more than 2,000 other potential, or “candidate” planets.

And it did it all using what's called the transit method.

Kepler pointed a highly sensitive light sensor, called a photometer, at a single patch of sky, and it checked for changes in the stars' brightness.

If a planet passed in front of a star, or transited, it would block out a tiny bit of that star's light, decreasing its brightness by a little tiny bit.

And if this decrease occurred periodically, it was likely an orbiting planet, dimming the star's light again and again as it traveled around the star.

Kepler's sensor could detect those tiny dips in light, and by measuring their strength and duration, astronomers could calculate the size of the planet that caused them, and its distance from the star.

Problem is, smaller planets create much smaller decreases in light, which makes them much harder to find.

A planet with the volume and orbit of Earth, for example, would create a dimming that would only last hours — detectable only with the latest sensors.

But Kepler has yielded incredible results nonetheless, discovering hundreds of worlds that -- for better or worse -- we’ve tended to describe in terms of our own solar neighborhood: gaseous hot Jupiters; big, rocky super-Earths, cold mini-Neptunes.

Unfortunately, in May 2013, one of the mechanisms onboard Kepler that astronomers used to maneuver its sensors, called a reaction wheel, failed. The first of its four reaction wheels had actually stopped working the previous July, but after the second went, Kepler’s primary mission was done for.

NASA’s still coming up with ways to keep using the hobbled observatory, but it’s time for something new.

Kepler has not only given us about a thousand-planets’ worth of knowledge about the universe, it also made us a lot better at simply looking for them. Minerva wouldn’t be able to do what it’s about to do, if it weren’t for everything Kepler taught us about the transit method.

But we’ve also been finding planets since before Kepler’s time, using a totally different technique.

It’s a little more complicated and mathy, but it gets the job done. And we know this, because it’s what Mayor and Queloz used to find the first exoplanet in the 90s.

It’s called the radial velocity method, and it works by measuring a planet's gravitational effect on the orbit of its star.

I’m gonna say that again -- it measures the orbit of a star, not the orbit of the planet.

This is really cool: strictly speaking, planets don’t really orbit stars.

Instead, both planets and stars orbit their common center of mass. It’s a lot like what we here on Earth call the “center of gravity.”

It’s not the geometrical middle of an object, or the half-way spot between two objects. Instead, it’s the central point of all the mass that’s distributed through the entire system -- in this case, the combined mass of a star and a planet.

Because a star is so much more massive than a planet, their common center of mass is usually pretty close to the center of the star. So oftentimes, it just looks like a planet is orbiting a star while the star spins in place.

But the bigger a planet is, the farther away that center of mass is from the core of the star. Since both the planet and the star are orbiting around this central point, it gives the star an off-center-looking orbit that’s sometimes described as a “wobble,” and it’s a telltale sign that a planet is nearby.

And to determine how big the planet is, we can measure the speed of the star’s orbit -- that’s its radial velocity.

Using spectrometers, we can detect shifts in the wavelengths of the light that the star emits as it moves toward and away from us. And the faster a star is orbiting, the bigger its companion planet must be.

Now, it takes a really massive planet to have a measurable effect on a star’s orbit, of course, so this technique works best in finding really large exoplanets. Mayor and Queloz's planet was around the size of Jupiter, for example -- hundreds of times larger than Earth.

So, using either of these techniques, we still have a hard time detecting planets smaller than about five times the size of Earth. And even then the data that each method yields is kind of sketchy. Given all that we can't see, it takes a long time to be sure that an exoplanet we think we’ve detected actually exists.

To double- and triple- check the measurements, we need to wait for a second, and then a third transit of a planet, or orbit of a star — and as we know from our own solar system, sometimes orbits take a long time.

And then what do you do when the time comes around, and all the telescopes are booked?

Astronomers have to share the very few observatories that are powerful enough to detect exoplanets, and... sharing is hard.
 
That's where Minerva comes in.

Astronomers from Caltech, Harvard, the University of Montana, and other U.S. universities are devoting this observatory exclusively to exoplanet research — and because it’ll be using both the transit and radial velocity methods, that means the two sets of data can be combined to verify findings.

Four optical telescopes, each with an aperture of 0.7 meters, are being installed on California's Mount Palomar. They'll all be equipped with a sensor and a camera, and linked to a single high-resolution spectrometer.

This way, astronomers can either put all four scopes together and measure a chunk of the sky with a big, combined aperture of 1.4 meters, or they can use them individually to take separate readings using different methods.

Minerva will only look at a few stars at a time, starting with ones within 75 light-years, because those stars would likely have our closest exoplanet neighbors.

Compared to Kepler, which watched 150,000 stars at a time, Minerva’s purview may be kinda small. But Minerva won’t take its mechanical eyes off those stars for three whole years — not once!

The telescopes and the spectrometer will be controlled by computers that will position them every night, so Minerva won't need humans to constantly make adjustments.

Instead, us humans will be freed up to do the math!

So what’s next? Well, the brass ring in exoplanet research is detecting potentially habitable worlds.

Right now, we have enough data to guess how likely it might be for, say, liquid water to be on a planet, based on how big it is, how big its star is, and how far apart they are.

But when it comes to actually detecting the chemistry of these distant planets, like whether it has CO₂, or O₂, or water vapor in its atmosphere, we need some very powerful equipment.

Spectrometers can detect the presence of chemicals in a planet’s atmosphere, and we’ve just started to apply this technique to some of our nearest neighboring exoplanets.

But Minerva will be helping us discover planets that we can't even see directly, so observing what they're made of is a ways off.

Happily, we're living at the dawn of a whole new era of super powerful telescopes -- not just Minerva, but, beginning in 2018, the James Webb Space Telescope will be in orbit. And in addition to its mission of peering back into the earliest reaches of the universe, it’ll also be enlisted to help study the atmospheres of the most enticing worlds we can find.

So, some time in the not-too-distant future, we'll be able to look at exoplanets themselves, and maybe not too long after that, we’ll be able to see what’s in their atmospheres.

So, for everyone since the time of Giordano Bruno who’s fantasized about finding distant planets around other stars … you have been vindicated!

And for all of us here who just can’t wait to see them... just a little while longer.

Thank you for watching this episode of SciShow Space, and I want to give an extra super special thanks to our Subbable subscribers. You make this channel possible, and my hope is that this channel, in turn, keeps you fascinated and curious and passionate about the universe.

If you want to learn more about space with us, just go to youtube.com/scishowspace and subscribe!