5,000th exoplanet found!

The number of known exoplanets has topped 5,000.

There are thousands of strange, new worlds. It is amazing.

The odometer clicked over with the addition of 65 new planets, most of which were confirmed by NASA's K2 mission.

This has been a long time coming. The first confirmed exoplanets were announced in 1992. The remnant core of a massive star that exploded as a supernova was found around a pulsar. It was pretty much the last place anyone would expect to see a planet, let alone two, the first two ever found. The best guess is that they formed from the debris of the star that didn't get blown away completely, which formed a disk around the pulsar and formed planets. A third person was found in that system. The tale of the discovery of these planets is a great example of how science works.

It was amazing and revolutionary, but it was still not quite right. We wanted to know if planets could be in the vicinity of stars like the Sun.

In 1995 a planet with half the mass of Jupiter was discovered. The host star 51 Pegasi is about 50 light years away from Earth and similar to the Sun, though older and likely to be taking its first tentative steps to becoming a red giant. Even though it was found around a star, the planet was still a shock because it was very close to the star. That means it is hot, with a temperature of 1,000 degrees.

Astronomers were skeptical since a gas giant couldn't possibly form that close to the star. Some planets can get very close to their host star, as shown by the physical models. Jupiter was thought to be moving inward toward the Sun early on, but the gravity of Saturn pulled it back in a complicated dance.

Astronomers were skeptical about the existence of exoplanets. The method used to find them is called the radial velocity method, which means that the planet's gravity causes the star to make a small circle as well. As the star moves around in a circle, it moves towards the Earth for half of the time, and that causes a shift in the star's light. This can be measured, but is usually so small that it is difficult to observe. There are many other factors that can make it look like a planet when there isn't one.

That ended with the planet HD. It was discovered using the radial velocity method, but calculations indicated it was very close to edge on. It should pass between us and the star once per orbit. That causes a mini-eclipse and a drop in the star's light. Astronomers observed the star at the time it was calculated and saw its light fall. It was a great moment, and the skepticism of the existence of exoplanets evaporated overnight.

We needed a dedicated space observatory to look for planets. It could look at a single spot in the sky and look for transits around many, many stars all at the same time, because it had no atmosphere to fuzz out starlight. The observatory was launched and observed over 150,000 stars in a single patch of sky. It found over 2,500 exoplanets, half of which are known at the moment. After four years in space, the second of the four reaction wheels that point at its target was lost, but a clever technique developed by engineers allowed the mission to be extended in what was dubbed K2

NASA followed this up with the launch of the TESS, which scans the entire sky to look for exoplanets.

We have learned a lot. One of the most common types of planets is larger than Earth but smaller than Neptune, which we call mini-Neptunes. There is no such planet that goes around the Sun, unless Planet 9 turns out to be one. Our solar system is not like a common kind of planet. We don't know why this is.

We can see planets that are larger than the Sun. The discovery of the TRAPPIST-1 system, a star not much bigger than Jupiter yet hosts at least seven Earth-sized planets, shows that tiny, cool, and dim red dwarfs are good at making Earth-sized worlds.

We've found planets around white dwarfs, the dead cinders of stars that were once like the Sun, and in fact the oldest known evidence for an exoplanet was in data taken of a white dwarf in 1917. It wasn't known at the time that the spectrum of the white dwarf van Maanen 2 showed evidence of a planet, but a century later we knew what to look for.

We have found planets around the nearest star to the Sun, Proxima Centauri, and a planet in the constellation 40 Eridani A.

Not all stars host planets, but some have systems like ours with multiple planets. There are hundreds of billions of planets in the universe.

I used to wonder if Star Trek was too optimistic. It underestimated how many there are.

The Nancy Grace Roman Space Telescope will be able to detect chemical elements in exoplanet atmospheres, as well as detecting thousands more exoplanets.

There are five-thousand planets. It is a drop in the bucket compared to what is out there. There are hundreds of billions of galaxies like ours.

The future of discovery looks very bright and full of planets.

Per ardua planetarum!

My friend and exoplanet astronomer,Jessie Christiansen, who manages NASA's Exoplanet Database, is a newly appointed TED fellow, and who answers the questions I pepper her with. She did a Q&A about the discovery of 1.609 kilometerstone.