It was the first world, other than Earth and the Moon, I ever saw. I knew nothing of the other planets in the Solar System, was completely unaware of their moons. I was five years old, and the idea of a universe filled with other worlds transfixed me. I wanted to visit Tatooine, visit Yavin IV. Star Wars had completely mesmerized me, made me fall in love with space.
A few months later, I found Star Trek: The Next Generation.
Week after week, I watched humans — actual humans from Earth — visit planet after planet, meeting alien races. Star Trek introduced me to Mars, to Jupiter and Saturn. I began devouring space books. I learned about the barren Mercury, the Venusian inferno, the Martian desert, the gas giants. I learned about icy Pluto, the Solar System's seemingly lone sentinel against the ocean of interstellar space.
I also learned that they were the only planets that humans knew of. In real life, there was no Tatooine orbiting two suns. No Vulcan only sixteen light-years away. No Qo'nos or Romulus. None of the countless stars of the universe had planets. In the past, scientists such as Giordano Bruno and Isaac Newton postulated that the stars had planets, but there was no proof.
By 1992, I grew disillusioned with space. There was just nothing else worth learning. There were just nine planets and a bunch of icy and rocky moons, all alone in the universe. Little did I realize that same year, astronomers made a discovery that changed the field forever.
It was January 9, 1992. Pole Aleksander Wolszczan and Canadian Dale Frail discovered two planets orbiting a pulsar, PSR 1257+12. It was a momentous find; until that fateful day, scientists had only believed planets were possible around stars on their main sequence, like the Sun. The discovery forced astronomers to rethink their preconceptions, to look at all categories of stars. Two years later, a third planet was discovered in the system. The planets were believed to have been formed by secondary planet formation, the result of a supernova remnant or quark nova.
But this was only the beginning. On October 6, 1995, the Swiss astronomers Michel Mayor and Didier Queloz discovered a gas giant orbiting in a close orbit around 51 Pegasi, 50 light-years from Earth. It was the first time a planet had been discovered around a Sun-like star. The gas giant was the first "hot Jupiter" detected by astronomers, but far from the last.
In the next three years, more planets were detected, including 47 Ursae Majoris b, which was the first planet found to be orbiting at a "normal" distance from its star (a gas giant orbiting 2.11 AUs from its star); and Gliese 876 b, the first planet found around a red dwarf (and a world that orbits its star at a distance closer than Mercury orbits the Sun). Both planets, along with the earlier 51 Pegasi b, were found using the radial velocity method — in which astronomers measure the slight radial movement of a star.
In 1999, three planets were discovered orbiting Upsilon Andromedae — the first multi-planetary system discovered around a main sequence star. What's more, Upsilon Andromedae d was found to be in the system's habitable zone. Despite Upsilon Andromedae d being a gas giant, its presence in the habitable zone has raised the possibility of habitable moons.
After years of using the radial velocity method of discovering exoplanets, astronomers used the transiting method for the first time to discover HD 209458 b in 1999. This planet was also the first planet to have a confirmed atmosphere, to be detected by multiple methods, the first exoplanet to have confirmed weather, to have hydrogen, carbon and oxygen in its atmosphere, and the first exoplanet to have its mass measured.
Three years later, Iota Draconis b was discovered, the first planet to be detected around a giant star, the orange giant Iota Draconis.
A year later, in 2003, astronomers discovered PSR B1620-26 b, one of the oldest planets at 12.7 billion years. The planet is over twelve thousand light-years away, orbiting a pulsar and white dwarf.
Over the next three years, planets were found around brown dwarfs. Multiplanetary systems without any gas giants were found. Extrasolar Neptunes were discovered. The first Super-Earths (in the sense that they were closer in size to Earth rather than Neptune or Jupiter) were discovered. By 2007, dozens of worlds had been detected.
I remained completely unaware of any of this, having abandoned astronomy for the study of history and parallel universes. But in 2007, a discovery was made that brought me not just back into astronomy, but put me on the path to becoming the scientist I am today. It was April 2007, and Gliese 581 c and d had been discovered. 581 c and d were both designated as Super-Earths, and both determined to be within the habitable zone and thus potentially habitable. It should be noted that originally, 581 c was deemed to be potentially habitable while 581 d was determined to have been too cold. Subsequent observations have changed this; 581 c is thought to be like Venus, while 581 d is considered to be potentially warm enough to be habitable. Another Super-Earth in the system, 581 g, was found to be right in the middle of the habitable zone. However, since the planet's existence has not been confirmed, it cannot be considered here. But I was hooked, and began searching for every shred of extrasolar planet news I could find.
The discoveries kept coming. Planets closer and closer to the size of Earth were found, such as COROT-7b and Gliese 581 e in 2009. In October of that same year, 30 new planets were announced. Hundreds of planets had been found, around a vast variety of stars. Brown dwarfs, red dwarfs, giants, pulsars, Sun-like stars...it seemed as though there was no barrier to planetary formation.
And then came Kepler. Mika McKinnon's excellent article about Kepler can be found here, and since she deserves to have people read it, I won't get into the details of the Kepler Mission. Instead, I'll review its most exciting discovery, at least to me: Kepler 16b, which orbited two stars.
Two suns. Tatooine, that first planet I had seen all those years ago, was no longer pure fantasy. There are real planets out there, orbiting two stars. Some are probably barren lumps of rock. Others are gas giants. But some, like Kepler 16b, are in the habitable zone, and thus may have habitable moons. In fact, all of the circumbinary planets found by Kepler are either close to or within the habitable zone.
All of them.
This is an amazing time to be alive. Over 1,800 confirmed planets have been found. By year's end, another thousand will probably be confirmed. Thousands more are waiting in the wings. And once the James Webb Telescope, TESS (Transiting Exoplanet Survey Satellite), and the European Space Agency's PLATO (Planetary Transits and Oscillations of stars) satellite are launched, we could be seeing discoveries in the tens or hundreds of thousands. Astronomers have theorized that there could be hundreds of billions of planets; maybe even trillions, counting rogue planets. 40 billion of those could be Earth-sized and habitable. Factor in moons of gas giants in habitable zones, and the number of habitable worlds could be in the hundreds of billions. Twenty years ago, there were only nine planets. Now there are billions.
It's quite a crowded sky...and I don't know about you, but I take a great deal of comfort in that.