2000 known planets, and counting.


We're going to learn about many new planets in your lifetime. Here's a primer on how we got to over 2000, as well as a plug for a project that'll find hundreds, if not thousands of new planets.

Hi there internet friend,

I want to tell you about an exciting project coming soon to a neighborhood near you. I'm stoked on this and you should be too.1

This project is a telescope. It'll be mounted on a satellite that will be fired to space in mid-2017. It's going to find planets that orbit nearby stars. I've talked a bit about these "exoplanets" in earlier posts, but what follows is an (incomplete) bit of context on the "search for planets". If you're less interested in history, scroll down for the "why do I care?"

The search for other planets

  • 50,000BC, behavioral modernity, to 1200BC: you would have to be observant (or a Polynesian navigator) to know that there was more than the Sun, the Moon, twinkling stars, and that fat band of light in the night sky. If you were observant, you would notice a few different-colored dots that moved. None of the other whitish-yellow move, but these ones do and did! These are Mercury, Venus, Mars, Jupiter and Saturn.
  • 3500 years ago (1200BC): first known records of planets in Babylonian star catalogs. These catalogs reference older catalogs, but this is more-or-less the start of recorded astronomy. If you're lucky enough to access these texts, you'd know about 7 "celestial bodies" (Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn). You probably wouldn't think of our Earth as one of them.
  • Big step forward: 1610 AD. Dutch spectacle-makers invent the telescope, and the designs find their way to Galileo in Italy. He uses them to build his own telescope and finds four of Jupiter's moons in a single year. Makes history. Throughout the 17th century, Cassini and Huygens discover some of Saturn's moons, and in
  • 1781 Herschel discovers Uranus. First new planet since... well, since the first time anyone thought that the the funny-moving, funny-colored dots were something special.
  • 1846 Galle mathematically predicts Neptune's existence and points Le Verrier to its discovery.
  • Post-1850: more moons; Pluto (Tombaugh). Telescopes improve, and it turns out that the Solar System's outer gas giants have a lot of moons. By now, we've figured out that moons and planets are different, if for any reason because the planets orbit the sun, while the satellites (moons) orbit host-planets. We've also become familiar with our own blue dot's place in this arrangement: third-closest to the Sun.

Ok. Let's take a breather, in maybe 1991. Smells Like Teen Spirit just broke, and we know about the 9 'planets' you heard about in school: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto.2 We've even sent robotic emissaries to explore them (e.g., the Voyagers to the outer planets, Pioneers to Venus, and the Vikings to Mars). On the ground, a few astronomers have claimed detection of planets around other stars, but none of these claims stand up to further observation. Things are about to change.

  • 1992: Frail and Wolszczan discover two planets orbiting a pulsar (a highly magnetized, very dense star). These planets, poetically named PSR B1257+12b and PSR B1257+12c after their host star, are the first confirmed exoplanets in an oncoming torrent.
  • 1995: First planet around a "normal" (main sequence) star (Mayor and Queloz, both from the University of Geneva). Dubbed 51 Pegasi b, it's an abnormal planet: it orbits its star in about 4 days (1/20th that of Mercury), but has a mass comparable to Jupiter's.
  • Post-1995: More planets! A mix of ground-based and space-based searches yield over two thousand known extrasolar planets. About half of these are discovered by a space-based telescope called Kepler.
  • From 2009 to 2013, Kepler records the brightness of around 150,000 stars on a roughly 10-by-10 degree patch of sky. When planets pass in front of the stars from our line of sight, the shadow cast on us is observed as a predictable brightness dip (check out Planethunters to see it for yourself). This leads to now over 1000 of the total 2000 known, with many more candidates counting.

This is all great, but why should you care about other planets?

The first indications of life on other planets could likely come from this planet discovery effort.

This deserves another post in itself. The basic idea is: a planet passes in front of its host star (and we observe the "transiting" system as a single point of light on the sky). Some of the light passing through the exoplanet's atmosphere is absorbed by its gases, causing an observable shift in the spectrum of light from the system as we observe it. Earth's atmosphere has "biosignature gases" - gases that couldn't maintain the abundances they do without a lifecycle replenishing them.3

Studying the atmospheric spectra of exoplanets is a major step towards answering an age-old question: are we alone?

The more we know about other planets, the more we can understand our own.

Relational knowledge is a core component of human understanding. How can I understand how much I like a food without some reference to other foods? Similarly, how could I hope to understand Earth's story without knowing the stories of other planets? How did they form? What are they made of? Are Earth-like planets common? Preliminary answers to these questions are already coming from exoplanet-searching efforts.

This is a frontier of exploration and discovery.

Fairly self-explanatory. Simply put: early cartographers drew incomplete world maps. The field of exoplanets produces galaxy maps, at the level of "if I had an interstellar spacecraft, where would I visit?"

Now, here's that promised plug:

A major exoplanet-searching telescope called TESS (Transiting Exoplanet Survey Satellite) will launch in 2017 and discover hundreds of new planets. Many of these planets will resemble Earth, and their atmospheres will be easier to study than almost any known exoplanets.

Here are some related links:

I'm going to be writing about TESS, along with the field of exoplanets and my journey through it, on this blog-space. Swing by every now and then, we'll learn together!

  1. I'm biased, because I'll be contributing to this project, but see for yourself. 

  2. Meta-acronym: my very easy method just speeds up naming planets. Pluto stays, for the acronym. 

  3. The specifics of what exactly the best biosignatures are is still being worked out. However, a good example of life on Earth radically changing its atmosphere is in the appearance and maintenance of oxygen