Something that’s really exploded over the past twenty years is the number of exoplanets—planets outside our solar system orbiting distant stars and pulsars. Unlike discoveries of new comets and asteroids, which sometimes happen through ‘citizen science,’ exoplanets are discovered by astronomers dedicated to the task.

In the 1970s, such planets were thought likely to exist, but we had no proof. We had trouble seeing to the edge of our own solar system, as current work on ‘Planet Nine’ makes plain. Back in the 1970s, there was no Hubble space telescope. There was no array to listen for alien life. Heck, the discovery of Pluto, which was a planet for a while before it wasn’t anymore, happened in my own parents’ childhoods. Dad was 12, Mom was 6. That was 1930.

Pluto lost its planetary status in 2006, becoming the first ‘dwarf planet.’ The guy whose work spurred that is Caltecher Mike Brown, twitter handle @plutokiller. He works on Planet Nine now, and there’s a great little picture book about him and Pluto, which makes a great gift for any of the youngest astronomers in your life.

Anyway, Neptune was discovered in 1846 and Uranus in 1781. These dates are recent history when compared to the discovery of the inner rocky planets and Jupiter and Saturn. These were discovered as early as ~2000 BC, by Babylonian astronomers. Wow!

Each discovery helped us understand our solar system more fully. Each shed light on how solar systems form to begin with. Orbital physics, ecliptics, accretion discs—we’ve learned about the physical universe through investigation of our local neighborhood.

But exoplanets are the new thing, and they’re being discovered with increasing frequency. Questions they raise include:

  1. How common are planets, outside our solar system?
  2. Is our solar system ‘average’?
  3. What about Earth? Is it ‘average?’
  4. Does life exist anywhere but here?
  5. Is the make-up of the universe homogenous or wildly varied?

The first exoplanets were confirmed in 1992, as two planetary bodies orbiting a pulsar. Maybe you remember it. I was a graduate student, at the University of Wisconsin, working on my PhD in genetics. I remember how giddy everyone was at this baby step toward more fully measuring the universe. And it wasn’t just the astronomy students who were excited, but all of us. We were over the moon; a cause for celebration and wonder.

Three years later more exciting news: A mega planet with an orbit of four days had been confirmed. I recall thinking that it was an exoplanet, sure, but not the first one discovered, and not an interesting one (!!), in part because the 1990s were such an exciting time in astronomy anyway. Reports of this megaplanet had to compete with the appearance of a newly discovered comet, Hale-Bopp. Also, Hubble was up in the sky, NASA’s shuttle missions were enjoying their hey-day, and the Jet Propulsion Lab’s mission to Saturn—the Cassini mission—was preparing to launch. What a decade.

Exoplanet discovery became routine, with a new report on a new planet coming about once per year. These early discoveries relied on a technique called ‘radial velocity.’ If a star in the night sky ‘wobbles’ just right, it becomes a candidate for hosting a planet, and the periodicity of the wobble indicates the time required for the planet to orbit its star. Radial velocity took a star-by-star approach; it was a tedious process, but the goal made up for that—new planets! To date, close to 1000 planets have been found using radial velocity.,

But we can also find planets through direct imaging. If a planet is large and hot enough—think Jupiter—and if our instruments are working at peak capacity with limited distortions in the viewing field, we can image exoplanets. Around a hundred planets have been found this way. And, there are other ways to discover exoplanets, which rely on the gravitational pull of various bodies to one another.

Then, big news came along in 2009 with a space telescope called Kepler. Over the course of the following decade, this telescope monitored the brightness of stars within a patch of the galaxy. Using photometry (a measure of brightness), Kepler predicted when an object obscured, or basically eclipsed, a star. This periodic drop in brightness gave evidence of new exoplanets, and of half a million stars imaged, Kepler found over twenty five hundred planets.

To date, in 2021, we’ve got 4,864 confirmed exoplanets, in 3,595 different planetary systems. Over 800 systems have two or more planets identified. This is our galaxy, and last month came the first report of a possible planet in a separate galaxy. This is our universe!

Which ones, do you think host life? Which ones might have a breathable atmosphere? Which ones would we visit if we could? Are any as beautiful as Earth? All sorts of questions come up.

When I was a kid, we thought there might be aliens (we called them Martians; we called them little green men) within our own solar system. I don’t think kids today really think that’s the case, and so far, our search for extraterrestrial intelligent life has turned up squat. But with thousands of planets and counting, maybe we will find worlds with life—worlds that are habitable. Most of the planets discovered to date will be hostile worlds, presumably as sterile as Venus or Neptune. A scant few might host life.

People are sorting through the exoplanets to decide which ones those might be. Here’s one schematic from Potentially Habitable. This group pores through the data to put odds on habitability:

Dozens of exoplanets, based on size and distance from their star and a few other things, might host life. To put those back into context of all the diversity ‘out there,’ here’s artist Martin Vargic’s conceptualization of what we’ve found, to date:

Exciting, yeah? Where do we go from here?

At least for some astronomers, including MIT’s Sara Seager,  the answer to that is this: we might shift from large scale censuses back toward a planet-by-planet analysis. With improving optics and analysis, and with the knowledge of precisely where these planets lie in the sky, we can begin to ask more pointed questions about them. What can we say about the atmospheres of these worlds? Can we detect biosignatures, like methane or oxygen? What makes up the cores of these planets?

All of this discovery, for me, provides a deep sense of wonder and awe while also anchoring me into how wonderful our own planet—our home—is. The fact that we measure the universe to understand our place within it is one of the best parts of being human and alive. That we share this joy with others is another part of that.  

So to wrap up:

  1. Planets abound in our galaxy.
  2. They come in all shapes and sizes.
  3. There’s so much more to be discovered.

We’re always on the threshold of new discoveries. I hope the next one is extraterrestrial life.

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