As recently as the mid 1990’s if you wanted to talk about real planets you were limited to our own solar system – Mercury, Venus, Earth, etc. While it seemed wildly unlikely that our Sun was the only star to be orbited by planets, that wasn’t the same thing as knowing for sure that there were other planets around other stars, or knowing the slightest thing about them.
But beginning in 1996 that all changed with the announcement that astronomers had found positive proof of “exoplanets,” an appropriately exotic sounding label for planets orbiting stars other than our Sun. By 2001 as the number of cataloged exoplanets reached double-digits, I was marveling that there were more planets known outside of our solar system than there were within it. Since then the number of identified and cataloged exoplanets has completely exploded. We now know of nearly a thousand confirmed exoplanets, and new exoplanets are being discovered at a rate of nearly once per day. The pace of discovery in this field is breathtaking.
The first exoplanets were discovered by detecting a wobble in their central star caused by the exoplanet’s gravitational tug-of-war with the star as it orbited. To produce a detectable wobble in a star the exoplanet had to be huge and extremely close to its star. As a result, almost all of the first exoplanets discovered were commonly referred to as “Hot Jupiters,” truly hellish places where the chances of them being home to some kind of life were virtually nil.
In 1999 a new way to detect exoplanets was discovered by observing how a planet passing in front of its central star causes that star’s observed brightness to dim by about 0.01%. It’s kind of like figuring out how to ultra-precisely measure changes in the light from an extremely distant lighthouse so that you can detect the dimming and brightening of the light as a gnat flies in front of it.
For this observation to work the plane of the orbit of the exoplanet around its star has to be almost level with our line of sight so that the planet periodically passes directly in front of the star as seen by us. Only about one star in 200 might be oriented in such a way as to have the orbits of its planets line up with our view of the star.
And even with 1-in-200 odds you’re going to be looking at a lot of stars that might have planets available to this clever new way of finding exoplanets.
Which is exactly what the Kepler space telescope began doing in May of 2009.
The Kepler instrument stares, unblinking, at a field of more than 100,000 stars in the constellation of Cygnus the Swan. The size of Kepler’s field of view is about the size of your clenched fist held out at arm’s length. That’s the equivalent of about 1/400th of the sky.
Ultrasensitive photoreceptors connected to Kepler’s 1.4 meter telescope continuously measure the light coming from each of these stars as computers onboard the spacecraft watch for telltale periodic dimming and brightening of a star that indicate the presence of an orbiting planet.
Since it began operating just over four years ago the Kepler mission has found hundreds of exoplanets, and has identified thousands of “Objects of Interest” that require further scientific investigation.
Several exoplanets that orbit stable stars in the star’s “habitable zone” have been found, and some of these are small enough that it’s likely they have liquid water oceans covering their surfaces. A prime example of this type of discovery is a world now known as “Kepler 62f,” a world in the habitable zone of its central star that’s only a skosh larger than our Earth.
And so now, less than 20 years since the first discovery of an exoplanet, we now see the emerging scientific methods of teasing out of the data on exoplanets information about the atmospheres (if any) of these distant worlds. Astronomers promise us that it’s only a question of how soon, not if, an Earth-like exoplanet will be discovered orbiting a Sun-like star. They are confident that we are not more than few years away from discovering an Earth-like exoplanet whose atmosphere reveals unambiguous evidence of biological activity.
We’ve not only had to add the term “exoplanet” to our vocabulary, but now also “exoplanetology,” “exoatmosphere,” “exoclimate” and the ultimate “exo,” “exobiology.”
If this subject is of interest to you, then you definitely want to plan to attend a special lecture at Clark Planetarium on the night of Thursday, August 15th. Dr. Jason Steffen, an astronomer who studies these “exo” worlds, will fill us in about the latest news in this exciting field.
See you then!
Exploring Distant Worlds with NASA’s Kepler Mission, Presented by Jason Steffen, PhD
Thursday, August 15th, 7:00pm – 8:00pm
NASA’s Kepler mission has revolutionized our understanding of planets and planetary systems. It is NASA’s first mission capable of finding Earth-size planets in or near the habitable zone, which is the range of distance from a star where the surface temperature of an orbiting planet might be suitable for liquid water.
Click here to purchase your tickets online. Tickets will be available for purchase on-site beginning Monday, July 1.