Posted by “Life is not a miracle. It is a natural phenomenon, and can be expected to appear whenever there is a planet whose conditions duplicate those of the Earth.”
-Harold Urey
One of the most exciting investigations going on right now in space is NASA’s Kepler Mission, which is on the hunt for planets beyond our Solar System!
Image credit: Dana Berry / NASA / Kepler Mission.
From high above the Earth’s atmosphere in outer space, Kepler points at a small region of our sky, sensitive to a remarkable 150,000 stars within our galaxy!
Image credit: NASA / Kepler, retrieved from the Astronomical Society of the Pacific.
Kepler has been observing these stars for years, now, monitoring the light coming from them. Stars — much like our Sun — are incredibly stable objects, with only the most minuscule of natural variations in their intrinsic brightness. But these stars do not simply exist in isolation; much like our own Solar System, a great many of these stars are suspected to have planets as well.
And with hundreds of thousands of stars at its disposal, it was suspected that over a thousand of them would have their planets oriented in such a way that, over time, from our point-of-view, these planets would transit in front of their star, temporarily blocking a small portion of their light.
Image credit: NASA / Tim Pyle, retrieved from GizMag.
And based on this, Kepler has not only found over a thousand candidate planets, many of which are comparable in size to Earth, they’ve found a great many star systems with multiple planets orbiting them, as the video below shows!
But you know how we are; we’re not satisfied with finding thousands of new planets, or even with finding hundreds of new complex Solar Systems.
We want to find planets similar to ours, with the capability of housing not only life, but life-as-we-know-it!
Image credit: Kepler mission / NASA, retrieved from cheller.phy.georgiasouthern.edu.
As far as we’re concerned, that means having rocky, Earth-like planets orbiting at the proper distance from their star to have liquid water flowing freely on their surface! In other words, if we plunked our planet down where this newly discovered world is found to orbit its star, would we still be able to sustain life-as-we-know-it?
The entire region around a star for which the answer, hypothetically, is yes defines what we call the habitable zone, which is (of course) different for every star!
Image credit: ESO / M. Kornmesser.
We’ve found planets before that appear to be in their star’s habitable zone, like Gliese 370 b, a rocky world which orbits a slightly cooler, orange star closer than the Earth-Sun distance, placing it right within the Goldilocks zone for habitability.
But we’ve never found a planet that could be considered the Earth’s twin, or even Earth’s cousin, orbiting a Sun-like star at roughly the same distance we orbit ours. That is, until now.
Image credit: NASA / Ames / JPL-Caltech.
According to the Kepler mission and NASA themselves, they’ve found exactly that: a rocky planet orbiting a star nearly indistinguishable from our Sun, at practically the same distance that we orbit our home star!
To no one’s surprise, this planet — Kepler-22b — is presently all the rage when it comes to exoplanet news. The search for extra-terrestrial intelligence, SETI, has started up again, looking for signs of intelligent life in the cosmos, and is starting here.
But what can we really say about this planet? Forget about intelligent life, how Earth-like is this world, really?
Image credit: NASA / Ames / JPL-Caltech.
This image, plastered everywhere on the internet, is simply an artist’s rendition! And yet, Kepler is incapable of measuring whether these features — oceans, clouds, a mostly transparent atmosphere — actually exist on this world. All it can truly tell, at this point, is what the radius and orbital distance of this planet is.
So how would we find out what the details of this planet are?
Image credit: Planets wallpaper, retrieved from hq-wallpapers4.us.
After all, there’s a huge variety among the known, rocky planets even within our Solar System! Does this planet rotate rapidly, like Earth (once a day), or much more slowly, like Mercury (59 days) or even Venus (just once every 243 days, and backwards at that)? Is there a substantial atmosphere like we have, a rarefied one (like Mars), a very hot, thick one (like Venus), or, like Mercury, none at all? And if it does have an atmosphere, what gases compose it?
Believe it or not, if we want to find these answers, all we need is to image this planet with a telescope, even if we can only get it to appear on camera as one pixel. This is totally doable; it’s something we’ve even done before! All you’d need to do is place a coronagraph — or an opaque piece of equipment to block the light of this planet’s parent star — in front of your telescope, and you’ll be able to image the planet itself!
Say hello to Fomalhaut b, an exoplanet orbiting the bright star Fomalhaut, 25 light years away.
Image credit: NASA, ESA, P. Kalas, J. Graham, E. Chiang, E. Kite, M. Clampin, M. Fitzgerald, K. Stapelfeldt and J. Krist.
Because the brightness of the planet, even if it only takes up one camera pixel, will change over time in an incredibly informative way. Here are some fascinating examples of how:
- If the planet orbited its star tidally locked in a 1:1 resonance (as the Moon is to Earth), we would see the same face of the planet at the same location in its orbit every single time. But if it isn’t locked, we can determine its period of rotation so long as it isn’t a perfectly bland, uniform planet.
- The phase of the planet, or how much of the illuminated hemisphere was visible at any given time, is something we can easily account for and understand. But understanding phase and rotation allows us to look for annual variation in planetary brightness, allowing us to detect atmospheric changes and/or catastrophic volcanic activity!
- If the planet has both oceans and land, then as it rotates, we’ll be able to note periodic fluctuations in the amount of light coming from it, just as the ocean- and land-dominated areas of Earth reflect different amounts of light! This is something we’ll be able to learn, once again, in correspondence with the planet’s rotation.
- If the planet has clouds, like Earth, we should see extra fluctuations in brightness on top of the planetary phase and rotation signals, dependent on cloud cover and reflectivity relative to the planet’s surface! The only things that can kill these ambitions is if the planet either has a very thick, dense, opaque atmosphere, like Venus, or is completely dead, uniform, bald and featureless, like some scrubbed-down version of Mercury. In these cases, the overall brightness of the illuminated hemisphere of the planet will never change. Practically, the only effect on the planet’s brightness will be its phase as it orbits its star.
- But whether that’s true or not for our world-in-question, planets — as we well know — can also have moons! If the planet has one or more sufficiently large moons orbiting it, periodic brightness fluctuations corresponding to the lunar transits in front of and behind their host planets will allow us to find them.
All of that comes from simply measuring the overall brightness of one trackable pixel as it orbits its star. One pixel gives you all that! And that’s not even demanding the best astronomical equipment. If you can attach a spectrograph to your data-taking telescope, you can go a step further, and determine what fraction of the light comes in from each particular wavelength of light!
Image credit: Mike Brown, retrieved from FIRE for Magellan telescopes / MIT.
And with that data — as we’ve done for objects in our own Kuiper Belt — we can determine whether it has an atmosphere, and if so, just what this planet’s atmosphere is made out of! Want to go looking for oxygen, nitrogen, carbon dioxide, or, as we’ve found on Pluto (above), methane? This is how you do it!
That’s all well and good, but Kepler-22b isn’t in our Kuiper Belt. It isn’t even at the distance of Fomalhaut; it’s 600 light-years away, orbiting the star Kepler-22! While this particular system is too far away to make these measurements in the near future, we are about to gain that capability for the nearest stars to us! How’s that?
Image credit: NASA / James Webb Space Telescope team.
The James Webb Space Telescope (JWST) will be able to make these measurements! In addition to all the other things JWST will do, it’s going to be a huge, spaceborne visible-and-infrared telescope outfitted with the right instruments to make these measurements, at least for the most nearby stars.
Want to look at this planet — Kepler-22b — with the level of detail I just described here? Well, remember what it is we’re looking for when, a decade from now, you hear NASA, the NSF, and the European Space Agency talking about a telescope for the next generation. Because we’re looking for no less than an understanding of our world’s place in the cosmos, and just how common or rare we really are. And to find out, the only thing we need are the right tools to allow us to look and see.
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