This artist's conception illustrates Kepler-22b, a planet known to comfortably circle in the habitable zone of a sun-like star. Image credit: NASA/Ames/JPL-Caltech
The search for extra-solar planets that the Kepler mission is doing just keeps cranking out discoveries. The latest one is special: the first planet that is close to the size of Earth that orbits its star in the so-called “Goldilocks zone”. Essentially, that’s the region around almost any star where liquid water could exist on the surface of a planet that happens to be orbiting ‘in the zone’.
The planet is called Kepler-22b, and it is about 600 light-years away from us. While the planet is larger than Earth, its orbit of 290 days around a Sun-like star resembles that of our world. The planet’s host star belongs to the same class as our Sun—a G-type star which is actually slightly smaller and cooler.
Now, this discovery of a planet in the right place around its star is interesting because—as we all know—water is one of the three requirements necessary for life to exist: water, warmth, and organic material (food). So, finding a planet in the sweet spot is a big first step in locating life on other worlds.
It does NOT mean that Kepler has found life. It just means it has found a planet in the right place to support conditions that might allow life. That sounds hand-wavy, but this is the way discoveries work. You have to figure out if the environment is right for life, and then go about trying to understand that environment. Now, we have to study the planet further to see if water exists there. It could be done by watching as the planet orbits between us and the star, and studying the star’s light as it passes through the planet’s atmosphere. That is a technique called spectroscopy, and it means that astronomers detect the light, let it pass through a “super-prism” that breaks up the light into its component wavelengths, and then study the data to see if it indicates that water is present. It would most likely be the presence of water vapor. The amount you find, along with some other characteristics, tell you about the amount of water in the system. So, Kepler’s discovery is a big first step.
Kepler is an interesting observatory. It doesn’t take pictures. It’s mainly interested in something called “light variation”. That is, it discovers planets and planet candidates by measuring dips in the brightness of more than 150,000 stars. If a it sees a periodic dip in the light intensity coming from a star, then there’s a very good possibility that a planet is crossing in front of the star (from our point of view), or “transiting” it. Kepler requires at least three transits to verify a signal as a planet. (And, by signal, we mean “a dip in the light intensity”.)
Once these candidate planets are announced, then a series of ground-based telescopes and the infrared-sensitive Spitzer Space Telescope look at them and provide data that helps astronomers verify that these things are planets. It’s a long-term task and one that’s keeping astronomers busy. Kepler finds many candidate planets, and each one needs to be meticulously checked out. Kepler has found 2,326 planet candidates. Of these, 207 are approximately Earth-size, 680 are super Earth-size, 1,181 are Neptune-size, 203 are Jupiter-size and 55 are larger than Jupiter. Today’s announced discovery is one of only 48 stars (of the many thousands that Kepler has studied) that have planets in their habitable zones. It is the first planet in a habitable zone that is near-Earth-sized, and that’s exciting. I hope it also means that there are many more of them out there, just waiting to be detected! Stay tuned.
This radar image of asteroid 2005 YU55 was obtained on Nov. 7, 2011, at 11:45 a.m. PST (2:45 p.m. EST/1945 UTC), when the space rock was at 3.6 lunar distances, which is about 860,000 miles, or 1.38 million kilometers, from Earth. Credit: NASA/JPL-Caltech (Click to enlarge.)
A couple of weeks ago many people were startled to learn that a space rock—an asteroid called 2005 YU55—was about to pass just inside of our Moon’s orbit. This tumbling piece of debris is big enough that a decent-sized ocean liner could fit inside it, and its 1.22-year orbit occasionally brings it close to Earth. This time, we were in no danger of an impact from it during the November 8th flyby.
Scientists took the opportunity to study the asteroid in great detail. The radio astronomy community was all over it. The Arecibo radio telescope, the Very Long Baseline Array, the Green Bank Telescope, and the Goldstone telescopes all focused on 2005 YU55. The Herschel Space Telescope also looked at the asteroid in far-infrared light, which helps us understand the temperature of the asteroid and what it’s made of.
In particular, astronomers used the Goldstone Deep Space Antenna to bounce radar signals off the asteroid and then examine the data to see what this baby looked like. The movie below shows a series of the highest resolution radar “images” ever taken of a near-Earth object. The movie consists of six frames made from 20 minutes of radar data, and is a work in progress. Word is there will be another, more detailed movie released here after astronomers get through analyzing all the data—perhaps in a week or two.
2005 YU55 rotates on its axis once every 18 hours, so what you see below is five repetitions of the same loop, and the loop shows the rotation faster than in real time.
What About NEOs?
So, I’ve had people ask me what NEOs mean. The close passage of this one raised concerns again about what we would do if such a rock were headed straight toward our planet. Obviously if it had hit Earth, 2005 YU55 would have dug out a crater about six kilometers across (nearly four miles) if it had impacted on solid ground. The consequences could have been pretty severe. Of course, the asteroid didn’t hit, for which we all breathed a sigh of relief.
But, that’s not to say that Earth is safe from a collision with one of these orbiting space rocks. It turns out the solar system is peppered with them, and in particular, the region we inhabit (the inner solar system) has a good-sized population of these rocks. They’ve BEEN around since the earliest history of the solar system. In fact, populations of such objects were spread out across much of the proto-solar nebula. They were the precursor “worldlets” that combined and collided to form the larger bodies such as Earth, the Moon, and so on. What we have now are the ones that didn’t participate in that early solar system tango to create worlds. They still zip around in their own orbits, and occasionally get close enough to another world (like Earth) to pose a collision threat.
There are communities of scientists who track these objects (once they’re discovered) and do a good job of assessing the chances of impact, near misses, and close encounters. You can read their work at the Web page for NASA’s Near Earth Object Program , the Minor Planets Center , and at the European Space Agency’s NEO’s pages here and here.
There are a number of search programs called asteroid surveys that constantly watch the sky and catalog just about everything that moves. They are scattered around the world, and you can see a list of the major ones here. These surveys aim to find as many NEOs as possible, down to the limits of what they can see. Planned future surveys will need to use ever-more sensitive detectors to find smaller and dimmer objects with orbits intersecting Earth’s.
So, what can we learn about these NEOs as they whiz by? The radar imaging you saw in the movie here tells scientists something about the surface characteristics of an object. That is, is it cratered, does it have other surface features like hills or outcrops? What is its shape? Sometimes they can figure out what its surface is made of—that is, the minerals that make up a rocky asteroid, for example. And, by sussing out the composition and “look and feel” of these asteroids, we learn more about the raw materials that made up Earth and other worlds. We find out what conditions were like in various parts of the solar system during the early days when these types of objects were forming, colliding, and contributing themselves to build larger worlds. So, in a sense, these asteroids are historical treasure troves that give us a look at the early history of the solar system. In another sense, the ongoing discovery of NEOs also tells us about their distribution—that is, how many of them there are and WHERE their orbits are in the inner solar system.
NEOs have always been there, folks. As I mentioned above, the solar system was born with an inventory of these guys, and over time they collide with planets and Sun. The inner solar system’s collection of NEOs is constantly being replaced by asteroids that migrate from the main Asteroid Belt, or from objects that are bumped from their orbits out near Jupiter and Saturn and sent inward toward the Sun.
Currently we’ve discovered most of the larger ones. In recent decades, we’ve developed much better detectors to find the smaller near-Earth objects (the size of city blocks, for example). Most are so small and so dim (their surfaces can be as dark as charcoal, which makes them hard to spot, particularly when they’re little guys).
Once a NEO is discovered, scientists have to make many observations of it to pin down its orbit very accurately. This is like watching a plane land: the more observations you have of that plane, the more accurately you can figure out its path to its landing site. In the days after a NEO discovery, scientists are very careful to point out that their calculations of the object’s orbit and trajectory are preliminary AND that the orbital parameters will change as more observations come in. This is completely normal and nothing to worry about. Yet, I often see people, particularly in the media or as part of the conspiracy theory crowd ignoring that fact and getting all upset because they think scientists are hiding information or aren’t telling the truth.
The truth is that calculating orbits, particularly when you want to figure out whether or not something will impact us, requires observations over a long period of time, and those observations should be very precise. It’s not an overnight job— it’s like any other quality work—it reflects the amount of time and effort put into it. We pay our scientists well to do their jobs, and so it’s only fair to LET them DO their jobs without having people screech about it.
I’ve also seen a lot of nonsense on the Web about how NEOs can change our magnetic fields or shift our polar axes or how they are being hidden by NASA/ESA/whoever. Such speculations are the work of people who either don’t know much about the reality of NEOs (or about the laws of physics for that matter) or don’t care to know because they can get more attention by making stuff up and then posting their “fantasies” on the Web. That’s the politest way I can term such nonsense. There’s good, solid science behind the discovery and characterization of NEOs, and I wish people would pay more attention to THAT. The universe is always much more fascinating and wondrous than our imaginations can dream up.
So, to sum up: NEOs are fascinating rocks from space. Sure, they can pose a threat, and we should be looking for ways to mitigate that threat. But, in the larger sense, NEOs hand us a unique chance to learn more about our neck of the woods, by giving us a look at what was once the undiscovered country of small bodies of the solar system.
(Special thanks to Dr. Paul Chodas at NASA/JPL for his insights on these NEOs. If you want to read more commentary about NEOs, check out David Ropeik’s discussion of impact risks here , and Alan Boyle’s comments on CosmicLog at MSNBC. Both of their blog entries were written after a workshop about communicating risks of NEO impacts, sponsored by the Secure World Foundation that I and a number of other scientists and writers attended this past week.)
