Once again, headlines are proclaiming the discovery of an "Earth-like" planet orbiting another star. Once again, closer inspection of the reports indicates that this new-found planet, based on the very little we know about it, is very unlike Earth. Didn't this just happen a few months ago?
- The New York Times: Astronomers Briefly Glimpse an Earthlike Planet (since changed to Search Finds Far-Off Planet Akin to Earth, which is a slight improvement, I guess)
- Nature (yes, the prestigious science journal!): Found: one Earth-like planet
- MSNBC: Scientists spot a new Earthlike planet
- Most dramatic of all, ABC Science Online (Australia): Earth's twin found at heart of Milky Way
So what do we really know?
This planet was found via a system of telescopes and instruments, set up over the last fifteen years or so, that monitor millions of stars regularly, looking for variations in their brightness. The particular system used here is called OGLE, for Optical Gravitational Lensing Experiment.
As predicted by Einstein's General Theory of Relativity, massive objects can act as lenses, bending and focusing light that passes near them. As a result of this gravitational lensing, if a too-dim-to-see object passes in front of a brighter, more distant star, the light of the background star brightens gradually before dimming back to normal, revealing the existence of the otherwise unseen foreground object. The amount of brightening and how quickly it happens (typically weeks to months) depend on the mass of the foreground object and how close it comes to directly passing in front of the background star. As a result, astronomers can use their measurements to estimate the mass of the unseen objecttypically a star.
Stars are small (relatively!) and the galaxy is very large, so these lensing events are not common, necessitating monitoring huge numbers of stars to catch a decent sample of events.
It turns out that during one of these events seen last summer, as the background star's light was gradually returning back to normal as the invisible foreground star passed by, a secondary brief blip in brightness occurred. This anomaly only lasted a few hours, and thus required monitoring from another telescope network, PLANET (Probing Lensing Anomalies Network), specially organized to respond quickly to track brief events. It is this secondary event that is the evidence for the new planet. The blip is consistent with a gravitational "micro-lensing" event from an object with a mass roughly five or six times that of Earth. (That's the actual data in the figure at right.)
From the analysis presented by the team of astronomers, the distance to this stellar system is known to within 30% or so. It is close to the center of our galaxy, twenty-thousand light-years away. This is much, much futher away than the vast majority of other extra-solar planets discovered in the last ten years by other techniques. Those are around stars in our own local neighborhood of the galaxy, within a few tens of light-years.
If we compare the distance of the typical known extra-solar planet, on the order of 100 light-years, to the distance between downtown San Antonio and Kerrville (60 miles), then the distance of this newly-discovered planet is half-way around the world. That's a long way off!
The only reason we can know anything about this planet is due to that freak lensing event as it passed virtually directly between us and a background star. Since that is not going to happen again for this object, we won't be able to learn anything more about it. At least not until we invent radically new astronomical instruments and techniques well beyond the capabilities of anything currently imagined.
The information the astronomers can get from this one-time event is extremely limited. The mass of the planet is only known to within a factor of fourit could be anywhere from 2.5 to 10 times the mass of Earth (and that is just the one-sigma confidence interval, for those familiar with such terms).
If it's on the bottom end of that range, its mass, if nothing else, could be described as "Earth-like."
But the star it circles is much smaller than our Sun, as revealed by its lensing effect on the background star. And, based on the delay between the peak of the central star's lensing event and the planet's lensing event, it is much further out from its sun than we are from ours. If we know anything about this planet, it is that it must be extremely cold. That should conclusively rule-out any notion that it is like Earth.
And just as easily as the planet could be only a few times Earth's mass, it may be 10 times Earth's mass. That and its frozen temperature would make it much more Uranus-like than Earth-like.
Unfortunately, we're not going to find out anything more.
The benefit of studying lensing events is not learning about individual systems, but getting a relatively unbiased sample of what kinds of objects are out there floating around in the galaxy, occasionally passing in front of background stars. As yet, lensing events have revealed only three planets, including this new discovery, so any statistical information that can be derived is extremely limited.
Of course, that's not stopping some people. Scientists love to speculate.
As Dr. Beaulieu explained in an e-mail message, it would have been much easier to see a giant gaseous type of planet. The long odds against detecting so small a planet as the new one argues for its commonness.
"If only a small fraction of the stars had such planets, we would have never detected this small planet," Dr. Beaulieu said.
Scott Tremaine, a theorist at Princeton, said, "The results suggest that rock-ice planets must be more common than gas giants."
Eventually, thanks to the more traditional techniques used to find planets in our neighborhood of the galaxy, we'll find a true Earth-like planet. One close enough to study in much more depth. I look forward to those headlines, perhaps a decade or so away.