With our planet looking a little worse for wear, there will undoubtedly come a time when we will seek out other planets to ravage, oops, I mean, settle. But unless humans evolve tough exoskeletons or thick fur, scientists need to be able to find planets that are similar to our own planet. This is exactly what an international team of astrophysicists have been doing, and recently they’ve managed to stumble upon the most Earth-like planet yet discovered. The researchers suggest that such a discovery is a step toward answering the big question of whether or not we are alone in the Universe. With researchers claiming that there are likely thousands of Earth-like planets within observation, there is every chance that life may exist elsewhere in the Universe. But will it be “life as we know it, Jim?”
The new planet, about five times as big as Earth, with the punchy and memorable title of OGLE-2005-BLG-290 Lb (OGLE), and its red dwarf parent star, five times less massive than our sun, lie in the constellation Sagittarius, about 25,000 light years distant in the central bulge of our Milky Way galaxy. OGLE seems to be comprised of rock and ice and orbits at three times the distance from Earth to the sun. Its cool parent star and large orbit implies that the surface temperature is -364 degrees Fahrenheit (-220 degrees Celsius) below zero. Astronomers predict that its rocky surface is probably buried deep beneath frozen oceans.
You might ask whether the big fuss about yet another planet is really warranted, considering that more than 200 planets within the Milky Way have already been found. The important difference is that the majority of planets found so far are gas giants, close to the size of Jupiter or Saturn, and they orbit their parent star at distances much less than the distance from Earth to the sun. What have been missing to date are planets that resemble Earth. The technique used to find OGLE - known as microlensing - means that discovering many more of these Earth-like planets is highly likely.
The gravitational microlensing technique is based on a concept first conceived by Albert Einstein early last century. When astronomers observe a star, the light waves generally come straight from the star to the telescope. However, if another celestial object passes directly in between - even if great distances separate the two - the gravity of the nearer object acts like a lens and magnifies the incoming light. Astronomers can use this distorting effect to determine how large an exoplanet is, and how far away it is from its star. "There's a deviation of light when a planet is in the way," said Kem Cook, an astronomer at the Lawrence Livermore National Laboratory. "In this instance, there was a half-day brightening that was indicative of a planet." Microlensing was believed to be more suited to detecting large gas giants rather than smaller rocky planets, but it seems that even relatively tiny, low mass objects can give a strong peak signal if alignment is perfect.
"The new discovery provides a strong hint that low-mass planets may be much more common than Jupiters," said researcher David Bennett of the University of Notre Dame. "Microlensing should have discovered dozens of Jupiters by now if they were as common as these five-Earth-mass planets. This illustrates the primary strength of the gravitational microlensing method: its ability to find planets of low-mass."
The observations made by Bennett and his team have elicited some startling conclusions from colleagues. Researchers believe that there is a strong likelihood that rocky planets may be even more common than their gas-giant brethren. This prediction would agree with one of the models for solar system formation, core accretion, which suggests that small, rocky, "failed Jupiters" should be far more common than the massive gaseous planets. "The fact that we stumbled on one [Earth-like planet] means there are thousands of them out there," said Cook.
So there may be thousands of them out there, but the $64,000 question is, “how many, if any, of these Earth-like planets can, or do, support life?” It’s a question that is not lost on the planet-hunting scientists. "This is an important breakthrough in the quest to answer the question 'Are we alone?'," said Michael Turner, assistant director for the National Science Foundation (NSF). But what may take some of the gloss off of the current research is that humans probably won’t make it to any of these life-sustaining planets. "The microlensing technique is not going to find nearby planets. We're not going to discover planets to which NASA can fly. Microlensing can tell us how common planets are in distant parts of the galaxy and probe details of planetary formation that other techniques cannot," explained Cook.
Another question to contemplate might be whether scientists would even recognize alien forms of life. Do they actually have a workable definition of life? Well, sort of. Researcher David Catling, jointly affiliated with the University of Washington's Astrobiology Program, is a man who contemplates these sorts of questions all the time. When asked, “what is life?” Catling explained that: “There is no satisfactory scientific definition of life. However, astrobiologists define life as we know it as a self-sustaining chemical system that is capable of undergoing Darwinian evolution. The evolutionary aspect of life is important because an informational system, i.e., some sort of genome, is almost certainly characteristic of life anywhere.”
“At present we can't be more precise about defining ‘life’. Most biology textbooks just list the properties of life - reproduction, metabolism, etc., without really defining what it is. I feel that we're in the same situation as 17th century scientists trying to define "water" before atomic theory nailed it as H2O. In those days, the best they could do was describe the properties of water: wetness, boiling, freezing point, etc.,” added Catling.
The important thing about Catling’s work is that he is not discussing piddling microbes, which he says “can live quite readily without oxygen,” but complex “animal-like” organisms that could live on an Earth-like planet. Catling’s paper on complex life forms, Why O2 Is Required by Complex Life on Habitable Planets and the Concept of Planetary “Oxygenation Time”, concludes that: “In summary, oxygen is the most feasible, most energetic source for driving the metabolism and growth of advanced life. Thus, in our view, an O2-rich atmosphere is very likely a necessary precursor to metazoan-like life on planets elsewhere.”
If they do ever find an Earth-like planet with an oxygen atmosphere, perhaps it’s a good thing that it’ll be out of our reach. We’d probably only make a mess of it. Flights of fancy aside, the international team of researchers who found OGLE are just happy that they have made a significant contribution to science. "The team has discovered the most Earth-like planet yet, and more importantly, has demonstrated the power of a new technique that is sensitive to detecting habitable planets," said the NSF’s Michael Turner.
Check out more exoplanet pictures and animations at the National Science Foundation.
Artist’s impression courtesy European Southern Observatory
