05/02/2007
Habitable? Possibly
I’m praying for a drought! For the second time in two weeks, torrential rains resulted in an overabundance of water in our basement/rec room. At least last week’s flooding was only about an inch deep and we had already thrown out or removed virtually everything that had been on the floor in our previous episode. Water is also involved, if only by inference, in the biggest story of the past week. Indeed, it could in the future turn out to be the biggest story of the century – the discovery of a planet outside our solar system that shows signs of life. We’re not there yet but the discovery announced last week that made headlines in the media is of the smallest planet outside our solar system in a “habitable zone” as it orbits its star. Our Editor, Brian Trumbore, mentions the planet in his Week in /Review column but I hope to delve a bit deeper into the matter.
I first learned of the discovery of the new planet in an article by Robert Boyd in the April 25 Star-Ledger and on Brian Williams’ NBC nightly news program. While the news accounts of the discovery did a good job of reporting the findings, I wanted to get a better idea of how the researchers came to the conclusion that a possible earth-like planet orbits the star designated as Gliese 581. I also wondered how certain they are of some of the conclusions reported in the media. A visit to the Web site of the European Southern Observatory (ESO) led me to the article, actually a Letter to the Editor submitted to the journal Astronomy and Astrophysics by Stephane Udry and his 10 European co-authors.
The method they used to detect the planet is what we called the “wobble” technique in earlier columns. A planet orbiting a star will, because of the gravitational attraction between the two bodies, push or pull the star very slightly closer to or farther from Earth as it orbits. The result is very slight shifts in the observed frequencies of the light emitted by the star as the planet orbits. It’s akin to the Doppler effect when a moving train goes by blowing its horn. You hear a shift in the sound wave frequencies as the train goes by. The bigger the planet, the bigger the shifts in the frequencies of the light.
The astronomical name for the wobble method is the Radial Velocity technique and the instrument used by the ESO workers is called HARPS (High Accuracy Radial Velocity for Planetary Searcher). HARPS, located in Chile, is an extremely sensitive spectrograph and can measure velocities with an astounding precision of a meter a second, which translates into about 2 miles per hour. In my younger days I could walk around 4 miles an hour. To measure velocities to that precision for objects 20 light- years (over a hundred trillion miles) away is astounding.
Two years ago, the ESO workers found a planet some 15 times more massive than Earth orbiting Gliese 581. This speedster whips completely around the star in only 5.4 days and is roughly the size of Neptune. The ESO data at the time also suggested the presence of another planet and further measurements have confirmed the existence of this planet. The planet has a mass that may be as small as only 5 times Earth’s mass and a diameter about twice that of our planet. The planet, which the European researchers call “super-Earth”, is farther away from Gliese 581 and zips all the way around Gliese 581 in 13 days. But there’s more. The researchers’ measurements indicated a third planet orbiting the star. This planet is bigger than super-Earth but not by that much, being 8 times more massive than Earth. It’s farther away from the star than the other two planets and orbits Gliese 581 in about 84 days.
Let’s concentrate on super-Earth. It’s 14 times closer to its star than we are to our star, the Sun. Being so close to its sun, you might well think that super-Earth would be awfully hot and could not harbor life as we know it. But let’s take a closer look at Gliese 581, one of a hundred closest stars to our Sun. Of these hundred closest stars, 80 of them are “red dwarfs”. Red dwarfs are smaller than our Sun and are at least 50 times fainter than our own star. Gliese 581 is a red dwarf, about a third the mass of our Sun. Not being as bright as the Sun, you can snuggle up much closer to 581 and still be in that magical “habitable zone” – a region where the temperatures are such that liquid water can exist. The astronomers calculate that super-Earth is at the “warm” edge of the habitable zone and that the other smaller planet is at the “cold” edge of the zone.
The researchers estimate that the mean temperature on super- Earth lies somewhere in the range from 0 to 40 degrees Celsius (32 to 104 degrees Fahrenheit), temperatures we experience here in New Jersey. Their models also suggest that super-Earth is a rocky planet or one covered with oceans. All these conclusions are based on measurements that detect velocity differences of only in the neighborhood of 5 miles per hour, a brisk walk. The extraordinary precision of these HARPS measurements account for the fact that, out of 13 known planets with masses less than 20 Earth masses outside our solar system, 11 were discovered by workers using HARPS.
There are some caveats that should be noted. The masses calculated from the velocity measurements are minimum masses. Why? We don’t know the angles of the orbits of the three planets as viewed from Earth. If a planet passes directly between us and the center of the star, it will pull the star towards us more strongly than if, in the worst case, it circles the star perpendicular to us. (In that case, it wouldn’t pull the star towards us at all, or extremely slightly at best.) Since the HARPS measurements detect only the small changes in velocity as the planets orbit the star, exact calculations of masses are possible only if we know the angles of the orbits to us. Hopefully, future telescopes will be powerful enough to actually see the planets orbiting. I’m guessing that the fact that the ESO workers detect the shifts in frequencies that they do gives them some confidence that the orbits around Gliese 581 are more in line with us than perpendicular to us.
The possibility that the third planet, the one near the cold edge of the habitable zone, is an artifact is considered in the Letter to the Editor. Apparently, there’s the possibility that dark spots on the surface of the rotating Gliese 581, similar I imagine to the sunspots on our Sun, could lead to erroneous conclusions from the velocity measurements. However, the authors point out that Gliese 581 is a very stable star and no large spots have been found. They do say, however, that the long-term stability of the star will have to be checked further.
The authors also caution that their estimates of temperatures on super-Earth depend on the planet’s composition and the composition and thickness of its atmosphere. Obviously, before we can celebrate even the possibility of life on super-Earth, there’s need for much more attention to super-Earth in future ground- or space-based projects. Such projects will certainly be aimed at studying the planet’s atmosphere for signs of CO2 or other compounds signifying the possibility that the “habitable” zone is truly inhabited by some form of life.
Finally, I was touched and delighted to see the videos on TV of 65-year-old Stephen Hawking floating in a plane engaged in maneuvers simulating the weightlessness of space travel. Hawking thinks that man will have to eventually leave Earth for another habitat in order to survive and it seems that he wanted to experience what it would feel like to be on that journey. Maybe someday a colony of intrepid space travelers will head off to super-Earth!
Allen F. Bortrum
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