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Longtime readers of these columns will know that I’m a sucker for any story related to astronomy, space or the universe. If you are such a reader and you watched the TV news or read a newspaper last week, you could probably predict the subject of this week’s column. I saw it first on Brian Williams’ NBC evening news program and then in the Star-Ledger. For me it was a story that I did not expect to see in my lifetime, given the fact I’ll be 81 next month. But there it was - astronomers have actually photographed four planets orbiting two stars hundreds of trillions of miles from Earth! Not only that, but in one case they’ve seen three planets, a whole solar system, orbiting one of the stars. Until now, the hundreds of planets detected orbiting stars other than our Sun have been discovered by wobbles in or dimming of the light from the parent stars. Nobody had actually seen a planet.
One would expect that the Hubble Space Telescope would be involved in such a momentous feat, and that’s true for one of the planets. However, the solar system was photographed using the huge ground-based telescopes on the summit of Mauna Kea in Hawaii. Both discoveries have been reported in the November 13 issue of Science. Normally, I would wait to receive my copy of Science before writing about something in that journal. However, I just cannot contain my excitement at such a momentous feat and have visited the Web sites of NASA and of the Keck Observatory for reliable information on the subject.
First, let’s look at the planet designated Fomalhaut b, which orbits the star Fomalhaut in the constellation Piscis Australis, known as the Southern Fish. This star is 25 light-years from Earth and Fomalhaut is 16 times brighter than our Sun. Such a hot star burns out much more quickly than our Sun and will last only a tenth the time before it dies. How did the Hubble team latch on to Fomalhaut as a possible candidate for finding a planet? Back in 1980, astronomers found that Fomalhaut was surrounded by dust or debris. Dust is what’s needed to make clumps of material that eventually grow into planets. The dust around Fomalhaut was detected from infrared data gathered by NASA’s Infrared Astronomy Satellite.
Normally, the light from such a distant star would overwhelm the light from a relatively small planet orbiting the star. To have any chance of seeing a planet the astronomers have to use a coronograph, an instrument that blocks the light from the star to allow the possibility of detecting the feeble light from a planet. If I recall correctly, a coronograph was first used to block the light of the Sun so as to be better able to see the details of the corona surrounding the Sun, essentially simulating a total eclipse of the Sun.
It was 2004 when a coronograph on the Hubble Space Telescope was used to block out the light from Fomalhaut and the first image in visible light of a large dust belt around Fomalhaut was found. The image was sharp enough to show the dust belt, 21.5 billion miles across, had a sharp inner edge and consisted of debris. The debris belt is likened to our solar system’s Kuiper Belt and its range of icy bodies ranging from the recently demoted former planet Pluto to dust particles. To astronomers the sharp inner edge indicated the possibility that a planet was sucking dust and debris toward it through gravitational attraction.
When the dust cloud was imaged in 2004 a few bright spots were noted as possible candidates as planets. Sure enough, in 2006, they found that one bright spot had moved, changing its position relative to the ring of dust, corresponding to an orbit of 872 years around the star. Analysis that took several months to complete indicates the planet to be as much as three times the mass of Jupiter and to lie nearly 11 billion miles from Fomalhaut. If you go on NASA’s Website site you can find pictures of the dust belt and of the planet, which shows up as a couple of pixels on the photos I saw. I was quite impressed how they managed to tease out of all that dust and debris the little bit of light that is the planet.
Because of the expected relatively short life of Fomalhaut, there isn’t likely to be any complex life on any other planets that may be orbiting it. With a total lifetime of only a billion years, even a planet on which life is conceivable is unlikely to have time to evolve advanced life. After all, it’s taken about 4 billion years for us humans and other mammals to come into being.
OK, let’s go to the Big Island of Hawaii and the Keck and Gemini telescopes on top of Mauna Kea. We’ve talked in the past about "adaptive optics", a major advance in astronomy that permits telescopes here on the ground to correct for the distortion of images due to Earth’s atmosphere. I won’t go into detail here but adaptive optics utilizes measurements of the turbulence or roiling of the atmosphere caused by such things as wind currents, temperature differences, etc. The measurements, made every fraction of a second, measure the distortion in an image of a so-called "guide star" in the heavens or perhaps a sort of artificial guide star made by beaming up a laser beam to activate sodium atoms, giving off light. The measurements are fed into a computer that adjusts the mirrors in the telescope to compensate for the distortion. The net effect is that the large telescopes on Mauna Kea can produce images that equal or even surpass those from Hubble in clarity and detail. The three-planet solar system was photographed using adaptive optics.
Unlike the Hubble discovery, if I understand correctly. the Mauna Kea photos of the three-planet solar system were made not in visible light but in the near-infrared. It’s similar I would say to looking at images with the night vision devices used by the military. If you look at the images on the Web site, the three planets are quite visible, orbiting the star HR8799. I suspect that now that the star has become famous it will be given a nickname of some sort. HR8799 is 140 light-years away, over 800 trillion miles. I’m blown away that we can see planets almost a quadrillion miles from our vantage point here on Earth. Hey, I’m having trouble comprehending the talk about trillions of dollars in the current fiscal fiasco.
The three planets are about 7 to 10 times the mass of Jupiter and orbit the star 24,37 and 67 times the distance of our Earth from the Sun. That leaves a lot of space for other smaller planets which, if present, are too small to detect at this time. We’re lucky in that HR8799 is very young, only a hundred million years old. This means the planets are also quite young and still quite hot from their formation; infrared and heat go together and hence they stand out, glowing in the infrared. The HR8799 solar system also has a dust cloud, as we have our Kuiper Belt, and one of the planets is in the dust cloud. Will it, as with Pluto in our Kuiper Belt, be demoted from planetary status sometime in the future?
Reading the article on the Keck Web site, it appears that applying adaptive optics to observations in visible light is more challenging than working in the infrared region of the spectrum. According to Taft Armandroff, Director of the Keck Observatory, the next generation of adaptive optics will allow almost perfect correction for atmospheric turbulence in the infrared and also extend the range into the visible light region as well as open up the field of view, which apparently is limited by current adaptive optics technology.
Finally, I have to acknowledge Bob, an avid reader of these columns and former Bell Labs colleague for an awful pun. At this week’s Old Guard meeting, Bob said he had read last week’s column on crystals and gathered that I enjoyed my work with crystals at Bell Labs. He wondered if at Bell Labs I was having a crystal ball! OK, you’re right, the quality of a pun is inversely proportional to how awful it is. Incidentally, wouldn’t it be great to have a crystal ball that could tell us when this sorry stock market is going to bottom out? We could make a fortune!
Allen F. Bortrum