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By Judith E Braffman-Miller
Expert Author Judith E Braffman-Miller
The first exoplanet to be spotted, orbiting a star like our own Sun, was discovered back in 1995. Since that historic finding almost a generation ago, exoplanet discoveries have poured in at a breathtaking pace and–as of this writing–more than 1,000 remote worlds circling distant stars have been confirmed. The search for a habitable planet, like our own world, remains the Holy Grail of dedicated planet-hunters. In November 2013, astronomers from the University of California at Berkeley and the University of Hawaii, Manoa, reported their estimation that one in five Sun-like stars have Earth-size planets with surface temperatures friendly to the evolution of life. Given that about 20 percent of stars are Sun-like, this amounts to several tens of billions of potentially habitable Earth-size exoplanets in our Milky Way Galaxy–alone !
“When you look up at the thousands of stars in the night sky, the nearest Sun-like star with an Earth-size planet in its habitable zone is probably only 12 light-years away and can be seen with the naked eye. That is amazing,” commented Berkeley graduate student Erik Petigura in a November 4, 2013 University of California, Berkeley, Press Release. Mr. Petigura led the analysis of data derived from NASA’s Kepler Space Telescope that arrived at this impressive number of potentially habitable Earth-like worlds. Kepler is now unfortunately crippled, with its four-year mission at an untimely end. Nevertheless, it still managed to provide enough precious data to answer its primary research question: How many of the 200 billion stars in our Galaxy could potentially host habitable planets ?
“It’s been nearly 20 years since the discovery of the first extrasolar planet around a normal star. Since then, we have learned that most stars have planets of some size orbiting them, and that Earth-size planets are relatively common in close-in orbits that are too hot for life,” explained Dr. Andrew Howard in the November 4, 2013 Berkeley Press Release. Dr. Howard, a former Berkeley post-doctoral fellow is now on the faculty of the Institute for Astronomy at the University of Hawaii. “With this result, we’ve come home, in a sense, by showing that planets like our Earth are relatively common throughout the Milky Way Galaxy,” he added .
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Petagura, Howard, and Dr. Geoffrey W. Marcy, who is a Berkeley professor of astronomy, and one of the most successful of planet-hunters, published their analysis in November 2013 in the online early edition of the journal Proceedings of the National Academy of Sciences .
Kepler revealed more than 3,500 candidate exoplanets in its first three years of operation, including both small and large planets, rocky and gaseous planets, and a total of 647 potential planets that appear to be Earth-size .
Also, in November 2013, an independent team of planet-hunters, behind the Kepler Space Telescope, announced that they had bagged another 833 potential exoplanets to consider adding to those already known .
This plethora of planets far exceeds what astronomers dreamed of before NASA launched Kepler in 2009. The telescope, which is in orbit around the Sun, discovers exoplanets by looking for them as they “transit”–that is, pass in front of–the brilliant, fiery faces of their stellar parents. This transit causes a brief dimming of the parent star’s light. “When I first started working with Kepler right before launch, I thought there would be maybe a thousand planets that Kepler would find,” said Dr. Jason Rowe at a November 4, 2013 press conference, held at the Kepler Science Conference in Moffett Field, California. Dr. Rowe is an astronomer at the SETI (Search for Extraterrestrial Intelligence) Institute in Mountain View, California .
“We’re finding that there’s a wide variety of systems out there. If you can imagine it, the Universe probably makes it,” Dr, Rowe added .
The best chance for finding life as we know it, dwelling on distant worlds around other stars, is to search for habitable environments that share the comfortable, life-friendly attributes of our own lovely planet: protective atmospheres, rocky compositions, the right amount of friendly star-shine, and plenty of water in its life-loving liquid phase. Astronomers define the habitable zone around a star as the range of distances, based on temperature, that are most likely to possess such a delectable plate of just right “Goldilocks” offerings. So far, Kepler has spotted 104 potential exoplanets that likely dwell in this “Goldilocks” region around their stars–10 of which are less than double the radius of our own planet. However, in order to know if these distant alien worlds actually do possess what it takes to brew up a batch of living things, follow-up observations from the next-generation of telescopes will be necessary .
Forty Billion Worlds !
The Berkeley team cautioned that Earth-size planets in “Goldilocks” orbits are not necessarily havens for life. This is because, even if such planets twirl around within the habitable zones of their stars, “Some may have thick atmospheres, making it so hot at the surface that DNA-like molecules would not survive. Others may have rocky surfaces that could harbor liquid water suitable for living organisms. We don’t know what range of planet types and their environments are suitable for life,” Dr. Marcy explained in the November 4, 2013 Berkeley Press Release .
Mr. Petagura’s analysis, however, is a giant step forward in reaching the main goal of the Kepler mission: to measure the percentage of Sun-like stars, dwelling in our Milky Way Galaxy, that possess Earth-size planet offspring. Sometimes termed eta Earth, it is an important factor in the Drake equation. The Drake equation is used to estimate the number of intelligent civilizations that might be partying around in our Galaxy .
“For NASA, this discovery is really important, because future missions will try to take an actual picture of a planet, and the size of the telescope they have to build depends on how close the nearest Earth-size planets are. An abundance of planets orbiting nearby stars simplifies such follow-up missions,” Dr. Howard continued to note in the November 4, 2013 Berkeley Press Release .
In October 2013, Dr. Marcy, Dr. Howard, and their colleagues provided hope that many life-friendly, Earth-type exoplanets spotted by Kepler actually are life-friendly, rocky worlds that could host lovely oceans of precious liquid water. They reported that one Earth-size planet–a roaster with a surface temperature of 2,000 Kelvin, and much too hot for life as we know it–is about the same density as our own planet and likely made up of rock and iron, like our own world .
“This gives us some confidence that when we look out into the habitable zone, the planets that Erik (Petagura) is describing may be Earth-size, rocky planets,” Dr. Howard continued to note in the November 4, 2013, Berkeley Press Release .
Many of the thousands of alien worlds that were spotted by Kepler are considerably larger than our own small planet–ranging from planets with dense atmospheres, like Neptune, to gigantic planets with extremely thick gaseous envelopes like Jupiter and Saturn. Some also whirl around in fast, close orbits that hug their parent stars so closely that they literally broil under their fiery stellar heat .
To sort out this abundance of brave new worlds, Mr. Petagura and his team used the two Keck Telescopes in Hawaii to gather spectra of as many stars as possible. This enabled them to calculate each star’s true brightness and determine the diameter of each transiting exoplanet, paying particular attention to those similar to Earth
in size .
Mr. Petagura, Dr. Howard, and Dr. Marcy focused on 42,000 stars that are like our own Sun or only slightly cooler and smaller. They found 603 exoplanet candidates circling them. Only 10 of these were similar to Earth in size–that is, they were one to two times the diameter of our own planet, and they circled their stellar parent at that “Goldilocks” distance where they were warmed to a blissfully comfortable temperature. The team’s definition of what constitutes habitable is a world that is bestowed with between four times and one-quarter the quantity of stellar light that Earth receives from its own Star, the Sun .
What distinguishes Mr. Petagura’s team’s study from previous analyses of Kepler data is that they subjected Mr. Petagura’s planet-hunting algorithms to a large number of tests in order to determine how many Earth-size, habitable zone, exoplanets they overlooked. Mr. Petagura actually introduced false exoplanets into the Kepler data to calculate which ones his software could bag and which it couldn’t .
“What we’re doing is taking a census of extrasolar planets, but we can’t knock on every door. Only after injecting these fake planets and measuring how many we actually found could we really pin down the number of real planets that we missed,” Mr. Petagura noted in the November 4, 2013 Berkeley Press Release .
Accounting for overlooked planets, as well as the fact that only a tiny percentage of exoplanets are oriented so that they transit in front of the glaring face of their parent-star as seen from Earth, enabled the team to calculate that 22 percent of all stars like our own Sun, inhabiting our Milky Way Galaxy, possess Earth-size exoplanets dwelling in their habitable zones .
All of the candidate, potentially habitable planets, found in the team’s survey, circle K class stars, which are slightly smaller and cooler than our Star. However, the astronomers’ analysis reveals that the result for K stars can be extrapolated to G class stars–like our Sun. Had Kepler not suffered its premature, tragic end, it would have gathered sufficient data to directly spot a handful of Earth-size exoplanets dwelling within the habitable zones of G class stars .
In January 2013, the team reported a similar analysis of Kepler data for hot roasting exoplanets that hug their stellar parents. This more recent, more complete analysis reveals that “nature makes about as many planets in hospitable orbits as in close-in orbits,” Dr. Howard said in the November 4, 2013 Berkeley Press Release .
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various newspapers, magazines, and journals. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published