A Brief History Of Gliese 581d and 581g, The Planets That May Not Be

Tuesday, July 8, 2014

A Brief History Of Gliese 581d and 581g, The Planets That May Not Be

A Brief History Of Gliese 581d and 581g, The Planets That May Not Be:

Artists impression of Gliese 581g. Credit: Lynette Cook/NSF

Two potentially habitable planets in the Gliese 581 system are just false signals arising out of starstuff, a new study said. Gliese 581d and 581g are (study authors said) instead indications of the star’s activity and rotation. It’s the latest twist in a long tale about the system as astronomers struggle to understand how many planets could be orbiting the star.

“Our improved detection of the real planets in this system gives us confidence that we are now beginning to sufficiently eliminate Doppler signals from stellar activity to discover new, habitable exoplanets, even when they are hidden beneath stellar noise,” stated Paul Robertson, a postdoctoral fellow at Penn State University, in a press release.

“While it is unfortunate to find that two such promising planets do not exist, we feel that the results of this study will ultimately lead to more Earth-like planets.”

Planets were first announced around the system in 2007 (by a research team led by Geneva’s Stephane Udry) including Gliese 581d. The system has been under heavy scrutiny since a team led by Steven Vogt of the University of Santa Cruz announced Gliese 581g in September 2010. Both 581d and 581g were considered to be in the “habitable” region around the dwarf star they orbited, meaning the spot that’s not too far or close to the star for liquid water to exist.

Potentially habitable exoplanets and exoplanet candidates as of July 3, 2014. Gliese 581d and 581g are crossed off in the catalog. Click for larger version. Credit: PHL @ UPR Arecibo

About two weeks after the discovery, another team led by Geneva University’s Francesco Pepe said it could not find indications of Gliese 581g in data from HARPS (High Accuracy Radial Velocity Planet Searcher), a telescope instrument frequently used at the European Southern Observatory to confirm exoplanets. It also cast doubt on the existence of Gliese 581f, announced by a team led by Geneva’s Michel Mayor in 2009. Other researchers examined the system, too, with mixed results.

Two years later, Vogt led another research team saying that analysis of an “extended dataset” from HARPS did show Gliese 581g. But in a press release at the time from the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo, its director (Abel Mendez) said the discovery would continue to be controversial. At the time he added the planet to the list of potentially habitable exoplanets the laboratory maintains. As of yesterday, both 581d and 581g are crossed off.

The uncertainty arises from the delicacy of looking for signals of small planets around much larger stars. Astronomers typically find planets through watching them pass across the face of a star, or measuring the tug that they exert on their parent star during their orbit. It is the nature of the tug on Gliese 581 that is so interesting astronomers.

The orbits of planets in the Gliese 581 system are compared to those of our own solar system. The Gliese 581 star has about 30 percent the mass of our Sun, and the outermost planet is closer to its star than the Earth is to the Sun. The 4th planet, G, is a planet that could sustain life. Credit: Zina Deretsky, National Science Foundation

“These ‘Doppler shifts’ can result from subtle changes in the star’s velocity caused by the gravitational tugs of orbiting planets,” wrote Penn State in the press release yesterday. “But Doppler shifts of a star’s ‘absorption lines’ also can result from magnetic events like sunspots originating within the star itself — giving false clues of a planet that does not actually exist.”

The researchers now say that only three planets exist around this star. It’s impossible to fully represent the debate in a single short news article, so we encourage you to look at some of the original literature. Here is a list of papers related to Gliese 581g and another for Gliese 581d. The new paper is available online in Science.

Also, here are some past Universe Today stories about the system:

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The National Aeronautics and Space Administration (NASA) is the United States government agency responsible for the civilian space program as well as aeronautics and aerospace research. President Dwight D. Eisenhower established the National Aeronautics and Space Administration (NASA) in 1958 with a distinctly civilian (rather than military) orientation encouraging peaceful applications in space science. The National Aeronautics and Space Act was passed on July 29, 1958, disestablishing NASA's predecessor, the National Advisory Committee for Aeronautics (NACA). The new agency became operational on October 1, 1958.

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The Oxford English Dictionary defines a UFO as "An unidentified flying object; a 'flying saucer'." The first published book to use the word was authored by Donald E. Keyhoe. The acronym "UFO" was coined by Captain Edward J. Ruppelt, who headed Project Blue Book, then the USAF's official investigation of UFOs. He wrote, "Obviously the term 'flying saucer' is misleading when applied to objects of every conceivable shape and performance. For this reason the military prefers the more general, if less colorful, name: unidentified flying objects. UFO (pronounced Yoo-foe) for short." Other phrases that were used officially and that predate the UFO acronym include "flying flapjack", "flying disc", "unexplained flying discs", "unidentifiable object", and "flying saucer". The phrase "flying saucer" had gained widespread attention after the summer of 1947. On June 24, a civilian pilot named Kenneth Arnold reported seeing nine objects flying in formation near Mount Rainier. Arnold timed the sighting and estimated the speed of discs to be over 1,200 mph (1,931 km/h). At the time, he described the objects' shape as being somewhat disc-like or saucer-like, leading to newspaper accounts of "flying saucers" and "flying discs". In popular usage the term UFO came to be used to refer to claims of alien spacecraft. and because of the public and media ridicule associated with the topic, some investigators prefer to use such terms as unidentified aerial phenomenon (or UAP) or anomalous phenomena, as in the title of the National Aviation Reporting Center on Anomalous Phenomena (NARCAP).

UFO PHOTO PHOTOGRAPHY

UFO - Unidentified Flying Object An unidentified flying object, or UFO, in its most general definition, is any apparent anomaly in the sky that is not identifiable as a known object or phenomenon. Culturally, UFOs are associated with claims of visitation by extraterrestrial life or government-related conspiracy theories, and have become popular subjects in fiction. While UFOs are often later identified, sometimes identification may not be possible owing to the usually low quality of evidence related to UFO sightings (generally anecdotal evidence and eyewitness accounts). Stories of fantastical celestial apparitions have been told since antiquity, but the term "UFO" (or "UFOB") was officially created in 1953 by the United States Air Force (USAF) to serve as a catch-all for all such reports. In its initial definition, the USAF stated that a "UFOB" was "any airborne object which by performance, aerodynamic characteristics, or unusual features, does not conform to any presently known aircraft or missile type, or which cannot be positively identified as a familiar object." Accordingly, the term was initially restricted to that fraction of cases which remained unidentified after investigation, as the USAF was interested in potential national security reasons and/or "technical aspects" (see Air Force Regulation 200-2). During the late 1940s and through the 1950s, UFOs were often referred to popularly as "flying saucers" or "flying discs". The term UFO became more widespread during the 1950s, at first in technical literature, but later in popular use. UFOs garnered considerable interest during the Cold War, an era associated with a heightened concern for national security. Various studies have concluded that the phenomenon does not represent a threat to national security nor does it contain anything worthy of scientific pursuit (e.g., 1951 Flying Saucer Working Party, 1953 CIA Robertson Panel, USAF Project Blue Book, Condon Committee).

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Photos of Nature | Nature Photography What is The Universe

Tuesday, July 8, 2014