“Red dwarfs the dim bulbs of the cosmos have received scant attention by SETI scientists in the past,” said Jon Richard, of SETI, a private, non-profit organization which stands for Search for Extraterrestrial Intelligence. “That's because researchers made the seemingly reasonable assumption that other intelligent species would be on planets orbiting stars similar to the sun.”

Yesterday's announcement that astronomers using European Southern Observatory (ESO) telescopes in Chile shown above and other facilities have found clear evidence of a planet orbiting the closest star to Earth, Proxima Centauri may ultimately prove to be a habitable planet could that harbor an advanced technological civilization. The long-sought world, designated Proxima b, orbits its cool red -dwarf parent star every 11 days and has a temperature suitable for liquid water to exist on its surface. This rocky world is a little more massive than the Earth and is the closest exoplanet to us -- and it may also be the closest possible abode for life outside the Solar System.

Red Dwarfs “may be one instance in which older is better,” said Seth Shostak, senior astronomer and director of California-based SETI. “Older solar systems have had more time to produce intelligent species.” A super-Earth known as Kapteyn b that orbits an 11.5 billion-year-old red dwarf, for example, makes the star and the planet 2.5 times older than Earth.

The SETI Institute belives that planetary systems orbiting red dwarfs — dim, long-lived stars that are on average billions of years older than our sun — are worth investigating for signs of advanced extraterrestrial life. The star that's closest to our sun, Proxima Centauri, is a red dwarf. A variety of observing efforts, including Cornell's Pale Red Dot initiative, are looking for habitable planets around Proxima Centauri .

The two-year project involves picking from a list of about 70,000 red dwarfs and scanning 20,000 of the nearest ones, along with the cosmic bodies that circle them using the SETI Institute's Allen Telescope Array in the High Sierras of northern California, a group of 42 antennas that can observe three stars simultaneously.

“We'll scrutinize targeted systems over several frequency bands between 1 and 10 GHz,” said SETI scientist Gerry Harp. “Roughly half of those bands will be at so-called ‘magic frequencies' — places on the radio dial that are directly related to basic mathematical constants. It's reasonable to speculate that extraterrestrials trying to attract attention might generate signals at such special frequencies.”

For a long time, scientists ruled out searching around red dwarfs because habitable zones around the stars are small, and planets orbiting them would be so close that one side would be constantly facing the star, making one side of the planet very hot and the other quite cold and dark. But more recently, scientists have learned that heat could be transported from the light side of the planet to the darker side, and that much of the surface could be amenable to life.

“In addition, exoplanet data have suggested that somewhere between one sixth and one half of red dwarf stars have planets in their habitable zones, a percentage comparable to, and possibly greater than, for Sun-like stars,” said the statement.
The brightest of Red Dwarfs are a tenth as luminous as the sun, and some are just 0.01 percent as bright, but account for three-quarters of all stars, with 6 percent or more of all red dwarfs having potentially habitable, Earth-sized planets.

Guillem Anglada-Escudé from Queen Mary University of London who led the Pale Red Dot project explains the background to this unique search: "The first hints of a possible planet were spotted back in 2013, but the detection was not convincing. Since then we have worked hard to get further observations off the ground with help from ESO and others. The recent Pale Red Dot campaign has been about two years in the planning."

During the first half of 2016 Proxima Centauri was regularly observed with the HARPS spectrograph on the ESO 3.6-metre telescope at La Silla in Chile and simultaneously monitored by other telescopes around the world. This was the Pale Red Dot campaign that was looking for the tiny back and forth wobble of the star that would be caused by the gravitational pull of a possible orbiting planet.

The Pale Red Dot data, when combined with earlier observations made at ESO observatories and elsewhere, revealed the clear signal of a truly exciting result. At times Proxima Centauri is approaching Earth at about 5 kilometers per hour -- normal human walking pace -- and at times receding at the same speed. This regular pattern of changing radial velocities repeats with a period of 11.2 days. Careful analysis of the resulting tiny Doppler shifts showed that they indicated the presence of a planet with a mass at least 1.3 times that of the Earth, orbiting about 7 million kilometers from Proxima Centauri -- only 5% of the Earth-Sun distance.

Guillem Anglada-Escudé comments on the excitement of the last few months: "I kept checking the consistency of the signal every single day during the 60 nights of the Pale Red Dot campaign. The first 10 were promising, the first 20 were consistent with expectations, and at 30 days the result was pretty much definitive, so we started drafting the paper!"

Red dwarfs like Proxima Centauri are active stars and can vary in ways that would mimic the presence of a planet. To exclude this possibility the team also monitored the changing brightness of the star very carefully during the campaign using the ASH2 telescope at the San Pedro de Atacama Celestial Explorations Observatory in Chile and the Las Cumbres Observatory telescope network. Radial velocity data taken when the star was flaring were excluded from the final analysis.

Although Proxima b orbits much closer to its star than Mercury does to the Sun in the Solar System, the star itself is far fainter than the Sun. As a result Proxima b lies well within the habitable zone around the star and has an estimated surface temperature that would allow the presence of liquid water. Despite the temperate orbit of Proxima b, the conditions on the surface may be strongly affected by the ultraviolet and X-ray flares from the star -- far more intense than the Earth experiences from the Sun [4].

Two separate papers discuss the habitability of Proxima b and its climate. They find that the existence of liquid water on the planet today cannot be ruled out and, in such case, it may be present over the surface of the planet only in the sunniest regions, either in an area in the hemisphere of the planet facing the star (synchronous rotation) or in a tropical belt (3:2 resonance rotation). Proxima b's rotation, the strong radiation from its star and the formation history of the planet makes its climate quite different from that of the Earth, and it is unlikely that Proxima b has seasons.

This discovery will be the beginning of extensive further observations, both with current instruments and with the next generation of giant telescopes such as the European Extremely Large Telescope (E-ELT). Proxima b will be a prime target for the hunt for evidence of life elsewhere in the Universe. Indeed, the Alpha Centauri system is also the target of humankind's first attempt to travel to another star system, the StarShot project.

Guillem Anglada-Escudé concludes: "Many exoplanets have been found and many more will be found, but searching for the closest potential Earth-analogue and succeeding has been the experience of a lifetime for all of us. Many people's stories and efforts have converged on this discovery. The result is also a tribute to all of them. The search for life on Proxima b comes next..."

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The Daily Galaxy via ESO, SETI Institute, and Red Dot Initiative

Image credits:  ESO and Red Dot Initiative

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"The echoing of FRBs is a very direct probe of dark matter," Munoz said. "While gravitational waves might 'indicate' that dark matter is made of black holes, there are other ways to produce very-massive black holes with regular astrophysics, so it would be hard to convince oneself that we are detecting dark matter. However, gravitational lensing of fast radio bursts has a very unique signature, with no other astrophysical phenomenon that could reproduce it."

In a paper published online by the journal Physical Review Letters the team of astrophysicists argues that these extremely bright and brief flashes of radio-frequency radiation can provide clues about whether a particular kind of ancient black hole is what makes up dark matter.

Julian Munoz, a Johns Hopkins graduate student and the paper's lead author, said fast radio bursts, or FRBs, provide a direct and specific way of detecting black holes of a specific mass, which are the suspect dark matter.

Munoz wrote the paper along with Ely D. Kovetz a post-doctoral fellow, Marc Kamionkowski, the William R. Kenan Jr. Professor of Physics and Astronomy, and Liang Dai, who completed his doctorate in astrophysics at Johns Hopkins last year. Dai is now a NASA Einstein Postdoctoral Fellow at the Institute for Advanced Study in Princeton.

The paper builds on a hypothesis offered in a paper published this spring by Munoz, Kovetz and Kamionkowski along with five Johns Hopkins colleagues. Also published in Physical Review Letters, that research made a speculative case that the collision of black holes detected early in the year by the Laser Interferometer Gravitational-Wave Observatory (LIGO) had actually revealed dark matter, a substance not yet identified but believed to make up 85 percent of the mass of the universe.

The earlier paper made what Kamionkowski called a "plausibility argument" that LIGO had found dark matter. The study took as a point of departure the fact that the objects detected by LIGO fit within the predicted range of mass of so-called "primordial" black holes. Unlike black holes that formed from imploded stars, primordial black holes are believed to have formed from the collapse of large expanses of gas during the birth of the universe.

The existence of primordial black holes has not been established with certainty, but they have been suggested before as a possible solution to the riddle of dark matter. With so little evidence of them to examine, the hypothesis had not gained a large following among scientists.

The LIGO findings, however, raised the prospect anew, especially as the objects detected in that experiment conform to the mass predicted for dark matter.

The Johns Hopkins team calculated how often these primordial black holes would form binary pairs, and eventually collide. Taking into account the size and elongated shape believed to characterize primordial black hole binary orbits, the team came up with a collision rate that conforms to the LIGO findings.

Key to the argument is that the black holes that LIGO detected fall within a range of 29 to 36 solar masses, meaning that many times the mass of the sun. The new paper considers the question of how to test the hypothesis that dark matter consists of black holes of roughly 30 solar masses.

That's where the fast radio bursts come in. First observed only a few years ago, these flashes of radio frequency radiation emit intense energy, but last only fractions of a second. Their origins are unknown, but believed to lie in galaxies outside the Milky Way.

If the speculation about their origins is true, Kamionkowski said, the radio waves would travel great distances before they're observed on Earth, perhaps passing a black hole. According to Einstein's theory of general relativity, the wave would be deflected when it passes a black hole. If it passes close enough, it could be split into two rays shooting off in the same direction - creating two images from one source.

The new study shows that if the black hole has 30 times the mass of the sun, the two images will arrive a few milliseconds apart. If roughly 30-solar-mass primordial black holes are dark matter, there is a chance that any given fast radio burst will be deflected in this way and followed in a few milliseconds by an echo.

Kaimonkowski said that while the probability for any such FRB echo is small, "it is expected that several of the thousands of FRBs to be detected in the next few years will have such echoes ... if black holes make up the dark matter."

So far, only about 20 fast radio bursts have been detected and recorded since 2001. The very sensitive instruments needed to detect them can look at only very small slices of the sky at a time, limiting the rate at which the bursts can be found. A new telescope expected to go into operation this year that seems particularly promising for spotting radio bursts is the Canadian Hydrogen Intensity Mapping Experiment. The joint project of the University of British Columbia, McGill University, the University of Toronto and the Dominion Radio Astrophysical Observatory stands in British Columbia.

"Once the thing is working up to their planned specifications, they should collect enough FRBs to begin the tests we propose," said Kamionkowski, estimating results could be available in three to five years.

The image at the top of the page shows he 305-m Arecibo telescope. From space, a sequence of millisecond-duration radio flashes are racing towards the dish, where they will be reflected and detected by the radio receivers. Such radio signals are called fast radio bursts, and Arecibo is the first telescope to see repeat bursts from the same source. (Danielle Futselaar).

The Daily Galaxy via Johns Hopkins University

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