Cosmic Diary Marchis
Source: SETI Institute Press-release
MOUNTAIN VIEW, CA – For the first time, an Earth-sized planet has been found in the habitable zone of its star. This discovery not only proves the existence of worlds that might be similar to our own, but will undoubtedly shape future investigations of exoplanets that could have terrestrial surface environments.
The new-found body, orbiting the red dwarf star Kepler-186 and designated Kepler-186f, is the fifth – and outermost – world to be discovered in this system. The results are described in an article appearing in Science.
“This is the first definitive Earth-sized planet found in the habitable zone around another star,” says lead author Elisa Quintana of the SETI Institute at NASA Ames Research Center. “Finding such planets is a primary goal of the Kepler space telescope. The star is a main-sequence M-dwarf, a very common type. More than 70 percent of the hundreds of billions of stars in our galaxy are M-dwarfs.”
Of the nearly 1800 confirmed exoplanets found in the past two decades, approximately twenty orbit their host star in the habitable zone – a range of orbital distances at which surface water on a planet with an atmosphere would neither freeze nor boil. However, all of these previously discovered worlds are larger than Earth, and consequently their true nature – rocky or gaseous – is unknown. On the basis of the observed dimming of starlight from Kepler-186, the authors estimate that this newly discovered planet is roughly the same size as the Earth.
Thomas Barclay, a staff scientist for the Kepler mission affiliated with both NASA and the Bay Area Environmental Research Institute, notes that “theoretical models of how planets form suggest that those with diameters less than 1.5 times that of Earth are unlikely to be swathed in atmospheres of hydrogen and helium, the fate that’s befallen the gas giants of our own solar system. Consequently, Kepler-186f is likely a rocky world, and in that sense similar to Venus, Earth and Mars.”
The new planet orbits at a distance of 0.36 astronomical units from its star, or slightly closer than Mercury is to the Sun. Its orbital period is 130 days.
Traditionally, planets orbiting red dwarf stars were considered to be poor candidates for life. The objection was that star-hugging planets in the habitable zone would become tidally locked, and suffer a synchronous or pseudo-synchronous rotation that could make climate on these planets untenable. However, more recent modeling studies suggest that such worlds are not necessarily inhospitable, since atmospheric winds or ocean currents could even out extreme temperature variations . In addition, Kepler-186f is far enough away from its host star that it is unlikely to be locked. This greater distance also reduces the danger to any potential life forms posed by stellar flares, which are more common for dwarf stars.
Since 2012, the SETI Institute’s Allen Telescope Array has been observing Kepler
candidate exoplanets looking for signals that would indicate extraterrestrial intelligence. A search for emissions from Kepler-186f has been made over the very wide frequency range of 1 to 10 GHz, but none have so far been found. These observations will be repeated. Note that a detectable signal would require a transmitter approximately 10 to 20 times more powerful than the planetary radar system at Arecibo, in Puerto Rico.
According to Quintana, at 490 light-years Kepler-186f may be too dim for follow-up surveys to probe its atmosphere, even with next-generation telescopes. “However, our research tells us that we should be able to find planets around bright stars that will be ideal targets to observe with James Webb.” NASA’s Webb space-based telescope, now under construction, will be able to directly image planets around nearby dwarf stars, and use spectral analysis to characterize their atmospheres.
Finding Kepler-186f is a first, but “it’s not a record we wish to keep,” Quintana says. “We want to find more of these.”
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The mission of the SETI Institute is to explore, understand and explain the origin, nature and prevalence of life in the universe. The Institute is a private, nonprofit organization dedicated to scientific research, education and public outreach. It comprises three centers, the Center for SETI Research, the Carl Sagan Center for the Study of Life in the Universe and the Center for Education and Public Outreach. Founded in November 1984, the SETI Institute today employs over 150 scientists, educators and support staff. For more information, www.seti.org 650-961-6633.
Some may say that our universe is full of beauty, others argue that it is our solar system that surprises us the most, but ultimately I will say that it is the world of small solar system bodies which is strikingly full of diversity. Today’s announcement of the discovery of rings around the Centaur Chariklo by an international team of astronomers is a vivid proof that small solar system bodies have not yet revealed all their secrets.
My recent work has made me realize that asteroids (also called small solar system bodies or minor planets) are in fact real mini-geological worlds. Planetary scientists found out about their complexity very recently after visiting them with robotic spacecraft that took snapshots of their surfaces. These images showed the presence of geological activity on their surface (craters & landslides) and morphological features (ponds of dust, and pebbles). More recently, studies based on powerful observations with large ground-based telescopes, revealed that several asteroids have moons. The follow-up of their moons’ orbits revealed a complex interior for these asteroids that is possibly differentiated, similarly to the major planets.
The announcement of a ring around a Centaur is, without doubt, another step ahead in blurring the differences between minor and major planets.
Centaur asteroids are a special population of minor bodies which orbit among the giant planets. They possess the characteristics of comets, since some of them have shown cometary activity, and can also look like asteroids. They are most likely made of a mixture of ices and rocks. They were discovered quite recently and because they are far away from us, we don’t know much about them beyond their orbits around the sun, the colors of their surfaces, and the average size for the largest ones.
One Centaur named Chariklo, or asteroid number 10199, was discovered in 1997 by the Spacewatch survey. It has never been the target of the Hubble Space Telescope because time on this telescope is extremely difficult to obtain, due to the high competition with other programs. Also, to my knowledge, none of the 8-10m class telescopes equipped with adaptive optics has also observed it, most likely because of its faintness (V~18.3 in visible). Consequently, we didn’t know much about this asteroid, with the exception of its average size of 260 km.
A team of astronomers designed a clever way to study distant asteroids, like Centaur asteroids. Since their orbits are well-known after several years of observations, they can predict when a Centaur asteroid will hide the light of a bright star and where and when this event will happen. On 3 June 2013, Chariklo passed in front of a star and the path of this occultation crossed South America. Based on this prediction, seven telescopes observed the disappearance of this star that lasted a few seconds. But what really stunned the astronomers was that the star also vanished a few seconds before and after the main occultation.
After putting together those occultation chords, they derived the size and shape of Chariklo, and realized that these secondary occultations could be best explained by assuming that Chariklo is surrounding by a dense ring of dust. Using data from the largest telescope involved in these observations (the 1.54m Danish telescope located at La Silla Observatory in Chile), they found out that there are in fact 2 rings around Chariklo.
This is not a direct image per se, but their work, which is based on multiple chords taken from a wide variety of instruments and different observers, is quite reliable.
The structures in the disk around Chariklo suggest that the system is quite complex. We can assume that its rings are shepherded by a system of moons, not yet detected, like the F ring of Saturn is shepherded by Pandora and Prometheus.
The team also discussed the consistency of their results with previous observations of Chariklo. Between 1997-2008, they calculated that the system was seen edge-on, so the ring of dust was occulting the primary and should have dimmed its brightness significantly. This attenuation of Chariklo’s brightness was reported in the literature and no explanation had been found at the time. Additionally, spectroscopic variations of Chariklo were seen during the same period. The disappearance of these absorption bands, when the ring was edge-on, could be explained by assuming that it is made of ices and organics.
“How did such a system form?” is the bonus question of this study. The presence of those signatures of ices in the spectrum of the Chariklo system implies that the system is young, so how are we so extremely lucky to see it? The team discussed a collisional origin but other scenarios could be considered. I hypothesize that the system has a complex moon system and the rings are replenished by cometary activity from the surface of the primary.
This centaur asteroid will be a great target for extremely large telescopes. The rings are 0.040″ away from the primary so will be difficult to detect with current 8-10m class telescopes, which can only marginally reach this angular resolution. Extremely large telescopes, like the future TMT, E-ELT or GMT, would be capable of imaging the system and revealing those moons.
I wonder how many of these asteroid ring systems will be found in the future thanks to the development of large aperture telescopes on the ground and in space. The development of instrumentation has revealed how common moons are around asteroids, so we can extrapolate that asteroid ring systems may be common in the distant and cold regions of our solar system.
Chariklo’s ring system, and those that remain to be found, are not only a curiosity for astronomers, they are also a time-capsule that allow us to peek into the collisional history of our solar system. Their study could also help us understand the composition of the asteroid itself.
One simple question that came to my mind when I read the article is why we found this ring system around a centaur asteroid? It would have been easier to find it around an asteroid in the main-belt since they are closer to us and we observe the larger ones quite often with large ground-based telescopes. I suspect that this is related to the composition of centaur asteroids and that the ring is indeed a secondary product of their surface activity.
I am done speculating for the moment, and will simply enjoy this rare moment when a new sub-field of research in planetary science is being born.
David Morrison, director of the Carl Sagan Center of the SETI Institute, has written a biographical memoir of Carl Sagan (1934-1996), founder of the modern disciplines of planetary science and exobiology.
In the biographical memoir, Morrison shares his thoughts on the life of Carl Sagan, “one of the world’s best-known scientists and a true celebrity” whose work still resonates today in the field of planetary science. Morrison, who was one of the first students of Sagan’s in the 1970s at Harvard University, reflects not only on the life of his mentor and friend, but also on his controversial “out of the box” thinking.
Carl Sagan was a founding member of the Division for Planetary Science of the American Astronomical Society, the Committee for Skeptical Inquiry, and Icarus, the foremost professional journal in planetary science. Later in the 1980s he founded the Planetary Society, a public-membership organization that supports planetary exploration, and became a supporter of the search for extraterrestrial intelligence, work which became the basis of his best-selling novel, Contact (1985). In 1996, Sagan died from complications of a rare blood disease, shortly after joining the Board of Trustees of the SETI Institute.
This biographical memoir will interest any fan of planetary science, astrobiology, the SETI Institute and science in general.
Let’s end with a quote by Carl Sagan which summarizes his life’s work and achievements “Even today, there are moments when what I do seems to me like an improbable, if unusually pleasant, dream: to be involved in the exploration of Venus, Mars, Jupiter, and Saturn; to try to duplicate the steps that led to the origin of life on an Earth very different from the one we know; to land instruments on Mars to search there for life; and perhaps to be engaged in a serious effort to communicate with other intelligent beings, if such there be, out there in the dark of the night sky.”
- Beginning Astrobiology Talk by D. Morrison http://www.youtube.com/watch?v=1lmfGQzOIjQ
- Carl Sagan Biographical Memoire http://www.nasonline.org/publications/biographical-memoirs/memoir-pdfs/sagan-carl.pdf