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  • richardmitnick 12:43 pm on June 21, 2021 Permalink | Reply
    Tags: "Gap in Exoplanet Size Shifts with Age", As time goes on larger planets lose their atmospheres which explains the evolution of the radius valley the researchers suggested., , , , Changes with Age, , , , It’s been proposed that some planets lose their atmospheres over time which causes them to change size., Most planets develop atmospheres early on but then lose them effectively shrinking in size from just below Neptune’s (roughly 4 times Earth’s radius) to just above Earth’s., Photoevaporation, That deluge of data has inadvertently revealed a cosmic mystery: Planets just a bit larger than Earth appear to be relatively rare in the exoplanet canon., Today thousands of exoplanets are known to inhabit our local swath of the Milky Way., Twenty-six years ago astronomers discovered the first planet orbiting a distant Sun-like star.   

    From Eos: “Gap in Exoplanet Size Shifts with Age” 

    From AGU
    Eos news bloc

    From Eos

    6.21.21
    Katherine Kornei

    1
    Planets just slightly larger than Earth are unusually rare in the Milky Way. Credit: iStock.com/oorka.

    Twenty-six years ago astronomers discovered the first planet orbiting a distant Sun-like star. Today thousands of exoplanets are known to inhabit our local swath of the Milky Way, and that deluge of data has inadvertently revealed a cosmic mystery: Planets just a bit larger than Earth appear to be relatively rare in the exoplanet canon.

    A team has now used observations of hundreds of exoplanets to show that this planetary gap isn’t static but instead evolves with planet age—younger planetary systems are more likely to be missing slightly smaller planets, and older systems are more apt to be without slightly larger planets. This evolution is consistent with the hypothesis that atmospheric loss—literally, a planet’s atmosphere blowing away over time—is responsible for this so-called “radius valley,” the researchers suggested.

    Changes with Age

    In 2017, scientists reported [The Astronomical Journal] the first confident detection of the radius valley. (Four years earlier, a different team had published a tentative detection [The Astrophysical Journal). Defined by a relative paucity of exoplanets roughly 50%–100% larger than Earth, the radius valley is readily apparent when looking at histograms of planet size, said Julia Venturini, an astrophysicist at the ISSI:International Space Science Institute in Bern (CH), Switzerland, not involved in the new research. “There’s a depletion of planets at about 1.7 Earth radii.”

    Trevor David, an astrophysicist at the Flatiron Institute (US) in New York, and his colleagues were curious to know whether the location of the radius valley—that is, the planetary size range it encompasses—evolves with planet age. That’s an important question, said David, because finding evolution in the radius valley can shed light on its cause or causes. It’s been proposed that some planets lose their atmospheres over time which causes them to change size. If the timescale over which the radius valley evolves matches the timescale of atmospheric loss, it might be possible to pin down that process as the explanation, said David.

    In a new study published in The Astronomical Journal, the researchers analyzed planets originally discovered using the Kepler Space Telescope. They focused on a sample of roughly 1,400 planets whose host stars had been observed spectroscopically. Their first task was to determine the planets’ ages, which they assessed indirectly by estimating the ages of their host stars. (Because it takes just a few million years for planets to form around a star, these objects, astronomically speaking, have very nearly the same ages.)

    The team calculated planet ages ranging from about 500 million years to 12 billion years, but “age is one of those parameters that’s very difficult to determine for most stars,” David said. That’s because estimates of stars’ ages rely on theoretical models of how stars evolve, and those models aren’t perfect when it comes to individual stars, he said. For that reason, the researchers decided to base most of their analyses on a coarse division of their sample into two age groups, one corresponding to stars younger than a few billion years and one encompassing stars older than about 2–3 billion years.

    A Moving Valley

    When David and his collaborators looked at the distribution of planet sizes in each group, they indeed found a shift in the radius valley: Planets within it tended to be about 5% smaller, on average, in younger planetary systems compared with older planetary systems. It wasn’t wholly surprising to find this evolution, but it was unexpected that it persisted over such long timescales [billions of years], said David. “What was surprising was how long this evolution seems to be.”

    These findings are consistent with planets losing their atmospheres over time, David and his colleagues proposed. The idea is that most planets develop atmospheres early on but then lose them effectively shrinking in size from just below Neptune’s (roughly 4 times Earth’s radius) to just above Earth’s. “We’re inferring that some sub-Neptunes are being converted to super-Earths through atmospheric loss,” David told Eos. As time goes on larger planets lose their atmospheres which explains the evolution of the radius valley the researchers suggested.

    Kicking Away Atmospheres

    Atmospheric loss can occur via several mechanisms, scientists believe, but two in particular are believed to be relatively common. Both involve energy being transferred into a planet’s atmosphere to the point that it can reach thousands of degrees kelvin. That input of energy gives the atoms and molecules within an atmosphere a literal kick, and some of them, particularly lighter species like hydrogen, can escape.

    “You can boil the atmosphere of a planet,” said Akash Gupta, a planetary scientist at the University of California-Los Angeles (US) not involved in the research.

    In the first mechanism—photoevaporation—the energy is provided by X-ray and ultraviolet photons emitted by a planet’s host star. In the second mechanism—core cooling—the source of the energy is the planet itself. An assembling planet is formed from successive collisions of rocky objects, and all of those collisions deposit energy into the forming planet. Over time, planets reradiate that energy, some of which makes its way into their atmospheres.

    Theoretical studies [The Astrophysical Journal] have predicted that photoevaporation functions over relatively short timescales—about 100 million years—while core cooling persists over billions of years. But concluding that core cooling is responsible for the evolution in the radius valley would be premature, said David, because some researchers have suggested that photoevaporation can also act over billions of years in some cases. It’s hard to pinpoint which is more likely at play, said David. “We can’t rule out either the photoevaporation or core-powered mass loss theories.”

    It’s also a possibility that the radius valley might arise because of how planets form, not how they evolve. In the future, David and his colleagues plan to study extremely young planets, those only about 10 million years old. These youngsters of the universe should preserve more information about their formation, the researchers hope.

    See the full article here .

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    Eos is the leading source for trustworthy news and perspectives about the Earth and space sciences and their impact. Its namesake is Eos, the Greek goddess of the dawn, who represents the light shed on understanding our planet and its environment in space by the Earth and space sciences.

     
  • richardmitnick 9:42 am on September 28, 2020 Permalink | Reply
    Tags: "The first Ultra Hot Neptune LTT 9779b is one of nature’s improbable planets", , , , , Photoevaporation, ,   

    From University of Warwick: “The first Ultra Hot Neptune LTT 9779b is one of nature’s improbable planets” 

    From University of Warwick

    21 September 2020

    Peter Thorley
    Media Relations Manager (Warwick Medical School and Department of Physics)
    peter.thorley@warwick.ac.uk
    Mob: +44 (0) 7824 540863

    1

    International team including University of Warwick astronomers discovers a new class of planet, the Ultra Hot Neptune.

    ● The planet was found in the Neptunian Desert, where such objects are rarely found.

    ● Could be a transitional planet, a deflated gas giant.

    ● Provides a unique opportunity to study the atmospheres of hot Neptune-type planets.

    An international team of astronomers, including a group from the University of Warwick, have discovered the first Ultra Hot Neptune planet orbiting the nearby star LTT 9779.

    The world orbits so close to its star that its year lasts only 19 hours, meaning the stellar radiation heats the planet to over 1700 degrees Celsius.

    At these temperatures, heavy elements like iron can be ionized in the atmosphere and molecules disassociated, providing a unique laboratory to study the chemistry of planets outside the solar system.

    Although the world weighs twice as much as Neptune does, it is also slightly larger and so has a similar density. Therefore, LTT 9779b should have a huge core of around 28 Earth-masses, and an atmosphere that makes up around 9% of the total planetary mass.

    The system itself is around half the age of the Sun, at 2 billion years old, and given the intense irradiation, a Neptune-like planet would not be expected to keep its atmosphere for so long, providing an intriguing puzzle to solve; how such an improbable system came to be.

    LTT 9779 is a Sun-like star located at a distance of 260 light-years, a stone’s throw in astronomical terms. It is super metal-rich, having twice the amount of iron in its atmosphere than the Sun. This could be a key indicator that the planet was originally a much larger gas giant, since these bodies preferentially form close to stars with the highest iron abundances.

    Initial indications of the existence of the planet were made using the Transiting Exoplanet Survey Satellite (TESS), as part of its mission to discover small transiting planets orbiting nearby and bright stars across the whole sky.

    NASA/MIT TESS replaced Kepler in search for exoplanets.

    Such transits are found when a planet passes directly in front of its parent star, blocking some of the starlight, and the amount of light blocked reveals the companion’s size. Worlds like these, once fully confirmed, can allow astronomers to investigate their atmospheres, providing a deeper understanding of planet formation and evolution processes.

    The transit signal was quickly confirmed in early November 2018 as originating from a planetary mass body, using observations taken with the High Accuracy Radial-velocity Planet Searcher (HARPS) instrument, mounted on the 3.6m telescope at the ESO la Silla Observatory in northern Chile.

    ESO/HARPS at La Silla.

    ESO 3.6m telescope & HARPS at Cerro LaSilla, Chile, 600 km north of Santiago de Chile at an altitude of 2400 metres.

    ESO/Cerro LaSilla, 600 km north of Santiago de Chile at an altitude of 2400 metres.

    HARPS uses the Doppler Wobble method to measure planet masses and orbital characteristics like period. When objects are found to transit, Doppler measurements can be organized to confirm the planetary nature in an efficient manner. In the case of LTT 9779b, the team were able to confirm the reality of the planet after only one week of observations.

    Planet transit. NASA/Ames.

    The University of Warwick is a leading institution within the Next-Generation Transit Survey (NGTS) consortium, whose telescopes at Paranal in Chile made follow-up observations to help confirm the discovery of the planet.

    ESO NGTS an array of twelve 20-centimetre telescopes at Cerro Paranel, 2,635 metres (8,645 ft) above sea level.

    Dr George King of the University of Warwick Department of Physics worked on the analysis of the findings.

    He said: “We were very pleased when our NGTS telescopes confirmed the transit signal from this exciting new planet. The dip in brightness is only two tenths of one percent, and very few telescopes are capable of making such precise measurements.”


    Video credit: Ricardo Ramirez.

    Professor James Jenkins from the Department of Astronomy at the Universidad de Chile who led the team said: “The discovery of LTT 9779b so early in the TESS mission was a complete surprise; a gamble that paid off. The majority of transit events with periods less than one day turnout to be false-positives, normally background eclipsing binary stars.”

    LTT 9779b is a rare beast indeed, existing in a sparsely populated region of the planetary parameter space. “The planet exists in something known as the ‘Neptune Desert’, a region devoid of planets when we look at the population of planetary masses and sizes. Although icy giants seem to be a fairly common by-product of the planet formation process, this is not the case very close to their stars. We believe these planets get stripped of their atmospheres over cosmic time, ending up as so-called Ultra Short Period planets.” Jenkins explained.

    Calculations by Dr King confirmed that the atmosphere of LTT 9779b should have been stripped of its atmosphere through a process called photoevaporation. He said: “Intense X-ray and ultraviolet from the young parent star will have heated the upper atmosphere of the planet and should have driven the atmospheric gases into space.” On the other hand, Dr King’s calculations showed there was not enough X-ray heating for LTT 9779b to have started out as a much more massive gas giant. “Photoevaporation should have resulted in either a bare rock or a gas giant,” he explained. “Which means there has to be something new and unusual we have to try to explain about this planet’s history.”

    Professor Jenkins remarked: “Planetary structure models tell us that the planet is a giant core dominated world, but crucially, there should exist two to three Earth-masses of atmospheric gas. But if the star is so old, why does any atmosphere exist at all? Well, if LTT 9779b started life as a gas giant, then a process called Roche Lobe Overflow could have transferred significant amounts of the atmospheric gas onto the star.”

    Roche Lobe Overflow is a process whereby a planet comes so close to its star that the star’s stronger gravity can capture the outer layers of the planet, causing it to transfer onto the star and so significantly decreasing the mass of the planet. Models predict outcomes similar to that of the LTT 9779 system, but they also require some fine tuning.

    “It could also be that LTT 9779b arrived at its current orbit quite late in the day, and so hasn’t had time to be stripped of the atmosphere. Collisions with other planets in the system could have thrown it inwards towards the star. Indeed, since it is such a unique and rare world, more exotic scenarios may be plausible.” Jenkins added.

    Since the planet does seem to have a significant atmosphere, and that it orbits a relatively bright star, future studies of the planetary atmosphere may unlock some of the mysteries related to how such planets form, how they evolve, and the details of what they are made of. Jenkins concluded: “The planet is very hot, which motivates a search for elements heavier than Hydrogen and Helium, along with ionised atomic nuclei. It’s sobering to think that this ‘improbable planet’ is likely so rare that we won’t find another laboratory quite like it to study the nature of Ultra Hot Neptunes in detail. Therefore, we must extract every ounce of knowledge that we can from this diamond in the rough, observing it with both space-based and ground-based instruments over the coming years.”

    An Ultra Hot Neptune in the Neptune Desert is published in Nature Astronomy.

    See the full article here.

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    The establishment of the The University of Warwick was given approval by the government in 1961 and received its Royal Charter of Incorporation in 1965.

    The idea for a university in Coventry was mooted shortly after the conclusion of the Second World War but it was a bold and imaginative partnership of the City and the County which brought the University into being on a 400-acre site jointly granted by the two authorities. Since then, the University has incorporated the former Coventry College of Education in 1978 and has extended its land holdings by the purchase of adjoining farm land.

    The University initially admitted a small intake of graduate students in 1964 and took its first 450 undergraduates in October 1965. In October 2013, the student population was over 23,000 of which 9,775 are postgraduates. Around a third of the student body comes from overseas and over 120 countries are represented on the campus.

     
  • richardmitnick 11:40 am on December 4, 2018 Permalink | Reply
    Tags: A Neptune-like exoplanet’s atmosphere being eroded away, , , , , , Exploring the Escaping Atmosphere of HAT-P-11b, Photoevaporation   

    From AAS NOVA: “Exploring the Escaping Atmosphere of HAT-P-11b” 

    AASNOVA

    From AAS NOVA

    3 December 2018
    Susanna Kohler

    1
    Artist’s impression of the exoplanet HAT-P-11b and its host star. [Harvard Center for Astrophysics/D. Aguilar]

    The atmospheres of planets close to their host stars live a tenuous existence. New observations from the Hubble Space Telescope show signs of a Neptune-like exoplanet’s atmosphere being eroded away.

    Evaporation at Work

    Small planets observed to orbit closely around their host star fall into two main populations:

    those with radii smaller than 1.5 Earth radii, thought to be primarily rocky cores with little or no remaining atmosphere, and
    those with radii larger than 2 Earth radii, thought to retain some of their hydrogen and helium atmospheres.

    What causes the difference between these two populations? We think that all close-in exoplanets are sculpted by the energetic radiation of their host stars. This radiation can erode away the primordial atmospheres — and for the smallest planets, this will leave only their rocky cores behind.

    As we work to understand the detailed physics of this photoevaporation, it would be helpful to be able to directly watch a planet’s atmosphere escaping in this way. In a new study, scientist Megan Mansfield (University of Chicago) and collaborators present just the thing: observations of the escaping atmosphere of the exoplanet HAT-P-11b.

    Observations of a Hot Neptune

    3
    Artist’s illustration of WASP-107b, the first planet for which Hubble discovered helium escaping from its atmosphere. [ESA/Hubble, NASA, M. Kornmesser]

    HAT-P-11b is a Neptune-sized exoplanet that orbits very close to its host star in a system that’s located approximately 120 light-years from Earth. Using Hubble, Mansfield and collaborators discovered the subtle signature of helium escaping from the atmosphere of HAT-P-11b — making this the second planet for which this signature has been discovered by Hubble (the first was WASP 107-b) and one of only a handful of planets for which we’ve seen signs of atmospheric escape.

    By comparing these observations to models, Mansfield and collaborators estimate that HAT-P-11b is losing mass at a rate of roughly 10^9–10^11 g/s. This rate, while high, is still low enough that the planet has only lost a few percent of its mass over its history, leaving its bulk composition largely unaffected. This is consistent with what we would expect for a planet of its size: since it’s larger than 2 Earth radii, it should retain some of its hydrogen and helium atmosphere.

    3
    Narrowband spectrum of HAT-P-11b (blue and gray points) compared to three 1D models of hydrodynamic escape (red, green, and orange lines). [Mansfield et al. 2018]

    A New Approach

    Why are these escaping-helium detections important? Observations like this one represent a new method for exploring exoplanet atmospheres! The helium signature detected from HAT-P-11b had long been theorized as a way to study escaping atmospheres, but until Hubble’s recent observations of helium in the atmosphere of WASP 107-b, the potential of this approach remained untapped.

    Now two planets have been observed with this particular signal — and the signal from HAT-P-11b has been additionally confirmed with CARMENES instrument in Spain, marking the first time the same signature of photoevaporation has been detected by both ground- and space-based facilities.

    CARMENES spectrograph, mounted on the Calar Alto 3.5 meter Telescope, located in Almería province in Spain on Calar Alto, a 2,168-meter-high (7,113 ft) mountain in Sierra de Los Filabres



    Calar Alto 3.5 meter Telescope, located in Almería province in Spain on Calar Alto, a 2,168-meter-high (7,113 ft) mountain in Sierra de Los Filabres


    Calar Alto Observatory located in Almería province in Spain on Calar Alto, a 2,168-meter-high (7,113 ft) mountain in Sierra de Los Filabres

    Future observations like these — from both existing instruments and upcoming observatories like the James Webb Space Telescope — will hopefully continue to shed light on how atmospheres evaporate from small, close-in exoplanets.

    Citation

    “Detection of Helium in the Atmosphere of the Exo-Neptune HAT-P-11b,” Megan Mansfield et al 2018 ApJL 868 L34.
    http://iopscience.iop.org/article/10.3847/2041-8213/aaf166/meta

    See the full article here .


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    1

    AAS Mission and Vision Statement

    The mission of the American Astronomical Societyis to enhance and share humanity’s scientific understanding of the Universe.

    The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
    The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
    The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
    The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
    The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

    Adopted June 7, 2009

     
  • richardmitnick 2:11 pm on January 13, 2017 Permalink | Reply
    Tags: , , , , Photoevaporation, The Impact of Stars on Moons   

    From AAS NOVA: ” The Impact of Stars on Moons” 

    AASNOVA

    American Astronomical Society
    13 January 2017
    Susanna Kohler

    1
    Artist’s illustration of exomoons orbiting an exoplanet. A new study examines a way that exomoons might become unbound from their planets. [NASA/JPL-Caltech]

    In other solar systems, the radiation streaming from the central star can have a destructive impact on the atmospheres of the star’s close-in planets. A new study suggests that these exoplanets may also have a much harder time keeping their moons.

    Where Are the Exomoons?

    Moons are more common in our solar system than planets by far (just look at Jupiter’s enormous collection of satellites!) — and yet we haven’t made a single confirmed discovery of a moon around an planet outside of our solar system. Is this just because moons have smaller signals and are more difficult to detect? Or might there also be a physical reason for there to be fewer moons around the planets we’re observing?

    Led by Ming Yang, a team of scientists from Nanjing University in China have explored one mechanism that could limit the number of moons we might find around exoplanets: photoevaporation.

    1
    Artist’s illustration of the process of photoevaporation, in which the atmosphere of a planet is stripped by radiation from its star. [NASA Goddard SFC]

    Effects of Radiation

    Photoevaporation is a process by which the harsh high-energy radiation from a star blasts a close-in planet, imparting enough energy to the atoms of the planet’s atmosphere for those atoms to escape. As the planet’s atmosphere gradually erodes, significant mass loss occurs on timescales of tens or hundreds of millions of years.

    How might this process affect such a planet’s moons? To answer this question, Yang and collaborators used an N-body code called MERCURY to model solar systems in which a Neptune-like planet at 0.1 AU gradually loses mass. The planet starts out with a large system of moons, and the team tracks the moons’ motions to determine their ultimate fates.

    Escaping Bodies

    2
    Evolution of the planet mass (top) in a simulation containing 500 small moons. The evolution of the semimajor axes of the moons (middle) and their eccentricities (bottom) are shown, with three example moons, starting at different radii, highlighted in blue, red and green. The black dotted line shows how the critical semimajor axis for stability evolves with time as the planet loses mass. [Yang et al. 2016]

    Yang and collaborators find that the photoevaporation process has a critical impact on whether or not the moons remain in stable orbits. As the photoevaporation drives mass loss of the planet, the planet’s gravitational influence shrinks and the orbits of its exomoons expand and become more eccentric. Eventually these orbits can reach critical values where they’re no longer stable, often resulting in systems with only one or no surviving moons.

    The team finds that even in the best-case scenario of only small moons, no more than roughly a quarter of them survive the simulation still in orbit around their planet. In simulations that include larger moons further out, the system is even more likely to become unstable as the planet loses mass, with more moons ultimately escaping.

    What happens to the moons that escape? Some leave the planet–moon system to become planet-like objects that remain in orbit around the host star. Others are smashed to bits when they collide with other moons or with the planet. And some can even escape their entire solar system to become a free-floating object in the galaxy!

    Based on their simulations, the authors speculate that exomoons are less common around planets that are close to their host stars (<0.1 AU). Furthermore, exomoons are likely less common in solar systems around especially X-ray-luminous stars (e.g., M dwarfs) that can more easily drive photoevaporation. For these reasons, our best chances for finding exomoons in future missions will be around stars that are more Sun-like, orbiting planets that aren’t too close to their hosts.
    Citation

    Ming Yang et al, 2016 ApJ 833 7. doi:10.3847/0004-637X/833/1/7

    See the full article here .

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