Posted on by Jon Voisey

Update on Gliese 581d’s Habitability

An artist’s impression of Gliese 581d, an exoplanet about 20.3 light-years away from Earth, in the constellation Libra. Credit: NASA

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When last we checked in on Gliese 581d, a team from the University of Paris had suggested that the popular exoplanet, Gliese 581d may be habitable. This super-Earth found itself just on the edge of the Goldilocks zone which could make liquid water present on the surface under the right atmospheric conditions. However, the team’s work was based on one dimensional simulations of a column of hypothetical atmospheres on the day side of the planet. To have a better understanding of what Gliese 581d might be like, a three dimensional simulation was in order. Fortunately, a new study from the same team has investigated the possibility with just such an investigation.

The new investigation was called for because Gliese 581d is suspected to be tidally locked, much like Mercury is in our own solar system. If so, this would create a permanent night side on the planet. On this side, the temperatures would be significantly lower and gasses such as CO2 and H2O may find themselves in a region where they could no longer remain gaseous, freezing into ice crystals on the surface. Since that surface would never see the light of day, they could not be heated and released back into the atmosphere, thereby depleting the planet of greenhouse gasses necessary to warm the planet, causing what astronomers call an “atmospheric collapse.”

To conduct their simulation the team assumed that the climate was dominated by the greenhouse effects of CO2 and H2O since this is true for all rocky planets with significant atmospheres in our solar system. As with their previous study, they performed several iterations, each with varying atmospheric pressures and compositions. For atmospheres less than 10 bars, the simulations suggested that the atmosphere would collapse, either on the dark side of the planet, or near the poles. Past this, the effects of greenhouse gasses prevented the freezing of the atmosphere and it became stable. Some ice formation still occurred in the stable models where some of the CO2 would freeze in the upper atmosphere, forming clouds in much the same way it does on Mars. However, this had a net warming effect of ~12°C.

In other simulations, the team added in oceans of liquid water which would help to moderate the climate. Another effect of this was that the vaporization of water from these oceans also produced warming as it can serve as a greenhouse gas, but the formation of clouds could decrease the global temperature since water clouds increase the albedo of the planet, especially in the red region of the spectra which is the most prevalent form of light from the parent star, a red dwarf. However, as with models without oceans, the tipping point for stable atmospheres tended to be around 10 bars of pressure. Under that, “cooling effects dominated and runaway glaciation occurred, followed by atmospheric collapse.” Above 20 bars, the additional trapping of heat from the water vapor significantly increased temperatures compared to an entirely rocky planet.

The conclusion is that Gliese 581d is potentially habitable. The potential for surface water exists for a “wide range of plausible cases”. Ultimately, they all depend on the precise thickness and composition of any atmosphere. Since the planet does not transit the star, spectral analysis through transmission of starlight through the atmosphere will not be possible. Yet the team suggests that, since the Gliese 581 system is relatively close to Earth (only 20 lightyears), it may be possible to observe the spectra directly in the infrared portion of the spectra using future generations of instruments. Should the observations match the synthetic spectra predicted for the various habitable planets, this would be taken as strong evidence for the habitability of the planet.

Posted on by Jon Voisey

A Newly Discovered Planetary Nebula Teaches Us About Galactic Composition

Determining the chemical distribution of the galaxy is a tricky business. The ideal method is spectroscopy but since high quality spectroscopy takes bright targets, the number of potential targets is somewhat reduced. Stars seem like logical choices, but due to differential separation during formation, they don’t provide a true description of the interstellar medium. Clouds of gas and dust are the best choice, but must be illuminated by star formation. Another option is to search for newly formed planetary nebulae which are in the process of enriching the interstellar medium.

A new paper does just this, discovering a new planetary nebula in hopes of mapping the chemical abundance of the galaxy. The new nebula is almost the exact opposite direction of the galactic center when viewed from Earth. It lies at a distance of about 13 kpc (42,400 lightyears) from Earth making it one of the most distant planetary nebulae from the galactic center for which a distance has been determined and currently, the furthest with a measured chemical abundance.

The nebula was originally recorded on images taken by the INT Photometric Hα Survey (IPHAS) in 2003 but the automated program for detecting such objects initially missed the nebula due to its relatively large angular size (10 arcseconds). It was subsequently caught on visual inspection of the mosaics. Follow-up spectroscopy was conducted from 2005 to 2010 and reveal that the nebula is quite regular for planetary nebula, containing strong emission from hydrogen, nitrogen, oxygen, and silicon. The rate of expansion combined with its physical size suggests an age of nearly 18,000 years.

This newly discovered nebula provides a rare data point for the chemical abundance for the outer portions of the galaxy. While the galaxy is known to be enriched towards the galactic center, there has been debate about how quickly, if at all, it falls towards the galactic edge where star formation, and thus, enrichment, is less common. While there aren’t enough known nebulae to determine just yet (only four others are known at similar distances), this planetary nebula suggests that the abundance levels off in the galactic outskirts.

The authors also note that this nebula, as well as potentially the others, aren’t native to the Milky Way. They lie near a structure known as the Monoceros Ring, which is a stream of stars believed to be stretched out as the Milky Way devours the Canis Major Dwarf Galaxy.

Posted on by Jon Voisey

Transiting Super-Earth Detected Around Naked Eye Star

55 Cancri. Image credit: NASA/JPL

One of the first known stars to host an extrasolar planet, was that of 55 Cancri. The first planet in this system was reported in 1997 and today the system is known to host at least five planets, the inner most of which, 55 Cnc e, was recently discovered to transit the star, giving new information about this planet.

55 Cnc is an interesting system in many respects. Being a mere 41 lightyears from the Earth, the system is composed of a primary, yellow dwarf star in a wide binary orbit (1,000 AU) with a red dwarf. The planetary system lies within this orbit. The primary star is just brighter than 6th magnitude meaning it is visible to the naked eye under good viewing conditions.

One of these planets, 55 Cnc e, was discovered in this system via radial velocity measurements in 2004. At that point, the planet was reported to have a period of 2.8 days, and a minimum mass of 14.2 times the mass of the Earth. However, in 2010, Rebekah Dawson and Daniel Fabrycky from the Harvard-Smithsonian Center for Astrophysics argued that gaps in the observational period skewed the statistics and the true period the planet should be a short 0.7365 days.

One of the results of this was that the planet would have to orbit closer to the parent star. In turn, this increased the likelihood that the planet could transit the star from 13% to 33%. A team led by Joshua Winn from the Massachusetts Institute of Technology went searching for this faint transit and report its detection in a recent paper. But while the star itself is one of the brightest stars in our sky to harbor known extrasolar planets, the eclipse is far from visible without precise observations, changing by only 0.0002%, one of the smallest changes known. The timing of the eclipses confirms that correction by Dawson and Fabrycky and adds new information about the body.

Given the radius determined as well as the mass, the team was able to estimate the structure of the planet and report that the mass is 8.57 ± 0.64 Earth masses. The reported radius is 1.63 ± 0.16 times that of Earth, and the density is 10.9 ± 3.1 g cm-3 (the average density of Earth is 5.515 g cm-3). This places the planet firmly into the categories of a rocky super-Earth.

The team also explores whether or not the planet could retain an atmosphere in such a close orbit (only three times the radius of the star itself). At this close range, the planet would likely be tidally locked and with an albedo typical of rocky planets, the planet would likely have an average temperature of nearly 2970 K (5,000° F). If the planet were able to redistribute the heat, it may be as low as 2100 K (3,300° F). Either way, a planet of such mass would have difficulty retaining any primordial, gaseous atmosphere. However, the team reports that it may be possible for volcanic activity to create a thin atmosphere of high molecular weight components.

While this new report adds precious little in the grand scheme of the rapidly growing body of knowledge of exoplanets, the authors close with the note that, “there is some pleasure in being able to point to a naked-eye star and know the mass and radius of one of its planets.”

Posted on by VirtualAstro

Last & Best Chances to See NanoSail-D

Nanosail-D Pass Credit: Vesa Vauhkonen, Spaceweather.com

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Over the next few weeks, skywatchers will have excellent viewing opportunities for the NanoSail-D solar sail.

The satellite is coming to the end of its 95-day mission to test the viability of de-orbiting decommissioned satellites or space debris. NanoSail-D is now de-orbiting and slowly losing altitude in the Earths thin upper atmosphere.

As the satellite descends, viewing opportunities will improve.

To see NanoSail-D pass over, you will need to know exactly when it will be visible from your location. To do this, go to Heavens-above.com or Spaceweather.com where star charts with times and pass details will be displayed after you enter your observing site.

Once you know the time and location in the sky of the pass of the satellite, make sure you are able to get a good view of the part of the sky where the satellite due to appear. Give yourself plenty of time, go outside and get ready. I always set a 30 second reminder on my watch or cell phone, so I don’t have to fumble around or guess the time.

To enjoy the NanoSail-D passes:

• Make sure you know the right place in the sky and the time of the pass, by checking on the web.
• Make sure you will be able to get a clear view of it from your viewing location.
• Set an alarm or get ready for the pass as it only lasts a few seconds.
• NASA expects NanoSail-D to stay in orbit through May 2011.
• If you are an astrophotographer, don’t forget, NASA and SpaceWeather.com are having an imaging contest of NanoSail-D. Find out more here.
• Most of all, get your friends and family outside with you to watch NanoSail-D and enjoy!

Artist concept of Nanosail-D in Earth orbit. Credit: NASA
Posted on by VirtualAstro

Planets Party In The Morning April 28-May 1

April's Morning Conjunction Credit: Adrian West

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Set your alarm clocks for an early treat about a half an hour before sunrise on Thursday April 28 through Sunday, May 1, 2011, as there will be a planetary delight in store! Go out and with either a pair of binoculars, a small telescope, or just use your naked eyes and find an unobscured view of the Eastern horizon to see a conjunction (objects near each other in the sky) of the planets Jupiter, Mars, Venus and Mercury, below and to the left of the thin crescent moon.

Bright Venus will be easy to spot first, then Mercury followed by Jupiter. The real challenge is to find Mars which will be very close to Jupiter. See the above diagram for help on where each object is located.

If you are unlucky on the first morning, try again the following day for a chance to see this rare planetary occurrence.

While observing this close to the Sun take care and never look at the sun directly with your eyes and never through an optical instrument, as this will permanently damage your eyesight or blind you!

Only special purpose made solar telescopes and filters are safe for viewing the sun.

Posted on by Jason Major

Kepler Discovers a Rare Triple Gem

Animation of HD 181068 (click to play)

 

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It may be visible to the naked eye, but it took the unblinking gaze of NASA’s Kepler space telescope to reveal the true triple nature of this star system.

Animation of HD 181068 (click to play)

Unofficially dubbed “Trinity”, object HD 181068 is a multiple star system comprised of three stars: a red giant more than twelve times the diameter of the Sun and two red dwarf stars each slightly smaller than the Sun. The red dwarfs orbit each other in tight rotation around a central point, which in turn orbits the red giant. The smaller stars complete a full orbit around the giant every 45.5 days and, from our point of view, pass directly in front of and behind the huge star.

The orbital eclipse events of HD 181068 last about 2 days. What’s surprising is that during these eclipses the brightness of the system is not affected very much. This is because the surface brightnesses of the three stars are very similar. The current metaphor is a “white rabbit in a snowfall”, wherein the two red dwarfs effectively become invisible when they pass in front of the red giant. It wasn’t until the Kepler mission that we had an observational tool precise enough to detect the structure of this intriguing star system, located 800 light-years away from our own.

“The intriguing nature of this unique system remained unnoticed until now despite the fact that it is nearly bright enough to be visible to the naked eye. We really needed Kepler with its unprecedentedly precise and uninterrupted photometric monitoring to uncover such a rare gem.”

– Aliz Derekas, Eotvos University and Konkoly Observatory, Budapest, Hungary

Another unexpected feature of Trinity is its “quiet” nature. Astronomers have known that red giant stars exhibit seismic oscillations, as does our own Sun. But these oscillations are not present in Trinity’s red giant. Scientists speculate that the two red dwarfs may be creating some sort of gravitational offset, effectively negating the red giant’s vibrations. More research will be needed to determine if this is in fact the case.

Find out more about HD 181068 and other recent Kepler discoveries on NASA’s mission site or in the press release issued by the Ames Research Center, or read the published report on Science.

Image credit: NASA/KASC

 

 

Posted on by Jon Voisey

April 9th Fireball

In my time watching the skies, I’ve seen quite a few meteors, fireballs, and bolides. The truly notable ones are few and far between, but last Saturday, I caught one that was among the most interesting I’ve seen. It was a slow moving, bright green one with a nice smoke trail that was easily as bright as Venus from where I saw it in the suburbs of St. Louis. I tweeted about it briefly but didn’t think much more about it until I got a response from another person that saw it along with a link to a collection of observations. As nice as the observation was for me, it was nothing compared to the view some others got.

Heading over to the American Meteor Society page for a meteor around this time, it looks like a meteor matching the one I saw generated a pretty good number of reports from across the country. Several have reactions similar to my initial one: This must be a firework. Many reports confirm the smoke trail and fragmentation as well. But the reports that are really fantastic are the ones from Canada.

At the Lunar Meteorite Hunters blog, several reports have been collected. Several of these reports from various locations in Ontario report the meteor being as bright as a full moon and lighting up the entire sky! One even notes that they could hear a fizzling noise, a rare phenomenon thought to occur when the passage through the atmosphere creates an ionized path that interacts with the Earth’s magnetic field creating radio waves that could induce physical vibrations in the air around the observer. Another comment reports a sonic boom around the same time (although sonic booms would occur well after the meteor was visible due to the sluggish nature of sound waves, much like the delay between lightning and thunder).

It doesn’t look like NASA’s All Sky Fireball Network caught this fireball, but an amateur observatory equipped with an all sky camera for detecting fireballs did catch the event.

The green color for such meteors is uncommon but not unprecedented. The presence of magnesium ions is responsible for this color. Interestingly, another famous meteor, the Peekskill meteor, also had a green color and rivaled the full moon in brightness. This meteor became famous because it was independently captured in at least sixteen videos (here’s one showing the green tint) as well as for surviving intact to the ground and damaging a car.

Meteors of this intensity are quite rare but bright fireballs like this seem to peak around the vernal equinox. In the weeks surrounding that day, the rate of such events increases around 10-30%.

Posted on by VirtualAstro

How to find Saturn in the Sky this Weekend

If you want to find the planet Saturn in the sky this weekend, but aren’t sure where to look, this guide should help you.

Saturn is visible all night long at the moment and is quite easy to find, as it is just past opposition which makes it quite bright.

Credit: Adrian West

Find the constellation of Leo the Lion (high in the Southern sky at around 10pm) by looking for the backwards question mark asterism (red in the diagram), which is the head of Leo. Find the last 2 stars in Leo’s body and draw an imaginary line through these 2 stars, and arc to the left and down until you reach a bright yellowish star. This is Saturn.

If you continue drawing this imaginary line a little further you will find the bright bluish white star Spica, in the constellation of Virgo.

Right now, Saturn should be an easy target to spot with the naked eye, but looks great through binoculars and is truly amazing through any telescope.

Posted on by Jon Voisey

Halos Gone MAD

Distribution of dark matter when the Universe was about 3 billion years old, obtained from a numerical simulation of galaxy formation. The left panel displays the continuous distribution of dark matter particles, showing the typical wispy structure of the cosmic web, with a network of sheets and filaments, while the right panel highlights the dark matter halos representing the most efficient cosmic sites for the formation of star-bursting galaxies with a minimum dark matter halo mass of 300 billion times that of the Sun. Credit: VIRGO Consortium/Alexandre Amblard/ESA

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One of the successes of the ΛCDM model of the universe is the ability for models to create structures of with scales and distributions similar to those we view in the universe today. Or, at least that’s what astronomers tell us. While computer simulations can recreate numerical universes in a box, interpreting these mathematical approximations is a challenge in and of itself. To identify the components of the simulated space, astronomers have had to develop tools to search for structure. The results has been nearly 30 independent computer programs since 1974. Each promises to reveal the forming structure in the universe by finding regions in which dark matter halos form. To test these algorithms out, a conference was arranged in Madrid, Spain during the May of 2010 entitled “Haloes going MAD” in which 18 of these codes were put to the test to see how well they stacked up.

Numerical simulations for universes, like the famous Millennium Simulation begin with nothing more than “particles”. While these were undoubtedly small on a cosmological scale, such particles represent blobs of dark matter with millions or billions solar masses. As time is run forwards, they are allowed to interact with one another following rules that coincident with our best understanding of physics and the nature of such matter. This leads to an evolving universe from which astronomers must use the complicated codes to locate the conglomerations of dark matter inside which galaxies would form.

One of the main methods such programs use is to search for small overdensities and then grow a spherical shell around it until the density falls off to a negligible factor. Most will then prune the particles within the volume that are not gravitationally bound to make sure that the detection mechanism didn’t just seize on a brief, transient clustering that will fall apart in time. Other techniques involve searching other phase spaces for particles with similar velocities all nearby (a sign that they have become bound).

To compare how each of the algorithms fared, they were put through two tests. The first, involved a series of intentionally created dark matter halos with embedded sub-halos. Since the particle distribution was intentionally placed, the output from the programs should correctly find the center and size of the halos. The second test was a full fledged universe simulation. In this, the actual distribution wouldn’t be known, but the sheer size would allow different programs to be compared on the same data set to see how similarly they interpreted a common source.

In both tests, all the finders generally performed well. In the first test, there were some discrepancies based on how different programs defined the location of the halos. Some defined it as the peak in density, while others defined it as a center of mass. When searching for sub-halos, ones that used the phase space approach seemed to be able to more reliably detect smaller formations, yet did not always detect which particles in the clump were actually bound. For the full simulation, all algorithms agreed exceptionally well. Due to the nature of the simulation, small scales weren’t well represented so the understanding of how each detect these structures was limited.

The combination of these tests did not favor one particular algorithm or method over any other. It revealed that each generally functions well with regard to one another. The ability for so many independent codes, with independent methods means that the findings are extremely robust. The knowledge they pass on about how our understanding of the universe evolves allows astronomers to make fundamental comparisons to the observable universe in order to test the such models and theories.

The results of this test have been compiled into a paper that is slated for publication in an upcoming issue of the Monthly Notices of the Royal Astronomical Society.

Posted on by Nancy Atkinson

Awe-Inspiring View of the Milky Way

The Milky Way as seen near the Very Large Telescope in the Atacama Desert. Credit: ESO/Y. Beletsky

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The Chilean Atacama Desert boasts some of the darkest skies on Earth – which is why it is home to several telescopes, including the Very Large Telescope. This beautiful panoramic image was taken there, showing the VLT’s Unit Telescope 1, and across on the other side of the image are the Large and Small Magellanic Clouds glowing brightly. Like an arch in between is plane of our Milky Way galaxy. This awe-inspiring image was taken by ESO Photo Ambassador Yuri Beletsky. These photographers specialize in taking images of not only the night sky, but also the large telescopes that give us eyes to see across the great distances of our Universe.

See this ESO page for a larger version of this image.