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Carnival of Space #100 April 28, 2009

Posted by CosmicThespian in Carnival of Space.
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The 100th edition of the Carnival of Space is online at the One Minute Astronomer blog.  There are 28 articles gathered from around the space blog-o-sphere for your reading enjoyment ranging from goings-on in our own solar system and practical advice for star party planners, to distant galaxy collisions and musing on neutron stars.  Plus a link to my own article on the recent discovery of two old, rich, and eccentric Jupiter-class worlds!

Go explore!

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Old, rich, and eccentric April 23, 2009

Posted by CosmicThespian in Discoveries, News, Planet Gallery.
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Add two more planets to the zoo of known worlds orbiting other stars!

A team of astronomers recently announced the discovery of two Jupiter-class planets; two gas giants in distant solar systems.

The discovery is part of two ongoing searches, one using the 0.6-meter telescope at Lick Observatory and the other with the massive 10-meter Keck Telescope.  Both of these efforts, started in 2004, are using the doppler shift method to search for the tell-tale wobble of a star induced by an orbiting planet.

The stars around which these worlds orbit are a bit different than our Sun.  Both are what astronomers call “subgiants”.  For most of a star’s lifetime, it sits quietly converting hydrogen into helium; the released energy from this “thermonuclear fusion” is what provides the energy needed to hold a star up and prevent it from collapsing on itself.  As a star ages, the hydrogen fuel in its core is eventually used up.  When this happens, the region where hydrgogen fusion occurs begins to move out from the star’s center.  This creates a “shell” of hydrogen fusion surrounding the now inert helium core.  The movement of hydrogen fusion from the core out to higher layers forces the star to balloon.  It puffs up and the temperature at its surface begins to drop.  It is at this point in a star’s life that we find the so-called subgiants: a star entering its retirement years, not quite big enough to be full-fledged giants, but bigger than the star was through most of its life.

The stars are also what astronomers call “metal-rich”.  Astronomers have a habit of calling any element that is not hydrogen or helium a “metal”.  Yes, we know it’s not the technical definition of a metal.  But astronomers are lazy.  The Universe is overwhelmingly composed of hydrogen and helium.  It’s just easier for us to group the couple of percent of matter that isn’t hydrogen or helium under one label.  So when an astronomer says a star is metal-rich, she means that the star has an overabundance of elements heavier than helium; specifically, an overabundance relative to our Sun.  This can be determined by looking at a star’s spectrum and figuring out what elements are present in its atmosphere.

One of the interesting findings over the past decade is that there appears to be a correlation between the amount of “metals” in a star’s atmosphere and the likelihood that star will have planets.  The more heavy elements, the greater chance of finding a planet.  This most likely has something to do with how planets form.  Those heavy elements (like carbon, oxygen, and silicon) make up the bulk of the rocky material that forms terrestrial planets and possibly the massive cores of gas giants.  The higher metal-content of these stars may reflect a larger source of material from which a developing solar system can draw to create new worlds.  Because of this correlation, some planet searches (like the ones that discovered these two new worlds) focus on “metal-rich” stars to increase the chances of finding something with the precious little telescope time they can get.

One of these planets orbits a star called HD 16175; a metal-rich subgiant located roughly 200 light years away in the constellation Perseus.  The planet is 4.5 times more massive than Jupiter. It orbits at about twice the Earth-Sun distance once every 2.7 years.  In our solar system, this would place this planet at about the inner edge of the asteroid belt.

The second planet was found around a star known as HD 96167; another metal-rich subgiant sitting at a distance of 270 light years in the dim constellation Crater, just south of Virgo.  This planet orbits only 30 percent further from its star than the Earth does from the Sun, which would put it about halfway between the orbits of Earth and Mars.  One year on this world, which has roughly 70 percent the mass of Jupiter, is about 1.3 Earth-years.

What really makes these planets stand out from anything in our solar system is that both of these worlds are on highly eccentric orbits.  By eccentric, I’m not referring to bizarre tastes in clothing or art; I’m referring to how “stretched out” their orbits are.  Most of the planets in our solar system have nearly circular orbits.  They are not perfect circles, however.  Their orbits are actually ellipses.  That means the distance between the Sun and any planet smoothly varies over the course of an orbit.  When a planet reaches its closest point to the Sun, we say the planet is at perihelion; when it’s at its furthest point, we say it is at aphelion.  The difference between perihelion and aphelion for most of the planets in our Solar System is pretty small.  For the Earth, the difference in distances is only about 3 percent.  Mercury, the planet in our Solar System with the highest eccentricity, has an aphelion distance which is 60 percent larger than its perihelion distance.

These newly discovered worlds, however, are on very stretched out orbits.  The planet around HD 16175 has a closest approach four times closer than its furthest approach while the one orbiting HD 96167 changes distance by nearly six times!  Imagine the Earth swinging from its current orbit to out past the orbit of Jupiter and back again every year and you have some idea of how crazy that is.

How planets end up on such crazy orbits is a matter that is currently being researched.  These two worlds aren’t alone; many of the new worlds we’re finding sit on highly eccentric orbits.  The leading hypothesis is that interactions between closely spaced planets might affect their orbits.  If two planets get too close, the lighter one can get ejected from the planetary system entirely while the remaining, more massive, world is left behind on a very elliptical orbit.  This is the same principle we use to slingshot probes out into deep space by stealing momentum from the planets.  We may be seeing the remnants of long-past interplanetary bumper cars!

The paper detailing these findings can be found here.

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Discovery of lightest known exoplanet April 21, 2009

Posted by CosmicThespian in Discoveries, News, Planet Gallery.
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A team of astronomers released two new bits of information on the Gliese 581 planetary system: one is  the discovery of a planet that is just two times more massive than Earth and the other is the realization that one of the worlds in this system is a candidate for hosting liquid water on its surface!

Gliese 581 is the name of a star 20.5 light years from the Sun in the constellation Libra.  That’s over 120 trillion miles away!  For the curious skygazer, Libra can be seen at this time of year rising in the east a little after 10 P.M; go take a look!

The star itself is what astronomers call a “red dwarf”.  The red color comes from the stars relatively cool temperature: about 5800 degrees Farenheit.  It’s also only about a third the mass of the Sun.  Red dwarfs are a favorite target of planet hunters because their lower mass means they are easier for planets to push around, thus giving them a larger wobble.  Think about what’s easier for you to push: a ping-pong ball or a basketball.  Apply the same force to both, and the ping-pong ball will go farther.  The same is true for stars and planets: put the same planet at the same distance from two different mass stars, and the lower mass star will respond with a larger wobble.  Larger wobble = easier to detect!

Gliese 581  has gained some noteriety in the past few years as the host of a multi-planet system; this most recent discovery brings the total number of worlds around this star to four.  The planets, known as b, c, d, and (now) e have masses 16, 5, 7, and 1.9 times the mass of the Earth, respectively.  The furthest planet out (d) orbits Gliese 581 in 66 days (Mercury, for comparison, orbits our Sun in 88 days).  The newest discovered planet is also the one closest to its sun; one year on Gliese 581 e is just over 3 days!  The astronomers responsible for this discovery believe that this world is most likely a rocky planet not unlike the four rocky worlds that orbit the Sun.  This remarkable discovery brings us one more step closer to the holy grail of exoplanet research: an Earth-like planet!

The same group was also able to refine the orbital parameters of another world in this system, Gliese 581 d.  In doing so, they found that this planet orbits in what astronomers refer to as the “habitable zone” of this star.  The habitable zone is the region around a star that is “just right” for liquid water to exist on the surface of a planet.  If a planet is closer than this region, it is too hot and water will simply evaporate existing only as steam; too far out, and water condenses into ice.  But at just the right range of distances, liquid water can flow.  As Stephen Udry, one of the astronomers on the team who announced these discoveries, says: “…it is the first serious ‘water world’ candidate.”

This diagram shows the distances of the planets in the Solar System (upper row) and in the Gliese 581 system (lower row), from their respective stars (left). The habitable zone is indicated as the blue area. (Image Credit: ESO)

This diagram shows the distances of the planets in the Solar System (upper row) and in the Gliese 581 system (lower row), from their respective stars (left). The habitable zone is indicated as the blue area. (Image Credit: ESO)

ESO Press Release.

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Carnival of Space #99 April 20, 2009

Posted by CosmicThespian in Carnival of Space.
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The 99th edition of the Carnival of Space is posted at Alice’s Astro Info.  At the top of the list is….yours truly!  And apparently I’m “piping hot”.  Look out astronomy blogging community — innumerable worlds is starting to grow!  Go check out this week’s Carnival and see what’s going on in the space-related blogosphere this week.

And to new visitors who have stopped by via the Carnival, welcome!  Kick back, relax, and hang out for a little while.  So glad to see you.

Clear skies!

Kepler First Light! April 16, 2009

Posted by CosmicThespian in News, Space Missions.
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Last week, the Kepler space telescope released its dust cover.  Today, a NASA press release announces that Kepler has taken its first images of the star field within which it will search for transiting exoplanets!

The image below illustrates where in the sky Kepler will spend the next three and a half years looking for these distant worlds.

keplerfov

The patch of sky straddles the constellations of Cygnus and Lyra, constellations which are visible high overhead in late summer evenings from the Northern Hemisphere.  This patch of sky covers about 100 square degrees, which is about the equivalent of looking at your outstretched hand held at arms length.  The images released to the public by the Kepler team consist of two images which cover this entire region plus three more images which  “zoom in” on areas within this field of view.

329156main_tres2nocalloutshot300-516

The above image subtends a tiny portion of Kepler’s field of view – only one-thousandth the size of the total search area.  Hundreds of stars are in this image, each a possible host to distant solar systems and planets like our own.  If they’re out there, Kepler will find them.  The work of the Kepler science team over the next few weeks will focus on calibrating the instruments.  They will start by looking for transiting planets which are already known to exist in order to measure the sensitivity and accuracy of the telescope.

Check out the other released images.  Kepler’s great planet hunt is about to begin!

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Changing phases of CoRoT-1b April 13, 2009

Posted by CosmicThespian in Discoveries, Planet Gallery, Space Missions.
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400 years ago, Galileo became the first person to observe the phases of Venus.  These observations were proof that the Sun was at the center of the Solar System, not the Earth, and a revolution in our understanding of the cosmos was begun.  Today we are able to observe the changing phases of planets orbiting distant stars!  In a recent paper, astronomers at the Leiden Observatory in the Netherlands published evidence of the different phases of an exoplanet called CoRoT-1b.

CoRoT-1b is the first planet discovered by the CoRoT mission.  CoRoT is a space telescope mission led by the French Space Agency (CNES) which will, among other things, search for exoplanetary transits – the brief dimming of a star’s light as the planet passes between us and its host star.  The planet was discovered in 2007 orbiting a star, CoRoT-1, that is very similar to our Sun.  Planet and star are located 1500 light years away in the constellation Monoceros which means the light we are now seeing left the system roughly 30 years after the fall of the Roman Empire!

While the star may look familiar to humans, the planet is unlike anything in our Solar System.  Almost the same mass as Jupiter and 50% wider, this distant gas planet orbits just 2 million miles from its sun which is only 2% of the distance between Earth and our Sun!  On such a tight orbit, the planet whips around its star in just one and a half days!!  For comparison, Mercury orbits 36 million miles from the Sun once every 88 days.

Sitting so close to its sun has set CoRoT-1b to a slow broil.  Daytime temperatures hover at just under 4000 degrees Farenheit! The intense heat from CoRoT-1 has caused its atmosphere to puff up giving it its unusually large size.

corot1phases

This figure, taken from the linked paper, shows what the authors believe is evidence of the changing phases of this bloated Jupiter as it swings around CoRoT-1.  The top panel is what astronomers call the star’s “light curve” – a plot of the star’s light as it changes over time.  Every dot is a different measurement of the star’s brightness.  The two big dips in the curve at the edges occur when the planet passes in front of the star and blocks a small fraction (about 2%) of its light.

The middle panel shows the same thing but “zoomed in” so that you can see more subtle changes in the star’s light.  They’ve also taken the observations and “binned” them; that is, they break up the observations into evenly spaced chunks (or bins) of time and then take the average brightness within each chunk.  What you see is that, between the transits, the combined light of the star and planet gradually increases by a whopping one-hundredth of a percent.  At a time exactly midway between the transits, there is a sharp decrease in the brightness back to the normal level of the star and then the brightness jumps up again and slowly decreases by the same amount until the next transit occurs.

That looks pretty wierd!

Until you look at the picture at the bottom!  The strange shape of the star’s light curve is consistent with the changing phase of CoRoT-1b as it orbits its sun!  During the transit, the planet has its back to us.  We are looking at its night time and the planet is blocking light from the star.  As soon it moves out of the way, the star light jumps back up.  As the planet swings around on its orbit, we see more and more of its day side (in the exact same way that we see the phase of the Moon or Venus change!).  Since the day side is brighter, the more of it we see, the more light we receive.  Just as we’re about to see the full face of the planet, it disappears behind the star.  The star has eclipsed the planet, an event astronomers usually call a secondary transit.  When that happens, we’re only seeing the light from the star.  When the planet reappears on the other side of the star, we suddenly get light from both star and planet and the light jumps up again.  As the planet continues back around, we see less and less of the day side and thus the combined light from planet and star decreases until the planet once again transits its star and the light plummets.

Observations like this are important tests of our planet detections.  Both doppler measurements and transits can be tricked by normal activity on the surface of the star.  These data provide additional confirmation that we are really seeing the light from a world 9 quadrillion miles away!

I think Galileo would be very proud.

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Kepler blows its top April 9, 2009

Posted by CosmicThespian in Space Missions.
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Kepler is a space mission to search for extrasolar planets by looking for periodic transits – the dimming of a star’s light as an orbiting planet occasionally passes between the star and observers on Earth. Launched on March 7 aboard a Delta II rocket, the telescope will trail behind the Earth on a heliocentric (Sun-centered) orbit for 3.5 years. During its mission, the telescope will stare at the same patch of sky in the constellation Cygnus, monitoring 100,000 stars for the occasional transit of a planetary companion. The instrument is designed to be able to detect Earth-sized planets orbiting on Earth-like orbits around distant stars! Project scientists estimate that over its lifetime, Kepler could discover around 50 Earth-like worlds in this region of the Galaxy and many hundreds of other types of planets. A new age of exoplanet discovery is about to begin!

This is the type of science that can only be done from space. Planetary transits last for only a few hours and may only occur once or a few times a year. To maximize detection, one needs to stare at the same stars for several years. This simply can’t be done from Earth. Earth-bound observers can not use their telescope during the day and most areas of the sky are unavailable to us for large portions of the year as the Earth travels around the Sun. Plus, the change in brightness of a star due to the transit of an Earth-like world is very tiny; roughly one part in 10,000! To be able to definitively detect such a miniscule dip in brightness, we need a telescope that sits above the interference of the Earth’s atmosphere where turbulence and other meteorological effects can either mask the signal or produce false detections. In space we don’t have to worry about day and night or deal with the atmosphere. We can just stare!

Yesterday, the Kepler space mission achieved an important milestone. The telescope cover was released from the instrument and light has begun to fall onto the electronic detectors that will capture the images. Now begins the several week long process of calibrating the instrument by using images of stars before actual science operations can begin. The dust cover is now drifting away from Kepler on its own orbit around the Sun. From the JPL press release:

“The cover released and flew away exactly as we designed it to do,” said Kepler Project Manager James Fanson of NASA’s Jet Propulsion Laboratory, Pasadena, Calif. “This is a critical step toward answering a question that has come down to us across 100 generations of human history — are there other planets like Earth, or are we alone in the galaxy?”

An animation of the dust cover release can be found here.

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The Wobbling Sun April 3, 2009

Posted by CosmicThespian in Tidbits, Toolkit.
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In a previous post, we discussed how a planet and its star orbit a common center (the barycenter).  This causes the star to wobble in space as the planet orbits and tugs on it.  In most cases, astronomers searching for extrasolar planets use this wobble to infer the presence of another world rather than directly seeing the planet itself.

I came across a paper a couple of weeks ago: “Exoplanets – search methods, discoveries, and prospects for astrobiology” (B.W. Jones, 2008).  The paper is a summary of exoplanet detection methods written for astrobiologists, scientists who work at the crossroads of physics, astronomy, biology, geology, oceanography, chemistry, etc. to understand the cosmic origins of life and the prospects for the development of life elsewhere in the Universe.  One of the figures he uses in the paper I found to be interesting and relevant to the past several posts on this blog:

sunwobbleThe figure shows the wobble of our Sun over a span of 50 years viewed from 30 light years away.  Our Sun’s wobble is complicated; it’s driven mostly by the competing gravitational tugs from eight worlds!  The dots show the location of the Sun at different years (starting in 1975) while the solid line traces out the path of the wobble.   The dashed line shows what the Sun’s wobble would be if Jupiter were the only planet in our solar system.  As you can see, Jupiter is responsible for much of the Sun’s apparent excursion across the sky.  This is because Jupiter is the most massive planet in the Solar System – roughly 300 times the mass of the Earth and 2.5 times more massive than all the other planets added together!  All of that mass means that Jupiter is the most influential planet on our Sun’s motion with the other seven major worlds contributing various amounts of perturbation on top of that motion.

The disk in the upper right shows how large the Sun would be on this scale.  It’s interesting to note that the amount the Sun moves over 50 years is roughly the size of the Sun itself.

The numbers along the side and the bottom indicate the angular scale on the sky; this box is 2 milliarcseconds on a side. That’s 2 millionth’s thousandths of an arcsecond or half a billionth millionth of a degree!  That is incredibly small.  It’s roughly equivalent to the thickness of a dime seen from 140 km, or just over 80 miles, away!

What’s also important to remember is that this is seen from only 30 light years away.  The further away you get, the smaller the wobble will appear.  30 light years encompasses only our very local neighborhood; the Milky Way galaxy is roughly 100,000 light years across!  Looking at a star that far away is peering only 3 hundredths of a percent across the length of our Galaxy!!

Clearly,  when we start detecting the wobbles of stars within 30 light years of our Sun, we will have barely scratched the surface of what could be out there!

Edit 4/22: Because I forgot how the metric system works.  (Thanks to GaryC for that catch!)

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Planet Hunting Toolkit V: Pulsar Timing April 2, 2009

Posted by CosmicThespian in Toolkit.
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The first planets discovered outside of our Solar System were found in what is probably the least likely place astronomers would have expected to find them: in orbit around the fading embers of a now dead star.

The most massive stars in the Universe do not fade away quietly. These stars, about eight or more times the mass of our Sun, end their lives in one of the most violent events in the Universe: a supernova! A supernova occurs when a massive star can no longer fuse elements in its core and provide the energy necessary to support its own bulk. When that happens, the star begins to implode. The core is crushed to a hyperdense state and the infalling material effectively bounces and sends a supersonic shockwave rippling back up through the star. The result is a cataclysmic explosion. The energy released in a supernova is roughly equal to the total amount of energy our Sun will radiate in its entire 10 billion year lifetime. Seen from across the Universe, a supernova can temporarily outshine its host galaxy. Imagine a single star shining with the brilliance of a hundred billion stars for nearly a month!

Multiwavelength image of Kepler's Supernova remnant.  [Credit: NASA/ESA/JHU/R.Sankrit & W.Blair]

Multiwavelength image of Kepler's Supernova remnant. (Credit: NASA/ESA/JHU/R.Sankrit & W.Blair)

For stars less than about twenty times the mass of our Sun, the stellar core is left behind to become what’s called a neutron star. The densities and temperatures inside the remnant core are so high that protons and electrons actually fuse together to form neutrons. The state of the matter beneath the surface isn’t well understood – neutron stars stretch the limits of our understanding of physics. What we do know is that a neutron star has roughly twice the mass of the Sun but is only about 15 miles in diameter which means it would fit comfortably within the limits of a small city. The gravity around a neutron star is so strong that an object dropped from just one meter above the surface will strike the star at 4.3 million miles per hour!! The density is such that one teaspoon of material from the surface would weigh the same as 15 times the world human population. It’s no understatement to say that neutron stars are among the most exotic objects we know of.

But, what does this have to do with finding planets?

Well, as the core collapses to form a neutron star, the conservation of angular momentum demands that it increase its rotation speed. This is exactly the same principle that causes a figure skater to spin faster as she brings her arms closer to her body. Additionally, the increased magnetic fields surrounding the star can form jets of highly energetic charged particles to shoot out from the magnetic poles. As the star spins around, this beam of particles sweeps out an arc through space, much like a lighthouse. If the Earth happens to lie in the path of the beam, the star will appear to us to be “pulsing”. Hence, astronomers call these objects pulsars.

Anatomy of a pulsar.  The cones indicate the presence of energetic particle beams that are swept through space as the star rotates.

Anatomy of a pulsar. The cones indicate the presence of energetic particle beams that are swept through space as the star rotates.

Pulsars can spin incredibly fast. The fastest pulsars have a rotation period of just a couple of milliseconds (the so-called millisecond pulsars). Imagine a star the size of a city spinning roughly a thousand times a second! But what’s of greater interest to us is how steady pulsars are. The accuracy of the pulsed beams coming from these stars can rival that of atomic clocks on the Earth. In fact, when the first pulsar was discovered, the steadiness of its radio pulses caused some to speculate (half-jokingly) that it could be a radio beacon from extraterrestrials. Hence, the object was nicknamed LGM-1, an acronym which stands for “Little Green Men”.

Composite X-ray and optical image of a pulsar in the Crab Nebula. [Credit: NASA/ESA]

Composite X-ray and optical image of a pulsar in the Crab Nebula. (Credit: NASA/ESA)

Because the pulsars are so steady in their timing, periodic deviations could signify the presence of an unseen body in orbit. Much like a star’s absorption spectrum will be doppler shifted by an orbiting world, a similar thing can happen to the pulse train from a pulsar. As the pulsar is tugged towards the Earth, the time between pulses will shorten slightly; as it moves away, the pulse train gets “stretched out” causing the pulsar timing to get longer. If this happens with a fixed period, that means the pulsar is moving back and forth in space which indicates that something – a planet – is pulling it around.

The very first planets ever discovered were found in orbit around a pulsar. In 1992, astronomer using the Arecibo Radio Telescope in Puerta Rico discovered just such a periodic variation in the timing of the pulsar PSR B1257+12, a millisecond pulsar 980 light-years away in the constellation Virgo. Analysis of the variations indicated that not one, but two planets were in orbit around the pulsar! Further study eventually revealed a third world in this exotic system.

Artist impression of the PSR B1257+12 planetary system. [Credit: NASA/JPL-Caltech/R. Hurt (SSC)]

Artist impression of the PSR B1257+12 planetary system. (Credit: NASA/JPL-Caltech/R. Hurt (SSC))

Only four planetary systems have been found around pulsars. Their existence is something of a mystery. It was surprising to astronomers to find planets in orbit around a neutron star; one would think that any planets wouldn’t have survived the supernova explosion. Either they were able to withstand the shockwave of the supernova or they formed from the material that was blown off after the explosion; either hypothesis is equally intriguing.

One of the most fascinating findings in the past fifteen years of exoplanetary research is that planets continuously turn up in places we would never expect to find them!

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