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First planet found via astrometry! June 1, 2009

Posted by CosmicThespian in Discoveries, News, Planet Gallery.
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A team of astronomers at the NASA Jet Propulsion Laboratory (JPL) announced last week that they are the first group to find an exoplanet using the astrometric wobble method!

As I discussed in one of my first blog articles, planets gravitationally tug on their host stars as they orbit around them.  Astronomers have long known that if they wanted to find distant planets, they would need to first find stars that appeared to wobble on the celestial sphere.  Unfortunately, these wobbles are impossibly small and up until now no team has ever been able to measure them.  The wobbles we have been detecting have been found via doppler shifts in the light from these planet hosting stars.

Drs. Steven Pravdo and Stuart Shaklan have broken through that barrier!  For the past 12 years, they have been experimenting with an instrument specifically designed to directly detect the slight wobble of distant stars on the Palomar Hale Telescope.  Over those twelve years, they’ve only had access to the telescope a few times each year.  But it appears all of their efforts have finally paid off!

The planet is a big one: six times more massive than Jupiter!  It orbits a very tiny star, VB 10, roughly 20 light years away in the constellation Aquila.  The size of its orbit is similar to Mercury’s, but because its sun is so very faint, the planet receives about as much light from its sun as Jupiter does from our own.

Schematic of the VB 10 system compared to our own.

Schematic of the VB 10 system compared to our own. Image credit: NASA/JPL-Caltech

The diminutive size of the star coupled with the mass of the planet are, no doubt, what led to a successful discovery.  If you recall from my original article on detecting wobbles, the name of the game here is the location of the center of mass of the system.  A high mass planet orbiting a low mass star will move the system’s barycenter further from the center of the star thus leading to a larger wobble of the distant sun.  Despite this advantage, the wobble is still incredibly tiny: the span of the wobble is no larger than the width of a human hair seen at over a mile-and-a-half away!

VB 10 is particularly tiny and for a time was the smallest star known.  It is a type of star known as a ‘red dwarf’.  It weighs in at just one-twelfth the mass of our Sun and isn’t much bigger than Jupiter!  In fact, despite the significant difference in mass between VB 10 and its new-found planet, the two would actually be roughly the same size!

This is an exciting new chapter in the unfolding saga of exoplanet discovery and is an important reminder of the potential of astrometric surveys.  Future space missions will take advantage of this technique to find Earth-like planets by the dozen in the next decade or so.  This journey of 1000 miles has taken its first step!

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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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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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