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Old, rich, and eccentric April 23, 2009

Posted by CosmicThespian in Discoveries, News, Planet Gallery.

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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4. ethanol - April 29, 2009

That eccentricity has to produce some interesting atmospheric dynamics. I wonder if the planet swells and shrinks over its orbit or whether it is too big for its temperature to be significantly effected over that time period.

5. cosmicthespian - April 30, 2009

That’s an interesting observation and one to which I don’t know the answer. I know that gas giants that are very close to their stars (within 0.1 AU) are observed to be bloated due to the high levels of stellar flux heating their atmospheres. These planets may orbit far enough from their stars so it may not be an issue. Also, I don’t know what the timescale is for a planet’s atmosphere to react to changing flux levels. If it only takes weeks or months to react, your scenario could have merit. If it’s a process that takes many years or decades, then the planet would be orbiting faster than its atmosphere could react.

I’m going to look into this some more…

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