Friday, October 19, 2012

A Comet Ride-by: Spotting Exo-Planets

I swore I'd just let this blog quietly fade in the sunset of years yore. But lately I've been chock-full of astronomical excitement, so had to snag a passing comet and get back here.

NASA Images: the sharpest picture yet of Mars, including its ice cap.

Planets, those fellow massive worlds, are thus called from the greek word for "wanderer" for how they move against the stars.  Until Gailieo, that was how we knew planets: observations, tracking their movements, trying to figure out why Mars went retrograde. They were gods to many religions. The Mayans developed complex calendars based on the orbit of Venus. Then-- technology!

Telescopes let us see their moons, even their complexion.  Space probes have visited, taken snapshots, even sampled their atmosphere. Now we have robots poking around Mars. And still, we observe and do math to figure out their quirks.

When it comes to exo-planets, meaning these worlds wandering around other stars, we have powerful telescopes, logging tons of data which computer munch, but machines do not discover the unexpected. They can't.

So, it all comes back to the human eye noticing oddly steady patterns in the night sky created by planets moving against stars. We recognize patterns far better than any computer; that sudden twitch in the grass could be a snake, or a tiger's tail.

Abrupt darkness on a star's brightness graph may well be a planet eclipsing the star during its orbit. Once the exo-planet is discovered, then it can be observed and further analyzed with the help of physics and machines.

But who will discover those planets? Planet-hunters, of course-- and you could be one.

And that's why I'm riding this comet over the moon. In the last few months we've discovered far more exoplanets than humanity has discovered planets in our solar system over millennia. They're popping out everywhere!

Alpha Centauri Bb, as we had hoped, has an earth-sized planet just 4.2 light years away from us, but it's orbiting at a Mercury-like distance, with an total year of 3.2 days.  We hope to find more planets in the star's habitable zone from their transit of the sun-- it just may take a few months, in case they're still on the other side.

Speed does matter, and size, in helping spot the planets. The first exoplanets discovered were gas giants, even bigger than Jupiter.  In addition to the increased light changes from such a big planet, a big planet can also create a wobble in a star's orbit.

Thus far we've discovered many planets in the habitable zone  (also known as the Goldilocks zone) of red dwarf stars. They're easier to spot red dwarf stars, at their biggest, have a surface mass less than half that of the sun, and temperatures to match. Since radiation obeys the inverse square law, that means the goldilocks zone is around 1/4  closer than our sun's-- for the biggest red dwarfs.

Most are far smaller. For instance, Barnard's star, a red dwarf only 6 light-years away, is 14.4% the mass of our sun.  Therefore its Goldilocks zone would be less than 2% the distance of our sun's zone, meaning any habitable planets would be orbiting much closer than Mercury does to our sun!  It is no suprise that millions of earth-size planets are estimated to orbit the habitable zone of red dwarf stars.  We've detected some habitable-zone planet around more sun-like stars, often "super-earths"
 The longer we study the skies and the data, the more we detect smaller planets , with the smallest being earth-twins orbiting around Kepler-20. Unless somebody's discovered even smaller, which is very likely; it just needs time to be confirmed.

But that's not all that's exciting me. You see, a lot of stars live with buddies. There are binary, trinary, quadruple, etc. star systems.  Finding multiple-sunned planets, of course would be great for people afraid of the night,  and vindicate generations of science fiction writers and filmmakers, but more importantly, such exoplanet discoveries help answer important questions of how planets may form and how they would orbit more than one star.

Composite picture of Asteroid 951 Gaspara and Phobos and Demios, Mar's two moons (NASA, 1977).

We have learned how complex orbitics can be from studying asteroids in horseshoe orbits, such as Cruithne, often referred to "Earth's Second Moon"-- which has a cycle in Earth's orbit which takes 770 years to complete, as this cool graphic shows.  

Now let your mind boggle at the recent discovery of PH-1  (yes, through Planethunter!) which is in a binary binary system:  PH-1 is orbited by two stars, and in turn orbits two other suns (meaning it has four suns.) 
Interestingly, PH-1 is 6.0 earths in radius, slightly fatter than Neptune, half the width of Jupiter, but we don't know its mass yet.  The complex orbitics make it hard to judge
 So we don't know yet if it is a gas giant or something different.  It seems unlikely it could be a diamond planet  (the barren core of a long-dead star) since such a planet would be very dense and far more massive.  But it'll be interesting to see the final mass estimates; it might tell us something about how this planet came to be.

The skies are full of wanderers.  Go ahead, catch a falling planet.

And I'm off on my comet again and may not be back for years.


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