Sunday, January 10, 2010

Second Smallest Exoplanet Found: HD 156668b (Caution! Geeky Content!)

"Distant Planet is Second Smallest Super-Earth " (January 7, 2010)

"A newly discovered planet light-years from Earth is just four times the mass of our home planet, making the second smallest extrasolar planet to be found to date.

"Astronomers using the Keck Telescope in Hawaii discovered the alien world, called HD156668b.

"The planet sits in a star system about 80 light-years from Earth in the constellation Hercules and orbits its parent star once every four days or so. It is the latest extrasolar planet to join the ranks of the so-called "Super-Earths," worlds slightly larger than our own.

" 'This is quite a remarkable discovery,' said astronomer Andrew Howard of the University of California at Berkeley. 'It shows that we can push down and find smaller and smaller planets.'...

"...The smallest extrasolar planet known today is called Gliese 581 e. It has a mass that is nearly twice that of Earth and orbits a planet-filled star system about 20.5 light-years away in the constellation Libra. ..."

Most of the 400-plus planets found (so far) orbiting other stars are big - often more massive than Jupiter.

HD 156668b joins a relatively small group called 'Super-Earths.' These planets are Earth-like in being rocky, like Earth: but bigger. The 'Super-Earths' found so far aren't likely to harbor life, since they're generally 'way too hot.

The article briefly explains how astronomers use Doppler shift in starlight to detect planets, and gives a pretty good background on where we're at in cataloging relatively nearby planetary systems.

"Nearby" is a relative term, of course. HD 156668b is very roughly 80 light years away. That sounds like a huge distance: and it is, sort of. Light from HD 156668 took about eight decades to get here.

But that's practically next door, on a galactic scale. To give an idea of how 80 light years compares to the size of our galaxy, each pixel in this diagram of the Milky Way galaxy is over 100 light years across:

(from Department of Physics, University of Oregon, used w/o permission)

HD 156668 is a spectral type K2 star, a bit cooler than our sun, but not all that much. With a 'year' about four days long, HD 156668b is likely to be a another very, very hot planet.

Which Reminds Me of the World Where It Rains Rocks

Doppler shift, or 'wobble,' techniques for finding planets work best when the planets are very close to their parent stars. Which explains why so many of the planets found have been in such tight orbits.

And, since they're so close to their suns, really hot.

One, COROT-7b, has an atmosphere that's most likely a mix of sodium, potassium, silicon monoxide and oxygen. The oxygen sounds hopeful - but look at the rest. The air there would essentially be vaporized rock: when it rained, pebbles would be falling from the sky. (October 1, 2009)

Smallest Exoplanet Found - So Far

Gliese 581 e, the innermost known planet in the Gliese 581 system, is not only the smallest known exoplanet - it's got more company than most we know of. The other planetary systems may have many more planets than the ones that have been detected - but we won't know until methods are found of taking closer looks.

(ESO, via, used w/o permission)
"This diagram shows the distances of the planets in our 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. The smallest known exoplanet to date is Gliese 581 e, which is nearly twice Earth's mass. Credit: ESO"

Back to HD 156668b - the Second Smallest Exoplanet - So Far

I did a little checking, and sure enough: like many other stars, HD 156668 shows up in several star catalogs. Besides its Henry Draper (HD) catalog number, 156668, the star is known as BD+29 2979 (Bonner Durchmusterung), G 181-34 (Giclas), and Hip 84607 (Hipparcos catalog). Looks like HD 156668 is about 78½ light years away. (

Finding those other catalog numbers for HD 156668 helped me find out where in the constellation Hercules it is: Right Ascension 259.418701 (17 hours, 17 minutes, 40 seconds, if I'm doing the math right (Astronomy Reader Forum)), declination 29.227207 degrees.

At 78 or so light years, HD 156668 won't be visible to the human eye. Our own star, Sol, is barely visible at 50 light years (Atlas of the Universe): and HD 156668 isn't quite as bright.

So Eggheads Found Some Hunk of Rock? So What?

Honestly, I don't see what practical value finding planets circling other stars has. Today. Or, for that matter, in the next year.

On the other hand, I don't think people have ever benefited from closing their eyes, sticking thumbs in their ears, and humming as loud as they can. And I think the universe is an exciting place - at any scale, microscopic to cosmic.

And, just as I think there's value in music and art, I think there's value in learning more about where we are.

Abstract Knowledge - Maybe More

The last I checked, physicists are still working through implications of some theoretical work by Miguel Alcubierre. If Alcubierre is right, we should be able to travel as fast as we want to: all we have to do is isolate a section of space-time and move it through the rest of space-time. Local velocity would be zero.

That "all we have to do" is a pretty tall order, though. Warping space-time might be possible by moving tiny black holes very fast. But we don't know how to do that. Besides, the power requirements are quite literally astronomical, and even if we could generate - or access - that much energy, there'd still be the matter of shielding the ship from radiation, as its warp field rips the interstellar medium apart at the subatomic level.

Warp ships may be possible - but I'm not holding my breath.

I think we're much closer to developing propulsion systems like the beamed core antimatter rocket engine. Despite the Star Trekish name, the beamed core antimatter rocket engine uses known physics, and technology that isn't outlandishly far from what we've got now. (see "CERN's Large Hadron Collider: A List of Posts ")

Again, I'm not holding my breath. But I wouldn't be terribly surprised if people were building interstellar spaceships in the next few hundred years that could reach Alpha Centauri in maybe 40 years.

That sort of travel time is too slow for tourism, but it's possible that by then we'll also have worked the bus out of maintaining stable, small, isolated habitats. If those (daunting) technical hurdles are cleared, I can imagine a small community - 500 to a few thousand people - willing to spend 40 years in transit, in hopes of setting up a new home.

It's been done before, although not too many people think about - for example - how the Hawaiian Islands were colonized. I'm talking about the people who had been in residence for maybe 1,500 years when Cook found the place: Polynesians, who were at least as good mariners as my Viking ancestors, and who somehow -
  • Found the Hawaiian Islands
    • No small feat, considering the size of the Pacific
  • Crossed a fair fraction of the largest ocean on Earth with
    • People
    • Equipment
    • Seed
    • Livestock
  • And founded a culture which was flourishing well over a millennium later
Their voyage almost certainly didn't take four decades: but you get the point. Communities have pulled up roots and moved before - and there's no reason to suppose that people won't do it again.

Message in a Virtual Bottle

I don't know when starships will be built, or what their engines will look like - although I can make an educated guess. But I think that it's more likely than not, that descendants of mine will see them built.

And perhaps be on them.

I'm descended from people who, not all that long ago, willingly left a safely familiar environment and settled near the center of the North American continent. Whether it's a yen for new horizons, a noble aspiration to fulfill human potential, or some kind of craziness, I don't know. The point is, they went across the Atlantic in none-to-reliable ships: and here I am.

Maybe, when the first ships turn toward the stars, they'll have a store of data on board that includes this post. (Think about it: uploading the Internet can be done - and would be a fairly straightforward way to bring a record humanity's accumulated wisdom and folly along.)

Again maybe, someone for whom I'm a great-to-some-power grandfather will be on board. For you, and all who travel with you, I leave this message:

Never stop learning. I wish you a safe voyage. My prayers are with you.

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