Friday, December 27, 2013

Dust and Fomalhaut: Learning that There's More to Learn

Scientists recently found dust circling Fomalhaut. There's a reasonable explanation for how it got there: but the explanation raises new questions.
  1. The Comets of Fomalhaut
  2. Luhman 16AB: Not Quite Stars, Maybe a Planet
  3. Colliding Comets? Something's Spreading Dust Around Fomalhaut

"Kuiper Belt?"

When I was growing up, Pluto was the most recently discovered planet. Since then, Pluto has been reclassified as a Kuiper belt object and the number of known planets passed 1,000 a few months back. As I've said before, it's an exciting era: for me. Your experience may vary.

We've learned that Sol, our star, isn't the only one with something like the Kuiper belt. These doughnut-shaped zones of leftover construction material are important in two of this post's news items, so here's a quick look at the borderlands of Sol:


(Data from the Minor Planet Center; Carl D. Murray and Stanley F. Dermott "Solar System Dynamics," Cambridge University Press (1999); Art and text from WilyD, Deuar and Serendipodous, all of the English Wikipedia; used w/o permission.)

What the colors mean:
  • Red = The Sun
  • Aquamarine = Giant Planet
  • Green = Kuiper belt object
  • Orange = Scattered disc object or Centaur
  • Pink = Trojan of Jupiter
  • Yellow = Trojan of Neptune
Numbers along the left and bottom edges show the scale in astronomical units. Colored spots show where known objects in the Solar system were in the year 2000. Four outer planets are blue, Neptune's trojans are yellow, Jupiter's are pink. Stuff between Jupiter's orbit and the inner edge of the Kuiper belt are called centaurs, and colored orange in this map.

The map shows gaps in the Kuiper belt that probably aren't there. That blank zone near the bottom, for example, is where the Kuiper belt runs in front of the Milky Way from our point of view. It's hard to pick out dimly-lit chunks of frozen gas scattered among clouds of stars.

Asteroids sharing a planet's orbit, about 60 degrees ahead of and behind the planet, are called trojans. The few known Neptunian trojans are hard to see on that reduced-scale picture. Here's a closer look:


(Data from the Minor Planet Center; Carl D. Murray and Stanley F. Dermott "Solar System Dynamics," Cambridge University Press (1999); Art and text from WilyD, Deuar and Serendipodous, all of the English Wikipedia; used w/o permission.)

Stars and the Search for Life

None of the articles I picked for this post deal with the search for life on other planets: understandably, since those stars probably don't support life.

Fomalhaut is a Class A star, much hotter than our sun, and much younger. Fomalhaut produces quite a lot more high-frequency ultraviolet radiation than our star, which might keep any sort of life from getting started.

If a planet like Earth is at a 'Goldilocks' distance from Fomalhaut, and if life can begin despite the radiation: there might be something like prokaryotes living there. That's a lot of "ifs." The earliest known life on Earth emerged when our planet was about as old as Fomalhaut is now, and that's another topic.

This week's other star, Luhman 16AB, isn't actually a star. It's a brown dwarf, roughly as hot as an oven. There are quite a few reasons why brown dwarfs probably can't support life, even if one has an Earth-like planet in its habitable zone. On the other hand, there are almost certainly an enormous number of brown dwarfs in the galaxy: so maybe a few of them got lucky, so to speak.

Fomalhaut isn't the only well-known star with a planet. Pollux, about 34 light years away in the constellation Gemini, has a planet with a nearly-circular orbit. The planet is roughly as far from Pollux as Mars is from our sun, and several times as massive as Jupiter.

We probably won't find any life orbiting Pollux. The star started as a main sequence Class A star, a bit like Fomalhaut, but ran through its supply of hydrogen and is now a K0 III giant. Any life that emerged on a Polluxian planet would have been fried long ago.

Looking at how many stars have planets, and what we're learning about the durability of life here on Earth, I think it's likely that there's life elsewhere in the universe. We may even have neighbors in the stars: free-willed rational creatures with bodies at least vaguely equivalent to ours.

If that's the case, and we meet them during my lifetime, it won't shake my faith.

More of my take on seeking truth:

1. The Comets of Fomalhaut


(From Amanda Smith, via ScienceDaily, used w/o permission.)
"Artist's impression of the Fomalhaut system. The newly discovered comet belt around Fomalhaut C is shown to the left. The comet belt around Fomalhaut A is in the distance to the right. The belt around Fomalhaut A is offset slightly, a signature of the elliptical orbits in the belt, which may have been caused by past interactions with the star Fomalhaut C. (Credit: Amanda Smith)"
"Companion's Comets the Key to Curious Exoplanet System?"
ScienceDaily (press release) (December 18, 2013)

"The nearby star Fomalhaut A hosts the most famous planetary system outside our own Solar System, containing both an exoplanet and a spectacular ring of comets. Today, an international team of astronomers announced a new discovery with the Herschel Space Observatory that has made this system even more intriguing; the least massive star of the three in the Fomalhaut system, Fomalhaut C, has now been found to host its own comet belt. The researchers published their results today in a letter to the journal Monthly Notices of the Royal Astronomical Society.

"Fomalhaut A is one of the brightest stars in the sky. Located 25 light years away in the constellation of Piscis Austrinus ... Fomalhaut C, also named LP 876-10, is a dim red dwarf star invisible without a telescope, and was only found to be part of the Fomalhaut system in October this year.

"Fomalhaut A's prominence made it a key target for the Hubble Space Telescope, which astronomers used to find the ring of comets, hints of and then a direct image of the planet, Fomalhaut b, in 2008 (astronomers use uppercase letters for stars, and lowercase letters are used for planets, so 'Fomalhaut b' is a planet, and 'Fomalhaut B' is the second star in the system)...."
As press releases go, this one's refreshingly well-written. I'm not sure that the Fomalhaut system is "the most famous planetary system outside our own Solar System," although Fomalhaut's name is arguably more memorable than KIC 11442793 or Gliese 667C.

The Fomalhaut system is well-known, though, at least among folks who keep track of exoplanet research. For one thing, Fomalhaut b is the first planet outside the Solar system to get its picture taken by astronomers.


(From NASA and ESA, via Wikimedia Commons, used w/o permission.)
"...This false-color composite image, taken with the Hubble Space Telescope, reveals the orbital motion of the planet Fomalhaut b. Based on these observations, astronomers calculated that the planet is in a 2,000-year-long, highly elliptical orbit...."
(Ray Villard Space Telescope Science Institute, Baltimore, Maryland; via Wikipedia.)

Another 'named' star. Pollux, a little under 34 light years away, may have a planet. Then again, it may not. One source says that analysis confirmed that Pollux b is real in June of 2006. Another analysis, submitted in July of 2006, said that observed fluctuations were real, that they might be caused by a planet circling Pollux: or something else.1

Both the CalTech/NASA Expolanet Archive and the Paris Observatory's The Extrasolar Planets Encyclopaedia list Pollux b as HD 62509 b. I checked, and Strasbourg University's SIMBAD listing confirms that Pollux is HD 62509 - star # 62509 in the Henry Draper catalog. And that's yet another topic.

Fomalhaut: Dust and Mysteries

"...The new discovery might hold the key to some of the mysteries of the Fomalhaut system. The lead author Grant Kennedy, an astronomer at the Institute of Astronomy at the University of Cambridge, said, 'It's very rare to find two comet belts in one system, and with the two stars 2.5 light years apart this is one of the most widely separated star systems we know of. It made us wonder why both Fomalhaut A and C have comet belts, and whether the belts are related in some way.' To get a feeling for how far 2.5 light years is, light from the Sun takes only 8 minutes to get to the Earth, and 5.5 hours to get to Pluto, and the nearest star to the Sun, Proxima Centauri, is only 4 light years away.

"This discovery may help solve the major mystery in the Fomalhaut system: the orbits of the comet ring and planet around Fomalhaut A are elliptical (which simply means that the orbits aren't circular). The elliptical orbits are thought to be the result of close encounters with something else in the system, perhaps with another as yet undetected planet or perhaps with one of the two other stars, B or C.

"The discovery of the comet belt around C is important because such encounters can not only make the comet belts elliptical, they can also make them brighter by causing the comets to collide more often, releasing massive amounts of dust and ice. Stars are rarely seen to have such bright comet belts, so their detection around both A and C suggests that they may have had their brightnesses enhanced by a previous close encounter between the two...."
(ScienceDaily)
The Fomalhaut star system is wonderfully odd. It looks like the three stars orbit a common center, even though light takes two and a half years to travel between Fomalhaut A and C.

There's an unexpected amount of small debris in the comet ring around Fomalhaut A, which may be caused by comets colliding many times each day: or by something else. I'll get back to that.

2. Luhman 16AB: Not Quite Stars, Maybe a Planet

"Possible Exoplanet Found Orbiting Nearby Binary Luhman 16AB"
Sci-News.com (December 17, 2013)

"Astronomers have detected what they think may be an exoplanet circling a brown dwarf pair called Luhman 16AB, the third-closest star system to the Sun.

"Luhman 16AB, also known as WISE 1049-5319, was discovered earlier this year by Prof Kevin Luhman of Penn State University and his colleagues.

" 'The distance to this brown dwarf pair is 6.5 light years – so close that Earth's television transmissions from 2006 are now arriving there,' Prof Luhman noted.

"Luhman 16AB is only slightly farther away than the second-closest star, Barnard's star, which was discovered 6 light-years from the Sun in 1916...."
Brown dwarfs are stars with the fires out, sort of. A brown dwarf is a ball of gas, mostly hydrogen, bigger than Jupiter: but not massive enough to start hydrogen fusion in its core. These not-quite-stars are about as hot as an oven, and have weather, with clouds the size of planets.

Some have planets, or whatever astronomers decide to call a thing that orbits another thing where the smaller thing is about as massive as Mercury, Saturn, or Jupiter; and the bigger thing isn't massive enough to be called a star.

The universe is simple in some ways, anything but in others, and that's yet again another topic.

Aside from being very close to us, by cosmic standards, Luhman 16AB is special because there may be a third object in the system: which brings me back to that possible planet.

Astronomers at the European Southern Observatory noticed that one of the the brown dwarfs wiggles, or exhibits periodic perturbations, depending on how many syllables you prefer.

It's too early to be sure, but they may have detected effects of a planet. If that's what they've been measuring, the planet's year is between two months and a year long.

3. Colliding Comets? Something's Spreading Dust Around Fomalhaut


(From ESA/Herschel/PACS/Bram Acke, KU Leuven, Belgium, via Space.com, used w/o permission.)
"This infrared image shows the young star Fomalhaut and its surrounding dust disc it as seen with ESA's Herschel space observatory. Astronomers suspect Fomalhaut's debris disc stems from dust particles created by prolific comet collisions, with an average rate of 2,000 daily crashes between comets of 1 kilometer across."
Credit: ESA/Herschel/PACS/Bram Acke, KU Leuven, Belgium
"Comet Demolition Derby Around Star Surprises Scientists"
Space.com (April 12, 2012)

"A young star that is home to at least one alien planet is also ringed by a vast, dusty cloud of comets, like our own solar system. But there's a big difference: There may be as many as 83 trillion comets there, with collisions destroying thousands each day, a new study suggests.

"In fact, there is so much dust around the star that the equivalent of 2,000 comets, each a half-mile (1 kilometer) wide, would have to have been obliterated every day to create the icy dust belt seen today, researchers say. In an announcement of the discovery, European Space Agency officials dubbed the demolition derby a 'comet massacre.'

"The dust also could have been created by few crashes of larger comets – perhaps just two collisions every day between comets 6 miles across (10 km) – but that's still a mind-boggling statistic, they added...."
Astronomers can't see the comets, much less individual dust grains: but are confident about the size of the dust because it's so cold, between minus 382 and minus 274 degrees Fahrenheit. That's minus 230 and minus 170 degrees Celsius.

Fomalhaut's A's dust ring is farther from its star than Sol's Kuiper belt, but Fomalhaut is brighter than our star: so those dust grains should get blown away by pressure from Fomalhaut A.

Fomalhaut is young, for a star: but stars can last a long time. Dust has been getting swept out of Fomalhaut's 'Kuiper belt' for about 440,000,000 years. After nearly half a billion years, the original dust should have long since been blown into the abyss between stars.

Something's replacing it, and debris from colliding comets are among the less-unlikely sources.

Debris, Data, and Explanations


(From NASA, ESA, and A. Feild (STScI); via Space.com, used w/o permission.)
"This illustration shows the size of the debris disc observed around the star Fomalhaut, as compared to the size of the Kuiper Belt and asteroid belt in our solar system."
"...The crashing comets encircle the star Fomalhaut about 25 light-years from Earth. Acke and his colleagues studied the comet belt with the European Space Agency's far-infrared Herschel space observatory, which spotted the telltale dust created by the constant collisions of comets in motion, the researchers said. [Latest photos from Herschel observatory]

"Depending on comets' sizes, there could be between 260 billion and 83 trillion comets in the dust belt around the star, the researchers found. If you combined the amount of material in Fomalhaut's dust belt, the mass would be the equivalent of 110 Earths, they added.

"Fomalhaut's comet belt arrangement is similar to the Kuiper belt of icy objects beyond the orbit of Neptune in our own solar system. Scientists have known about a dust cloud surrounding Fomalhaut since the 1980s, though now the Herschel observatory has revealed the ring in greater detail than ever before...."
(Space.com)
Maybe between two and 2,000 comets run into each other every 24 hours in Fomalhaut A's comet belt, or maybe something else is kicking up dust. This isn't dust in the British sense, by the way: rubbish readied for disposal. Fomalhaut's dust is very tiny particles, about a millionth of a meter across.

Happily, Fomalhaut is so close and so bright that observing it is comparatively easy. If comets have been in a sort of demolition derby there, scientists will almost certainly get enough data to explain why they're colliding: the comets, not the scientists.

If scientists learn that the dust couldn't come from comets - - - I'll be very interested in seeing what other explanations get published.


(From Grant Kennedy / Paul Kalas, via Sci-News.com, used w/o permission.)
"View of the Fomalhaut triple star system from Earth; the small inset shows a zoom of the newly discovered comet belt around Fomalhaut C as seen at infrared wavelengths by Herschel. Image credit: Grant Kennedy / Paul Kalas."
(Sci-News.com)

More about Fomalhaut:
Related posts, about science and:

1 Pollux almost certainly has a planet:

2 comments:

Brigid said...

Wrong word: "even if one has an Earth-like planet it its habitable zone."

The Friendly Neighborhood Proofreader

Brian Gill said...

Brigid,

Oops. Found, fixed, thanks!

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