A Bit More Detail

Assorted Personal Notations, Essays, and Other Jottings

Posts Tagged ‘planets

[BLOG] Some Monday links

  • Centauri Dreams takes a look at Pluto.
  • The Dragon’s Tales links to a paper suggesting that the “young faint sun paradox” can be explained by high but not very high levels of carbon dioxide and methane on the early Earth.
  • Eastern Approaches argues that Poland isn’t going to become the Saudi Arabia os shale gas any time soon.
  • Far Outliers takes a look at overlooked interracial fluidity and family in the American South.
  • Inkless Wells’ Paul Wells, writing at MacLean’s, wants greater press access to the Lac-Mégantic catastrophe.
  • Language Log takes a look at the failure of artificial intelligence as evidenced by the nonsensical conversations of a pair of Siri bots.
  • The Planetary Society Blog has a guest writer suggesting that even under NASA’s budget strictures, a Uranus probe could be possible.
  • Noel Maurer at The Power and they Money makes the case that arming the Syrian rebels shouldn’t be done, in that the outcomes produced by non-supply–a weakened regime or a weakened transition–are less threatening to American interests.
  • Towleroad links to a paper suggesting that homophobia is associated with fear of unwanted sexual advances.
  • Window on Eurasia quotes a Russian writer who argues that, if the Soviet Union had survived, immigration to Russia would have been substantially heavier and more politically controversial.

[BLOG] Some Wednesday links

  • Centauri Dreams’ Paul Gilster considers recent research suggesting that hypothetical “waterworlds”–broadly Earth-like planets with very large amounts of water, covering their rocky surfaces entirely–might in fact enter into “moist greenhouse” phases that would see their excess of water evaporate, leaving an Earth-like planet with an Earth-like combination of rocky and watery surfaces behind.
  • Geocurrents’ Nicholas Baldo highlights various bodies of watyer with exceptionally high salt content, from Antarctica to Turkmenistan.
  • Language Hat notes some interesting linguistic soupçons, unusual loanwords and the ease of post-war travel and the like, in Laurence Sterne’s A Sentimental Journey.
  • In one post at Lawyers, Guns and Money, Erik Loomis starts a discussion about continuing high levels of inequality in South Africa as evidenced by the recent massacre of miners, while Scott Lemieux starts a discussion about misogyny in the United States’ Republican Party as illustrated by Akin’s heinous comments.
  • New APPS Blog’s Mohan Matthen takes significant issue with Perry Anderson’s recent criticisms of Gandhi and Nehru in the London Review of Books.
  • The Power and the Money’s Noel Maurer argues from Canada’s existing experience embedded in international property rights protection law regimes that ratifying the International Centre for Settlement of Investment Disputes Convention won’t change anything.
  • Registan’s Casey Michel is unimpressed with pro-Wikileaks Uzbekistan critic Craig Murray’s use of his own wife’s rape to attack Assange’s accusers.
  • Supernova Condensate’s presents his own, purely mass-based, classification of celestial objects in response to the ongoing dwarf planet controversy.
  • Daniel Little at Understanding Society presents an interesting argument from a pair of sociologists in 2007 to the effect that political polarization can more often be not be an artifact of perception.

[BRIEF NOTE] What the disappearing debris disk around TYC 8241 2652 1 might mean

The sudden disappearance of the debris disk orbiting the distant star TYC 8241 2652 1 reported in Deborah Zabarenko’s Reuters article is fascinating, and has fascinating implications.

In a cosmic case of “now-you-see-it, now-you-don’t,” a brilliant disk of dust around a Sun-like star has suddenly vanished, and the scientists who observed the disappearance aren’t sure about what happened.

Typically, the kind of dusty haloes that circle stars have the makings of rocky planets like Earth, according to Ben Zuckerman, one of a team of researchers who reported the finding on Wednesday in the journal Nature.

Composed of warm dusty material, these disks can be seen by telescopes looking for infrared light. This one was first seen in 1983 by NASA’s Infrared Astronomical Satellite around the young star TYC 8241 2652. It glowed for a quarter-century before disappearing in a matter of 2-1/2 years.

An image taken May 1 by the Gemini observatory at La Serena, Chile, confirmed that the disk was gone.

Astronomers are accustomed to watching events that have unfolded over millions or billions of years, so seeing a bright ring depart from view in less than three years was an eye-blink in the astronomical context, Zuckerman said by telephone from the University of California-Los Angeles.

[. . .]

“So much dust orbiting so close to a young star implies that rocky planets similar to the terrestrial planets of our own solar system were in the process of forming around this star,” he said. But all of a sudden, this potential planet-maker was absent.

“We don’t really know where the dust came from in detail, and we certainly don’t know what caused it to disappear so quickly,” Zuckerman said.

ScienceNOW’s Ken Croswell suggests that the sudden disappearance of vast quantities of dust in the TYC 8241 2652 1 system, apparently 460 light-years away in the constellation Centaurus, has significant implications for the processes by which planets form. By all accounts, the system seems to have been typical insofar as very young stars and their systems go, making the disappearance all the more inexplicable.

Born about 10 million years ago, the TYC 8241 2652 1 system was chugging along just fine before 2009. Its so-called circumstellar disk glowed at the infrared wavelength of 10 microns, indicating it was warm and lay close to a star—in the same sort of region that, in our own sun’s neighborhood, gave rise to the terrestrial planets Mercury, Venus, Earth, and Mars. The infrared data reveal that the dust was about 180°C and located as close to its star as Mercury is to the sun.

By January 2010, however, nearly all infrared light from the dusty disk had vanished. “We had never seen anything like this before,” says astronomer Carl Melis of the University of California, San Diego. “We were all scratching our heads and wondering what the hell did we do wrong?” But subsequent observations with both infrared satellites and ground-based telescopes confirmed the surprising discovery, he says: “The disk was gone.”

Melis and his colleagues report the mystery online today in Nature—but they don’t know what caused it. “It’s very bizarre,” he says. “Nothing like this was ever predicted.” He says there’s no way something could eclipse the infrared-emitting disk for more than 2 years, because such an object would be immense. Furthermore, the star itself didn’t fade.

All this could mean that planets form much more quickly than anyone had suspected. (Or, possibly, that they are much rarer than thought.)

The most commonly held theory of planet formation is that minute particles of dust left over after a star forms clump onto each other, first through weak electrostatic interactions and later through gravitational forces. The aggregated dust particles eventually grow to become pebble-sized and then car- to house-sized objects. Ultimately, they become planets. The timescale at which this accretion occurs has been theorized and modeled mathematically, and Song said it is commonly thought to occur over hundreds of thousands of years, a time period that spans civilizations on Earth but is an astronomical blink of an eye.

“If what we observed is related to runaway growth, then our finding suggests that planet formation is very fast and very efficient,” [study co-author Inseok] Song said. “The implication is that if the conditions are right around a star, planet formation can be nearly instantaneous from astronomical perspective.”

[. . .]

Song added that a slightly different version of the “runaway accretion” theory suggests that dust grains accrete onto the central star in a very short timescale, implying that the star effectively eliminates planet-building material. If such events occur frequently, planet formation is much less likely than previously thought.

Another explanation for the sudden disappearance of the dust is that it was expelled from the sun’s orbit. Song explained that the particles are so small—a hundred times smaller than a grain of sand—that the constant stream of photons emanating from the sun could push them away and into each other, like pinballs, until they leave the suns’ orbit.

Because large clouds of dust can be formed when orbiting planets crash into each other, astronomers have often viewed the presence of such clouds as indirect evidence of unseen planets. If clouds of dust are only fleeting, however, then many more stars than previously thought could harbor planets.

“People often calculate the percentage of stars that have a large amount of dust to get a reasonable estimate of the percentage of stars with planetary systems, but if the dust avalanche model is correct, we cannot do that anymore,” Song said. “Many stars without any detectable dust may have mature planetary systems that are simply undetectable.”‘

Written by Randy McDonald

July 5, 2012 at 1:17 am

[BRIEF NOTE] On the HIP 11952 planetary system

HIP 11952 has made the news recently for its planets. This star 375 light-years away is noteworthy since, although it’s broadly similar to the Sol mass-wise–it’s only one-sixth less massive than our Sol, perhaps comparing to nearby famous Tau Ceti–it is a remnant of the very early universe. Our sun is 4.57 billion years old, our universe 13.75 billion (plus or minus 110 million years), but HIP 11952 formed in the youth of the universe, and is 12.8 billion years old. Because of its extreme age, when HIP 11952 was formed in the infant Milky Way it did so in an environment where there just hadn’t been enough nuclear fusion in stars to make the elements heavier and helium that dominate our terrestrial worlds; spectrograms indicate that this Population II star has 1% of the “metals” of our own sun. There was a consensus in astronomy that stars with so few metals shouldn’t have been able to support the formation of planets.

But HIP 11952 has two Jovian-mass planets regardless.

It is widely accepted that planets are formed in disks of gas and dust that swirl around young stars. But look into the details, and many open questions remain – including the question of what it actually takes to make a planet. With a sample of, by now, more than 750 confirmed planets orbiting stars other than the Sun, astronomers have some idea of the diversity among planetary systems. But also, certain trends have emerged: Statistically, a star that contains more “metals” – in astronomical parlance, the term includes all chemical elements other than hydrogen and helium – is more likely to have planets.

This suggests a key question: Originally, the universe contained almost no chemical elements other than hydrogen and helium. Almost all heavier elements have been produced, over time inside stars, and then flung into space as massive stars end their lives in giant explosions (supernovae). So what about planet formation under conditions like those of the very early universe, say: 13 billion years ago? If metal-rich stars are more likely to form planets, are there, conversely, stars with a metal content so low that they cannot form planets at all? And if the answer is yes, then when, throughout cosmic history, should we expect the very first planets to form?

Now a group of astronomers, including researchers from the Max-Planck-Institute for Astronomy in Heidelberg, Germany, has discovered a planetary system that could help provide answers to those questions. As part of a survey targeting especially metal-poor stars, they identified two giant planets around a star known by its catalogue number as HIP 11952, a star in the constellation Cetus (“the whale” or “the sea monster”) at a distance of about 375 light-years from Earth. By themselves, these planets, HIP 11952b and HIP 11952c, are not unusual. What is unusual is the fact that they orbit such an extremely metal-poor and, in particular, such a very old star!

For classical models of planet formation, which favor metal-rich stars when it comes to forming planets, planets around such a star should be extremely rare. Veronica Roccatagliata (University Observatory Munich), the principal investigator of the planet survey around metal-poor stars that led to the discovery, explains: “In 2010 we found the first example of such a metal-poor system, HIP 13044. Back then, we thought it might be a unique case; now, it seems as if there might be more planets around metal-poor stars than expected.”

What does this mean for planetary formation? What are the minimum requirements for planetary formation? When did the first planets in our galaxy form?

Centauri Dreams’ Paul Gilster notes that this has implications from the perspectives of galactic history and the possibilities of extraterrestrial life. If planets formed so early in the universe’s history, well, where is everybody?

The idea of ‘deep time’ exerts an abiding fascination. H.G. Wells took us forward to a remote futurity when his time traveler looked out on a beach dominated by a red and swollen Sun. But of course deep time goes in the other direction as well. I can remember wanting to become a paleontologist when I discovered books about the world of the dinosaurs, my mind reeling from the idea that the world these creatures lived in was as remote as any distant star. Paleontology was a grade-school ambition I never followed up on, but the Triassic and Jurassic eras still have a hold on my imagination.

In a SETI context, deep time presents challenges galore. Charles Lineweaver’s work offers up the prospect that the average Earth-like planet in our galactic neighborhood may well be far older than our own — Lineweaver calculates something like an average of 1.8 billion years older. Would a civilization around such a star, if one could survive without destroying itself for so long, have anything it wanted to say to us? Would it have evolved to a level where it had merged so completely with its environment that we might not be able to recognize its artifacts even if we saw them?

[. . .]

Are there, then, more planets around metal-poor stars than we have previously thought? We need to find more planetary systems in this age bracket to learn more, but Anna Pasquali (Heidelberg University), a co-author of the paper on this work, says “The discovery of the planets of HIP 11952 shows that planets have been forming throughout the life of our Universe,” a thought that reminds us to be careful about drawing hasty conclusions about planet formation. Don’t be surprised, either, if the idea doesn’t once again bring Dr. Fermi knocking at the door.

A commenter at Centauri Dreams suggests that gas giants, which are made substantially of the non-metals that were in exclusive abundance early in the universe’s history, would likely form more easily than rocky planets like the Earth. Against this, a commenter at the first news link wonders–jokingly–if HIP 11952 is the home system of the Vorlons.

Written by Randy McDonald

April 4, 2012 at 3:02 pm

[LINK] “Kepler Statistical Analysis Suggests Earthlike Planets Extremely Rare”

Space Daily’s John Rehling shares news of a recent analysis of data from the Kepler space telescope, charting extrasolar planets by the thousands, which suggests that Earth-like worlds are–contrary to earlier analyses–relatively rare. The Kepler program, which basically detected planets via their eclipses of their stars, was biased towards the detections of close-orbiting worlds and of large worlds (larger than the Earth). It turns out that doesn’t mean that Earth-like worlds, of the right size and in the right orbit, are common.

This release shows two favorable trends. As a statement from the Kepler team puts it, “With each new catalog release a clear progression toward smaller planets at longer orbital periods is emerging. This suggests that Earth-size planets in the habitable zone are forthcoming if, indeed, such planets are abundant.”

However, the fine details of this latest release favor more pessmistic projections. Although the release shows, in comparison to the previous release, one favorable trend regarding planet size and another favorable trend regarding planet distance from the star, these trends are, unfortunately, unfolding in an either-or respect: We see more Earth sized planets which are very close to their stars, and therefore likely very hot; and, separately, we see more giant planets which are located farther out from their stars.

This table shows the estimated frequency of planets per star in each bin [category]. The planet sizes are presented in terms of Earth radius, Re, while the orbital periods are in days (d). The bins are populated with frequency estimates based on the de-bias factor, times the observed candidate count, where that count was 4 or greater. In addition, more tentative numbers appear in parentheses in certain bins, based on a smaller n or the n that would have resulted from a single discovery. In each row, the maximum value appears in bold.

This shows that for each terrestrial planet size catgeory, we have observed the frequency max out at a very short period between 4 and 16 days, then exhibit decreasing frequency for longer periods.

[. . .]

For the bin corresponding precisely to the Earth, the projected frequency is 0.7%, a far cry from Traub’s projection of 34%, owing in part to differences in the size of the bin: If we consider the 3×3 collection of bins that surround Earth’s bin, the current projection rises to 7.8%. By including also the smaller Mars-sized planets, to 9.0%.

This does disprove some earlier optimistic estimates, although these estimates were made without any data at all.

In a memorable scene of Carl Sagan’s Cosmos, Sagan considers the Drake Equation and estimates that perhaps 1/4 of all stars have planets and that each such system might have two planets suitable for the development of life. That estimate was recorded before any planet outside our solar system had been discovered.

With the current Kepler release, we find that the number of earthlike planets per star is likely to be considerably lower than the 0.5 implied by Sagan’s estimate. Of course, the picture is considerably complicated: We may find habitable worlds, whether earth-like or Europa-like, orbiting giant planets. We may find earth-like worlds harbored as Trojans of giant planets in the habitable zone.

The frequency as a function of orbital period may have a second peak or flatten and fail to drop with still longer periods. Comfortable temperatures may also be found at planets close in to dim red dwarf stars, although that may likely result in the world being tidally locked, and therefore subject to one daylight hemisphere and one in eternal night. And if nothing else, the pessmistic characteristic of these results suggest that to find earth-like worlds elsewhere, we should prepare to look hard – and quite possibily very hard for decades if not centuries.

Written by Randy McDonald

March 9, 2012 at 4:29 am

[BRIEF NOTE] On the discovery of a possible super-Earth at Gliese 667

James Nicoll is one blogger of many who pointed to the discovery of a superterrestrial world that could support Earth-like conditions, as described in the arXiv-hosted paper “A planetary system around the nearby M dwarf GJ 667C with at least one super-Earth in its habitable zone”.

GJ 667C is an M-class dwarf, part of a triple star system some 22 light years from Earth. Hearing rumors that a ‘super-Earth’ — and one in the habitable zone to boot — has been detected around a nearby triple star system might cause the pulse to quicken, but this is not Alpha Centauri, about which we continue to await news from the three teams studying the prospect of planets there. Nonetheless, GJ 667C is fascinating in its own right, the M-dwarf being accompanied by a pair of orange K-class stars much lower in metal content than the Sun. The super-Earth that orbits the M-dwarf raises questions about theories of planet formation.

In the context of astronomy, “metal” refers to all elements heavier than hydrogen and helium. If stars that have relatively few heavy elements aren’t barred from having planets, but–maybe–just have smaller ones, this has obvious implications for planets across the universe.

See Sol Station and Wikipedia for more on the Gliese 667 system.

Written by Randy McDonald

February 3, 2012 at 9:15 pm

[BRIEF NOTE] On the Kepler-20 system

I really like living in an era when the discovery of Earth-like planets hundreds of light years away is almost quotidian.

NASA’s Kepler mission has discovered the first Earth-size planets orbiting a sun-like star outside our solar system. The planets, called Kepler-20e and Kepler-20f, are too close to their star to be in the so-called habitable zone where liquid water could exist on a planet’s surface, but they are the smallest exoplanets ever confirmed around a star like our sun.

The discovery marks the next important milestone in the ultimate search for planets like Earth. The new planets are thought to be rocky. Kepler-20e is slightly smaller than Venus, measuring 0.87 times the radius of Earth. Kepler-20f is slightly larger than Earth, measuring 1.03 times its radius. Both planets reside in a five-planet system called Kepler-20, approximately 1,000 light-years away in the constellation Lyra.

Kepler-20e orbits its parent star every 6.1 days and Kepler-20f every 19.6 days. These short orbital periods mean very hot, inhospitable worlds. Kepler-20f, at 800 degrees Fahrenheit (427 degrees Celsius), is similar to an average day on the planet Mercury. The surface temperature of Kepler-20e, at more than 1,400 degrees Fahrenheit (760 degrees Celsius), would melt glass.

“The primary goal of the Kepler mission is to find Earth-sized planets in the habitable zone,” said Francois Fressin of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., lead author of a new study published in the journal Nature. “This discovery demonstrates for the first time that Earth-size planets exist around other stars, and that we are able to detect them.”

The Kepler-20 system includes three other planets that are larger than Earth but smaller than Neptune. Kepler-20b, the closest planet, Kepler-20c, the third planet, and Kepler-20d, the fifth planet, orbit their star every 3.7, 10.9 and 77.6 days, respectively. All five planets have orbits lying roughly within Mercury’s orbit in our solar system. The host star belongs to the same G-type class as our sun, although it is slightly smaller and cooler.

The Kepler-20 planetary system is unusual, however. Ars Technica observes that the arrangement of the system’s planets is unexpected.

The big point of discussion that didn’t make it into the paper, however, was the truly unexpected nature of Kepler-20’s companions. As Harvard’s David Charbonneau put it, “the architecture of that planetary system is crazy.” With the new finds, we now have five planets that, as you move further from the host star, alternate between Neptune-sized and small, rocky bodies, with the furthest of the five orbiting closer than Mercury’s distance from the Sun.

Our models of planet formation can account for rocky inner planets, or systems where gas giants have moved in close to a star and booted anything else out of its way, but there’s nothing that can account for a collection of planets like this one.

Charbonneau said, “I really want to dare my fellow astronomers to explain how this system could have formed,” and admitted there was a bit of self interest in his challenge. He’s teaching planet formation at Harvard next year, and fully expects some freshman to ask him to explain Kepler-20’s oddball assortment of planets.

Written by Randy McDonald

December 21, 2011 at 12:42 am

Posted in Science

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[BLOG-LIKE POSTING] Notes on a diamond world

The newly-discovered planet of millisecond pulsar PSR J1719-1438 has gotten quite a lot of attention. In an era when exoplanets are found daily, it seems there’s still room for surprises.

The new planet is far denser than any other known so far and consists largely of carbon. Because it is so dense, scientists calculate the carbon must be crystalline, so a large part of this strange world will effectively be diamond.

“The evolutionary history and amazing density of the planet all suggest it is comprised of carbon — i.e. a massive diamond orbiting a neutron star every two hours in an orbit so tight it would fit inside our own Sun,” said Matthew Bailes of Swinburne University of Technology in Melbourne.

Lying 4,000 light years away, or around an eighth of the way toward the center of the Milky Way from the Earth, the planet is probably the remnant of a once-massive star that has lost its outer layers to the so-called pulsar star it orbits.

Pulsars are tiny, dead neutron stars that are only around 20 kilometers in diameter and spin hundreds of times a second, emitting beams of radiation.

In the case of pulsar J1719-1438, the beams regularly sweep the Earth and have been monitored by telescopes in Australia, Britain and Hawaii, allowing astronomers to detect modulations due to the gravitational pull of its unseen companion planet.

The measurements suggest the planet, which orbits its star every two hours and 10 minutes, has slightly more mass than Jupiter but is 20 times as dense, Bailes and colleagues reported in the journal Science on Thursday.

In addition to carbon, the new planet is also likely to contain oxygen, which may be more prevalent at the surface and is probably increasingly rare toward the carbon-rich center.

Sources give planet J1719-1438b a density in the area of 23 gram per cubic centimetre. This is dense. Earth, at standard pressure and temperature, has a density of some 5.5 grams per cubic centimetre, and diamond itself has a density of 3.5 grams per cubic centimetre. Who knows what sorts of conditions reign on that world?

J1719-1438b is just one body of many illustrating that the term “planet”, as denoting a specific class of bodies that formed in specific ways distinct from other classes of bodies forming in other specific ways, is useless. J1719-1438b probably started off as a star.

[J1719-1438b] orbits the pulsar in just 2 hours and 10 minutes, and the distance between the two objects is [600,000 kilometers] — a little less than the radius of our Sun. Second, the companion is so close to the pulsar that if its diameter was any larger than [60,000 km] — less than half the diameter of Jupiter — it would be ripped apart by the gravity of the pulsar.

“The density of the planet is at least that of platinum and provides a clue to its origin”, said Matthew Bailes from Swinburne University of Technology in Australia.

The team thinks that the planet is the tiny core that remained of a once-massive star after narrowly missing destruction by its matter being siphoned off toward the pulsar.

[. . .]

Pulsar J1719-1438 is a fast-spinning pulsar that’s called a millisecond pulsar. Amazingly, it rotates more than 10,000 times per minute, has a mass of about 1.4 times that of our Sun, but is only [20 km] in radius. About 70 percent of millisecond pulsars have companions of some kind: Astronomers think it is the companion that, as a star, transforms an old, dead pulsar into a millisecond pulsar by transferring matter and spinning it up to a very high speed. The result is a fast-spinning millisecond pulsar with a shrunken companion-most often a white dwarf.

“We know of a few other systems, called ultra-compact low-mass X-ray binaries, that are likely to be evolving according to the scenario above and may likely represent the progenitors of a pulsar like J1719-1438,” said Andrea Possenti, of INAF.

The image of a glistening diamond planet is irresistible. What would it look like? astronomers have been asked in the various articles?

Just what this weird diamond world is actually like close up, however, is a mystery.

“In terms of what it would look like, I don’t know I could even speculate,” said Ben Stappers of the University of Manchester. “I don’t imagine that a picture of a very shiny object is what we’re looking at here.”

From what I know of colour in diamonds, Stappers is right. Diamonds’ colours are lent by pollutants, hard radiation (like that produced by pulsars, say) turning diamonds green, the diamonds made up of irregularly-sized and -shaped carbon crystals known as carbondados being black in colour, and so on. J1719-1438b is a wonder, but its wonders are going to be far subtler than non-stop shine.

Written by Randy McDonald

August 31, 2011 at 3:57 am

[BLOG-LIKE POSTING] Why the Pluto controversy demonstrates the meaninglessness of the term “planet”

Pluto, second-largest dwarf planet in Sol’s planetary system and prototype of the plutoids (trans-Neptunian dwarf planets), is a rejected planet. You know it, I know it, everyone knows it. The news of the demotion didn’t move me particularly, even though I remember from Grade 1 or 2 listening to one of those books to be read along with an audiocassette and hearing the thin methanogenic winds when I turned to that page with its illustration of Pluto’s bleak eternal night. That Pluto was ever identified as a planet alongside the terrestrial worlds of the inner solar system and the gas giants of the outer system (middle?) seems to have been an accident, product of a lack of knowledge of what was out there in the Kuiper belt and Lowell’s fortuitous discovery. By the 1970s, as Duncan Lunan noted in his New Worlds for Old, justifying Pluto as a planet took some rather spectacular intellectual leaps.

Pluto was discovered in 1930, after its existence had been predicted (like Neptune’s in the 19th century) by analysis of the observed perturbations experienced by Uranus. Lowell predicted that “planet P” would lie 6,400 million kilometres from the Sun and have six times the mass of the Earth; Pluto’s mean distance from the Sun is 5,866,000,000 km, but, with the supposed mass, even a 5,760km diameter would give it fifty times the density of water and a surface gravity of 17g!

No ordinary material known to science could give Pluto so high a density. A core of “condensed matter”–nuclei stripped of their electrons and packed together, in white dwarf stars–might provide a solution, but could it be contained against electrostatic repulsion, by a total mass only six times that of the Earth’s? There is no evidence that even Jupiter contains condensed matter; Pioneer 10 data indicate the contrary, that density increases smoothly to the centre (204-205).

There’s dissenters. Alan Boyle writes at Wired Science, excerpted from his book, that the current definition of a planet as a body that’s round on account of its own gravity and has cleared its orbit of other material isn’t popular with some Pluto-as-planet proponents.

As a rule of thumb, if it’s big enough to crush itself into a round shape due to self-gravity, it’s big enough to be a planet. If it’s not big enough to get round, it’s a failed planet, taking on the potato or peanut shape normally associated with asteroids or comets. “These objects that we call planets have shaped themselves into spheres,” said Alan Stern, the planetary scientist who worked for seventeen years to get a probe sent to Pluto.

The significance of the shape isn’t merely that a round object makes for a pretty, planetlike picture. Rather, the important thing is that such a degree of self-gravity makes it possible for a planet to have a layered composition, an active geology, perhaps even volcanic activity beneath the surface, or an atmosphere above. “It’s about the physics,” Stern said.

Stern likes to talk of a Star Trek test for planethood: “The Starship Enterprise shows up at a given body, they turn on the cameras on the bridge and they see it. Captain Kirk and Spock could look at it and they could say, ‘That’s a star, that’s a planet, that’s a comet.’ They could tell the difference.”

Roundness would provide an instant way for Mr. Spock to tell. In contrast, Stern said, having to determine whether the round thing was one object among others at the same orbital distance would force Spock to put Kirk’s question on hold: “We have to make a complete census of the solar system, feed that into a computer, and do numerical integrations to determine which objects have cleared their zone.”

Using Star Trek, it should be noted, might not be the best of examples, especially given the intense politicization surrounding the issue. Regardless, a compromise definition that would hve extended the realm of planet didn’t come off: “Objects at least 800 kilometers wide with masses of at least 5 x 10 20 kilograms, or about 4 percent of Pluto’s mass, would be brought into the planet fold, with borderline cases decided as further observations became available. That would put Pluto as well as Xena in the pigeonhole for planets, along with the eight bigger planets and smaller Ceres, the rocky world that was hailed as a planet in 1801 but reclassified as an asteroid decades later.”

The issue rise provokes passions, clearly, with everyone fom romantic sentiment to nationalism playing a role. Me, I’m still a bit peeved that my favourite world of Ceres, a body hundreds of kilometres wide that’s a sphere on account of its sizable rocky mass, isn’t a planet.

In the end, “why bother”? Why isn’t the category of “dwarf planet” isn’t interesting in and of itself? Does “planet” really matter at all? Others have noted–Neil deGrasse Tyson, to name one example–that the term “planet” isn’t very descriptive, grouping small rocky bodies with thinnish atmospheres like Venus and Earth together with vast gas giants. More, even the category of gas giant is being subdivided, with the less massive Uranus and Neptune being classified as “ice giants.” An asteroid like Ceres is large enough to have internally differentiated. Can anyone say that moons like Titan and Enceladus and Europa and our own Moon aren’t worlds in themselves, with geological processes and internal differentiation and all?

“Asteroid,” “moon,” “planet”–these terms all seem excessively reductionist, don’t they, creating artificial separations and distinctions that really do little to further research and knowledge, if anything the reverse? Generic, non-judgmental “World” suits me fine. Could it suit astronomers? More, could it suit the general public?


Written by Randy McDonald

November 23, 2009 at 11:59 pm

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[BLOG-LIKE POSTING] Did you know that the first discovery of an extrasolar planet was Canadian?

Canadian? Well, at least by a Canada-based team.

The recent discovery of the planet CoRoT-7b, a planet discovered by the European Space Agency’s CoRoT space telescope which discovers planets by checking for dips in the luminosity of their parent star caused by the planet’s blocking the line of sight between Earth and said star, is hugely impressive. A rocky planet with a diameter almost twice that of Earth and perhaps five times as massive as our homeworld, orbiting a main-sequence yellow-orange dwarf star roughly 490 light years away, this planet is one of the more Earth-like world found, notwithstanding that a year of just 20 Earth-days has almost certainly given the world a surface of lava. This might not be as astonishing as the discovery of a planet with a mass six times that of Jupiter in the Andromeda Galaxy 2.3 million light years away (yes, you read me correctly), but still, the hunt for extrasolar planets has progressed at a remarkable speed. Soon, using one (or more) of any number of detection methods, astronomers will be able to find worlds similar to Earth or smaller; soon, we’ll be able to get some data, at least, on the frequency of life.

All this is momentous. But are the other firsts? There’s a few.

When were the first claims to have found extrasolar planets?

  • The first was in the mid-19th century, when various observers observed the binary star 70 Ophiuchi and believed to have found a planet of Jupiter’s mass or greater. These claims have been disproved, a pity I suppose since both stars of 70 Ophiuchi are relatively Sun-like and close to Sol. In the middle of the 20th century, astronomer suggested Peter van de Kamp suggested that the dim nearby red dwarf of Barnard’s Star had its own system, but these claims have been disproven.
  • When were the first planets roughly of Earth’s mass found?

  • These were worlds found in orbit of the pulsar PSR B1257+12, starting with two worlds in 1992 and ending up with three worlds and one dwarf planet by 2007. These worlds, belonging to a selected group of pulsar planets formed around rapidly rotating neutron stars, are confusing. No one knows how they could exist in orbit of stars which recently exploded, since–as Steven Dutch notes–it would take only a day to melt the Earth if our sun blew up. The common explanation is that they formed after the explosion.
  • What, you might ask, was the first discovery of a planet that was confirmed to actually exist?

  • It was the planet Gamma Cephei Ab, a world with a mass approximately 160% that of Jupiter orbiting the orange giant star Gamma Cephei A some 45 light years away. As one observer notes, “[t]he planet, with a mass of at least 1.59 times that of Jupiter, is one of the few known to lie within a double-star system, and orbits the main star Gamma Cephei A with a period of 2.47 years at an average distance of 2.03 Astronomical Units (304 million kilometers, 189 million miles), or 2.03 times the distance between the Earth and the Sun. A modest eccentricity brings the planet as close as 1.62 AU to its parent star and takes it as far as 2.43 AU.”
  • When was it first suspected to exist?

  • The initial announcement was made in July 1988.

    Who was it made by?

  • The announcement was made in the paper “A Search for Substellar Companions to Solar-Type Stars”, submitted to The Astrophysical Journal by Bruce Campbell, George Walker, and Stephenson Yang, all three working out of British Columbian academic institutions and making use of the Dominion Astrophysical Observatory in the British Columbian capital of Victoria.
  • In their 1988 paper, Campbell et al made use of the radial velocity technique to study 16 Sun-like stars, examining to see whether or not an orbiting planet was tugging on its parent star, using a hydrogen fluoride mixture to help “steady” the spectrum of the star so as to measure more precisely. Of all these stars, Gamma Cephei A returned the strongest signal of a planet. Alas, these claims weren’t very widely publicized.

    To be fair, there was some uncertainty. In the 1992 paper by Walker et al. “Gamma Cephei – Rotation or Planetary Companion?”, the claim was retracted, on the grounds that Gamma Cephei A’s post-main sequence fluctuations in luminosity were probably responsible for the signal of Gamma Cephei Ab. Later in 1995, the aforementioned French team of Mayor and Queloz announced the 51 Pegasi b, a gas giant planet half the mass of Jupiter orbiting a main-sequence yellow dwarf broadly similar to Sol in a tight orbit, using a refined version of the radial velocity method pioneered by Campbell et al.

    More than 200 exoplanets have been discovered since, all but a handful using variations on the precision radial-velocity technique pioneered at the University of Victoria. One of these planets orbits Gamma Cephei, just as Campbell and Walker suspected.

    “Everyone in the field recognizes that Campbell and Walker were the first ones to see evidence for a planet around a Sun-like star in 1992,” said UBC astronomer Jaymie Matthews. “Bruce and Gordon could have legitimately gone out and told everybody they’d found a planet.” He sighs. “But being good scientists — and maybe maybe being conservative Canadians — they didn’t make the proclamation that it was definitely a planet.”

    Their 1988 discovery was vindicated, happily, with the 2003 paper “Planet around [gamma] Cephei A”, which definitively excluded the possibility that the signal of a planet orbiting Gamma Cephei A was spurious and concludes that the “most likely explanation of the residual radial velocity variations is a planetary-mass companion with 1.7MJ and an orbital semimajor axis of 2.13 AU.” The authors note that the discovery of this planet, located in a a binary star system where the two stars approach each other quite closely, suggests something about the ease of forming planets. They also note that it’s important to study stars for extended periods if one’s planet-hunting, in order to come up with the best possible data.

    We did it. Canadians managed to make a major technological advance and in so doing helped to expand our species’ knowledge of the universe around us immensely. What, I ask you, isn’t very nice about that?

    Written by Randy McDonald

    September 19, 2009 at 8:40 am