Out of line

Astrophysicists have a fairly clear idea about how planetary systems form.

The whole thing starts with a nebula -- a cloud of interstellar gas and dust,  mostly made of hydrogen and helium -- that begins to contract under the influence of gravity.  Assuming it's large enough, that compaction raises its temperature; and because almost always, the cloud as a whole had some angular momentum to start with (i.e. it had a net spin around the nebula's center of mass, even if a small one) its rotational rate increases as the collapse proceeds.  That increase in spin rate flattens the cloud out -- think of a whirling blob of pizza dough in the hands of someone who knows how to make the perfect pizza crust -- resulting in a concentrated mass in the center (the future star) and a "protoplanetary disk."

The disk is never perfectly uniform, so the higher gravitational pull of the denser parts draws in more material, making them denser still -- a classic example of positive feedback.  Those lumpy bits form the planets, ultimately gaining sufficient mass to gravitationally clear the regions around their orbits.  When the star becomes dense and hot enough to initiate fusion, the light and heat blow away lighter elements (hydrogen and helium), leaving the inner regions enriched in heavier elements like carbon, silicon, magnesium, nickel, aluminum, and iron.

This model explains two things; why the planets in the Solar System all have relatively circular orbits that are aligned with each other and with the spin plane of the Sun, and why the inner planets (Mercury, Venus, Earth, and Mars) are dense and rocky, while the outer ones (Jupiter, Saturn, Uranus, and Neptune) are gas giants.

But.

When we get too confident, nature has this awkward way of saying, "You think you understand everything?  Ha.  A lot you know."  Back in the 1990s people looking for exoplanets started finding what are now nicknamed "hot Jupiters," which are gas giants locked in a tight orbit around their host stars.  Hot Jupiters seem to be pretty common; on the other hand, it may just be that they're simple to spot.  Given their size and mass, they are going to be easier to pick up both by the transit method (the dip in a star's brightness as its planet crosses in front of it) and the wobble method (stars having a slight back-and-forth "wobble" as the star and its planet orbit their common center of gravity; this effect is more pronounced for larger exoplanets and ones with closer orbits).  

So how does a gas giant form, and remain stable, so near to its host star?  Wouldn't the light and heat of the star blow away the lightweight gases, as they seem to have done in our own Solar System?

The answer is "we're not sure."

Another spanner in the works comes from planets that are misaligned -- that have rotational axes or orbital planes skewed with respect to the rotational plane of the star.  There are two examples in our own Solar System; Venus (which actually rotates backwards as compared to the other planets; its day is longer than its year) and Uranus (which lies on its side -- its rotational axis is tilted 82 degrees with respect to its orbital plane).

Neither of these has been explained, either.

But weirdest of all is when a planet's orbital plane is out of alignment with both the star's rotation and the orbits of other planets in the system.  This, in fact, is why the topic comes up; a paper this week in the journal Astrophysics presents some strange new data on the system AU Microscopii, suggesting that the planet AU Microscopii c has its orbital plane tilted by 67 degrees with respect to everything else in the system.  So as the other two planets, and the star itself, are all moving in a nicely aligned fashion, AU Microscopii c is describing these wild loops above and below the system's orbital plane.

You might be wondering how they figured out the orientation of the rotational axis of the star, since most stars look like points of light even in large telescopes.  And this part is really cool.  It's called the Rossiter-McLaughlin effect.  As a star rotates, from our perspective half of the star's disk is heading toward us while the other half is heading away.  So the light from the part that's coming toward us gets slightly blue-shifted, and the light from the other half is simultaneously red-shifted.  Now, imagine a large planet crossing in front of the star, orbiting in the same direction as the star is rotating.  First the blue-shifted part of the light will be partially blocked, then the red-shifted part, resulting in a spectrum alteration that will look like this:

[Image licensed under the Creative Commons Amitchell125, Animation of the Rossiter-Mclaughlin (RM) effect, CC BY-SA 4.0]

So we know the rotational plane of the star from the Rossiter-McLaughlin effect, and the orbital planes of the planets from the direction of the star's wobble.

And they don't line up.  At all.

This completely confounds our models of how planetary systems form.  Did a close pass by another heavy object yank one of the planets out of alignment?  Or an actual collision with something?  (That's one guess about why Uranus's axis is tilted.)  The answer is still "we don't know."  What seems certain is that the configuration is gravitationally unstable.  AU Microscopii is thought to be a young star, on the order of 24 million years old; the Solar System is over five hundred times older than that.  As I described in a post a couple of years ago, long-term stability usually requires some kind of orbital resonance, where the gravitational pull of planets acts to reinforce their trajectories, keeping them all locked in a tight celestial dance.  So it seems like the weird loop-the-loop described by AU Microscopii c is unlikely to last long.

But it's also an orbit that, based on what we know, shouldn't have happened in the first place.  So maybe it's not a good idea to place bets on what it's going to do in the future.

In any case, it's yet another example of how far we have to go in our understanding of the universe we live in.  That's okay, of course; it'd be boring if we had it all figured out.  Science is like some benevolent version of the Hydra from Greek mythology; for every one question we answer, we create nine more.

I think the scientists are going to be busy for a very long time to come.

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The cotton-candy planet

There's a general pattern you see in astrophysics, which arises from the fact that gravity is both (1) always attractive, never repulsive, and (2) extremely weak.

It's hard to overstate the "extremely weak" bit.  The next strongest of the four fundamental forces, electromagnetism, is 36 orders of magnitude stronger; that is, the electromagnetic force is 1,000,000,000,000,000,000,000,000,000,000,000,000 times more powerful than gravity.  This may seem odd and counterintuitive, since the gravitational pull on your body seems pretty damn strong (especially when you're tired).  But think about it this way; if you use a refrigerator magnet to pick up a paper clip, that little magnet is able to overcome the force of the entire Earth pulling on the clip in the opposite direction.

The practical result of these two features of gravity is that at small scales and low masses, the effects of gravity are essentially zero.  If I'm picking up a book, I don't have to adjust for the negligible gravitational attraction between myself and the book, only the attraction between the book and the enormous mass of the Earth.  On the largest scales, too, the effects of gravity more or less even out; this is called the flatness problem, and is something I dealt with in more detail in a recent post.  (Plus, on these cosmic scales, the force of expansion of spacetime itself -- something that's been nicknamed dark energy -- takes over.)

It's at mid-range scales that gravity becomes seriously important -- objects the size of planets, stars, and galaxies.  And there, the other feature of gravity kicks in; that it always attracts and never repels.  (Whatever Lost in Space may have had to say about anti-gravity, there's never been evidence of any such thing.)  So for objects between the size of planets and galaxies, gravity always wins unless there is some other force opposing it.

This, in fact, is how stars work; the pull of gravity from their mass causes the matter to collapse inward, heating them up until the fusion of hydrogen starts in the core.  This generates heat and radiation pressure, a balancing force keeping the star in equilibrium.  Once the fuel runs out, though, and that outward force diminishes, gravitational collapse resumes -- and the result is a white dwarf, a neutron star, or a black hole, depending on how big the star is.

All of this is just a long-winded way of saying that if you've got a mass big enough to form something on the order of a planet or star, it tends to fall inward and compress until some other force stops it.  That's why the insides of planets and stars are denser than the outsides.

Well, that's how we thought it worked.

The latest wrench in the mechanism came from the discovery of a planet called WASP-193b orbiting a Sun-like star about 1,200 light years away.  On first glance, WASP-193b looks like a gas giant; its diameter is fifty percent larger than Jupiter's.  So far, nothing that odd; exoplanet studies have found lots of gas giants out there.

But... the mass of WASP-193b is only one-seventh that of Jupiter, giving it the overall density of cotton candy.

So I guess in a sense it is a gas giant, but not as we know it, Jim.  At an average density of 0.059 grams per cubic centimeter, WASP-193b would float on water if you could find an ocean big enough.  Plus, there's the problem of what is keeping it from collapsing.  A mass one-seventh that of Jupiter is still an impressive amount of matter; its gravitational pull should cause it to pull together, decreasing the volume and raising the density into something like that of the planets in our own Solar System.  So there must be something, some force that's pushing all this gas outward, keeping it... fluffy.  For want of a better word.  

But what that force might be is still unknown.

"The planet is so light that it's difficult to think of an analogous, solid-state material," said Julien de Wit of MIT, who co-authored the study, in an interview with ScienceDaily.

[Image licensed under the Creative Commons NOIRLab/NSF/AURA/J. da Silva/Spaceengine/M. Zamani, Artist impression of ultra fluffy gas giant planet orbiting a red dwarf star, CC BY 4.0]

"WASP-193b is the second least dense planet discovered to date, after Kepler-51d, which is much smaller," said Khalid Barkaoui, of the Université de Liège's EXOTIC Laboratory and first author of the paper, which was published in Nature Astronomy last week.  "Its extremely low density makes it a real anomaly among the more than five thousand exoplanets discovered to date.  This extremely-low-density cannot be reproduced by standard models of irradiated gas giants, even under the unrealistic assumption of a coreless structure."

In short, the astrophysicists still don't know what's going on.  Twelve hundred light years from here is what amounts to a planet-sized blob of cotton candy orbiting a Sun-like star.  I'm sure that like the disappearing star from my post two days ago, the theorists will be all over this trying to explain how it could possibly happen, but thus far all we have is a puzzle -- a massive cloud of matter that is somehow managing to defy gravity.

As Shakespeare famously observed, there apparently are more things in heaven and earth than are dreamt of in our philosophy.

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Bridging the Great Divide

One of the main things that separates scientists from the rest of us is that they notice things we would just take for granted.

Gregor Mendel started in the research that eventually would uncover the four fundamental laws of inheritance when he noticed that some traits in pea plants seemed to skip a generation.  Percy Spencer was messing around with vacuum tubes, and noticed that in a certain configuration, they caused a chocolate bar in his pocket to melt -- further inquiry led to the invention of the microwave oven.  French physicist Henri Becquerel discovered radioactivity when he accidentally ruined some photographic plates with what turned out to be a chunk of uranium ore.  Alexander Fleming saved countless lives with the discovery of penicillin -- found because he wondered why a colony of mold on one of his culture plates seemed to be killing the bacteria near it.

I consider myself at least a little above average, savvy-wise, but I don't have that ability -- to look at the world and think, "Hmm, I wonder why that happened?"  Mostly I just assume "that's the way it is" and don't consider it much further, a characteristic I suspect I share with a lot of people.  So here's some recent research about something I've known about since I first started reading junior books on astronomy, when I was maybe ten years old, and never thought was odd -- or even worth giving any thought to.

There's a strange gap, something astronomers call "The Great Divide," between Mars and Jupiter.  The distance between Mars and Jupiter is over twice as great as the diameter of the entire inner Solar System.  In that gap is a narrow band called the Asteroid Belt -- and not a hell of a lot else.

Even more peculiar, when you think about it (which as I said, I didn't), is why inside of the Great Divide all the planets are small, dense, and rocky, and outside of it the planets are low-density gas giants (I do remember being shocked by the density thing as a kid, when I read that Saturn's overall density is lower than that of water -- so if you had a swimming pool big enough, Saturn would float).

[Image is in the Public Domain courtesy of NASA/JPL]

The problem with these sorts of observations, though -- even if you stop to wonder about them -- is that until very recently, we pretty much had a sample size of one Solar System to work with, so there was no way to tell if any particular feature of ours was odd or commonplace.  Even now, with the discovery of so many exoplanets that it's estimated there are a billion in our galaxy alone, we only have tentative information about the arrangement of planets around stars, to determine if there's any sort of pattern there, such as the apparent one in our neck of the woods.

Well, it looks like the physicists may have explained the Great Divide and the compositional difference of the planets on either side of it in one fell swoop.  A team from the Tokyo Institute of Technology and Colorado University have found that the Great Divide may be a relic of a ring of material that formed around the early Sun, and then was pulled apart and essentially "sorted" by the gravitational pulls of the coalescing planets.

The authors write:

We propose... that the dichotomy was caused by a pressure maximum in the disk near Jupiter’s location...  One or multiple such—potentially mobile—long-lived pressure maxima almost completely prevented pebbles from the Jovian region reaching the terrestrial zone, maintaining a compositional partition between the two regions.  We thus suggest that our young Solar System’s protoplanetary disk developed at least one and probably multiple rings, which potentially triggered the formation of the giant planets.

And once the process started, it accelerated, pulling dense, rocky material inward and lightweight, organic-chemical-rich material outward, resulting in a gap -- and an outer Solar System with gas giants surrounding an inner Solar System with small, terrestrial worlds.

"Young stellar systems were often surrounded by disks of gas and dust," said Stephen Mojzsis of Colorado University, who co-authored the paper, which appeared in Nature three weeks ago.  "If a similar ring existed in our own solar system billions of years ago, it could theoretically be responsible for the Great Divide, because such a ring would create alternating bands of high- and low-pressure gas and dust.  Those bands, in turn, might pull the solar system's earliest building blocks into several distinct sinks -- one that would have given rise to Jupiter and Saturn, and another Earth and Mars.

"It is analogous to the way the Continental Divide in the Rocky Mountains causes water to drain one way or another.  That's similar to how this pressure bump would have divided material in the early Solar System...  But that barrier in space was not perfect.  Some outer Solar System material still climbed across the divide.  And those fugitives could have been important for the evolution of our own world...  Those materials that might go to the Earth would be those volatile, carbon-rich materials.  And that gives you water.  It gives you organics."

And ultimately, it gives the Earth life.

So here we have a strange observation that most of us probably shrugged about (if we noticed it at all) that not only was instrumental to the formation of our own Solar System, but might (1) drive the arrangement of planets in star systems everywhere in the universe, and (2) has implications for the origin of life on our own -- and probably other -- worlds.

All of which brings to mind the wonderful quote by Hungarian biochemist Albert von Szent-Györgyi -- "Discovery consists of seeing what everyone has seen, and thinking what nobody has thought."

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This week's Skeptophilia book of the week is a dark one, but absolutely gripping: the brilliant novelist Haruki Murakami's Underground: The Tokyo Gas Attack and the Japanese Psyche.

Most of you probably know about the sarin attack in the subways of Tokyo in 1995, perpetrated by members of the Aum Shinrikyo cult under the leadership of Shoko Asahara.  Asahara, acting through five Aum members, set off nerve gas containers during rush hour, killing fifty people outright and injuring over a thousand others.  All six of them were hanged in 2018 for the crimes, along with a seventh who acted as a getaway driver.

Murakami does an amazing job in recounting the events leading up to the attack, and getting into the psyches of the perpetrators.  Amazingly, most of them were from completely ordinary backgrounds and had no criminal records at all, nor any other signs of the horrors they had planned.  Murakami interviewed commuters who were injured by the poison and also a number of first responders, and draws a grim but fascinating picture of one of the darkest days in Japanese history.

You won't be able to put it down.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]

Planetary genesis

In yesterday's post, I looked at some new photographs from the Hubble Space Telescope of one of the oddest objects in the universe -- the luminous blue variable Eta Carinae.  But that's not the only stunning new research coming from the astrophysicists.  No less than three studies in the last month have given us a new lens into something that has stirred our imaginations for years -- the characteristics of exoplanets.

The number of exoplanets -- planets around other stars -- has grown steadily since the first one was confirmed in 1995.  Today there are over four thousand exoplanets that have been discovered, and they include every possible twist on size and temperature, from "hot Jupiters" (gas giants that orbit so near their parent star that they complete one revolution in only a few days, and are so hot that they could liquify iron) to cool, rocky worlds like our own, to frozen blobs of methane and ammonia like Uranus and Neptune.  In fact, every time we find new worlds, it seems to open up new possibilities about what could be out there.

Let's start with a study that appeared in Nature Astronomy last week, led by Björn Benneke of the University of Montreal, which found a planet in that mid-range mass that doesn't exist in our Solar System -- a "sub-Neptune" or "super-Earth" that's somewhere between the mass of the Earth and the mass of Neptune (seventeen times Earth's mass).

What is extraordinary about this study is that the astronomers who studied this planet were able to determine the nature of its atmosphere from a hundred light years away.  The planet goes by the euphonious name GJ3470b, and its composition was unexpected.  Instead of being enriched in (relatively) heavy gases like methane and ammonia, like the gas giants in our own system, it was made almost entirely of the lightweight gases hydrogen and helium.  It also is so close to its parent star that it completes one revolution in only three days, so it's surprising that its proximity didn't result in the radiation and heat blowing away all of the lighter gases (which is apparently what happened to the inner planets in the Solar System), even considering that the star is a relatively dim red dwarf.

"We expected an atmosphere strongly enriched in heavier elements like oxygen and carbon which are forming abundant water vapor and methane gas, similar to what we see on Neptune," Benneke said in a press release.  "Instead, we found an atmosphere that is so poor in heavy elements that its composition resembles the hydrogen- and helium-rich composition of the Sun."

This brings up lots of questions about why planets have the atmospheres they have -- a relatively new field from which we are only beginning to have enough data to form some tentative hypotheses.

This brings us to the second study, also in Nature Astronomy, published last month and authored by a team led by Sebastiaan Haffert of the Leiden Observatory.  This team studied PDS70, a relatively young star that's 370 light years away, which is currently in the process of forming planets.  What's the coolest about this one is that we're able to observe the planets developing an atmosphere by siphoning off material from the outer layers of the star.

The authors write:

Newly forming protoplanets are expected to create cavities and substructures in young, gas-rich protoplanetary disks, but they are difficult to detect as they could be confused with disk features affected by advanced image analysis techniques.  Recently, a planet was discovered inside the gap of the transitional disk of the T Tauri star PDS 70.  Here, we report on the detection of strong Hα emission from two distinct locations in the PDS 70 system, one corresponding to the previously discovered planet PDS 70 b, which confirms the earlier Hα detection, and another located close to the outer edge of the gap, coinciding with a previously identified bright dust spot in the disk and with a small opening in a ring of molecular emission.  We identify this second Hα peak as a second protoplanet in the PDS 70 system. The Hα emission spectra of both protoplanets indicate ongoing accretion onto the protoplanets, which appear to be near a 2:1 mean motion resonance...  Finding more young planetary systems in mean motion resonance would give credibility to the Grand Tack hypothesis in which Jupiter and Saturn migrated in a resonance orbit during the early formation period of our Solar System.

Taking a step even further back into planet development, the third study, which appeared in Astrophysical Journal Letters in June, a team led by Takashi Tsukagoshi of the National Astronomical Observatory of Japan found a star that is in the early stages of planetary condensation from the "accretion disc" of dust and gas surrounding a young star.  The star, TW Hydrae, is two hundred light years from Earth, and the forming planet is currently a huge blob of luminescent gas whose diameter is about the distance from the Sun to Jupiter.  (The blob is located at a distance from TW Hydrae about equal to the orbit of Neptune.)

Cooler still is they have photographs:

[Image courtesy of the ALMA Radio Array and the National Astronomical Observatory of Japan]

The current supposition is that the blob will eventually condense into a gas giant about the size of Neptune.

The speed with which we're finding out new information about the formation of stars and planets further reinforces my general impression that exoplanet systems like our own are so common out there as to be nearly ubiquitous.  This, of course, further improves the likelihood that at least some of those planets host life.  Some of it, perhaps, intelligent.  Scientists are currently trying to figure out how to detect "biosignatures" on other planets, and it's harder than you'd think; consider that until a hundred years ago, our Earth would have been "radio silent" and therefore nearly invisible to alien astronomers except by the curious abundance of elemental oxygen in our atmosphere.  (Oxygen is so reactive that if there weren't processes continually pumping it into the atmosphere -- photosynthesis, in our case -- it would all eventually get locked up in stable molecules like carbon dioxide and silicon dioxide.)

So keep your eye on the skies.  When you look at the stars at night, consider that many -- probably most -- of the stars you're looking at have their own planetary systems.  And maybe, just maybe, there is an extraterrestrial out there contemplating the skies over its own home world who is looking back at you.

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This week's Skeptophilia book recommendation is pure fun for anyone who (like me) appreciates both plants and an occasional nice cocktail -- The Drunken Botanist by Amy Stewart.  Most of the things we drink (both alcohol-containing and not) come from plants, and Stewart takes a look at some of the plants that have provided us with bar staples -- from the obvious, like grapes (wine), barley (beer), and agave (tequila), to the obscure, like gentian (angostura bitters) and hyssop (Bénédictine).

It's not a scientific tome, more a bit of light reading for anyone who wants to know more about what they're imbibing.  So learn a little about what's behind the bar -- and along the way, a little history and botany as well.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]