Pas de deux

Ever heard of Antichthon? 

Sometimes called "Counter-Earth," Antichthon is a hypothesized (now known to be nonexistent) planet in the same orbit as Earth, but on the other side of the Sun.  And, therefore, invisible to earthbound observers.  It was first proposed by the fourth century B.C.E. Greek philosopher Philolaus, who argued against the prevalent geocentric models of the day.  Philolaus thought that not only was there another planet on the opposite side of the Sun from the Earth, he believed that the Sun and all of the planets were orbiting around a "Central Fire" exerting an unseen influence at a distance.  Thus, more or less accidentally, landing something near the truth, as the entire Solar System does revolve around the center of the Milky Way galaxy.

None of Philolaus's ideas, however, were based upon careful measurements and observations; another popular notion of his time was that celestial mechanics was supposed to be beautiful, and therefore you could arrive at the right answer just by thinking about what the most elegant possible model is.  (Nota bene: I took a class called Classical Mechanics in college, and what I experienced was not "beauty" and "elegance."  Mostly what it seemed like to me was "incredibly difficult math" and "intense frustration."  So honestly, maybe Philolaus was on to something.  If I could have gotten a better grade in Classical Mechanics by dreaming up pretty but untestable claims about planets we couldn't see even if we wanted to, I'd'a been all over it.)

Anyhow, Antichthon doesn't exist, which we now know for sure both because probes sent out into the Solar System don't see a planet opposite the Earth when they look back toward the Sun, and by arguments from the physics of orbiting bodies.  Kepler showed that the planets are in elliptical orbits, so even if Antichthon was out there, it wouldn't always be 180 degrees opposite to us, meaning that periodically it would peek out from behind the Sun and be visible to our telescopes.  Plus, an Earth-sized planet across from us would experience gravitational perturbations from Venus that would make its orbit unstable -- again, meaning it wouldn't stay put with the Sun in the way.

But there's no particular reason why there couldn't be two planets in the same orbit.  Way back in 1772, the brilliant astronomer and mathematician Joseph-Louis Lagrange found that there were stable points that small bodies could occupy, under the influence of two much larger orbiting objects (such as the Sun and the Earth).  There are, in fact, five such points, called "Lagrange points" in his honor:

[Image licensed under the Creative Commons Xander89, Lagrange points simple, CC BY 3.0]

And you can see that L3 is actually directly across from the Earth -- so Philolaus was before his time.  (Once again, though, not because he'd done the mathematics, the way Lagrange did.  It was really nothing more than a shrewd guess.)  In fact, there are three points that could result in a stable configuration of two planets sharing an orbit -- L3, L4, and L5.

The reason all this comes up is that scientists at the Madrid Center for Astrobiology have found for the first time a possible candidate for this elusive configuration -- around a T-Tauri type star called PDS 70 in the constellation of Centaurus.  The pair of planets, which appear to be gas giants, one of them three times the size of Jupiter, take 119 Earth years to circle their parent star once.

"Planets in the same orbit have so far been like unicorns," said study co-author Jorge Lillo-Box.  "They are allowed to exist by theory, but no one has ever detected them."

The discovery is so unusual that -- understandably -- the scientists are hesitant to state too decisively that it's proven.  Their paper, which appeared in the journal Astronomy and Astrophysics, indicates that they will continue to gather data from the ESO (European Southern Observatory) and ALMA (Atacama Large Millimeter Array) in Chile through 2026 to bolster their claim.

In any case, it's fascinating that a strange guess made 2,400 years ago by an obscure Greek philosopher, then shored up with rigorous mathematics by a French/Italian astronomer 250 years ago, has finally been shown to exist -- two planets locked in a celestial pas de deux, 370 light years away.  

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Homing in on Tatooine

I remember the first time I ran into the concept that the Earth's relatively circular orbit might not be universal amongst the planets out there in the universe.

I shudder to admit that it was on the generally abysmal 1960s science fiction series Lost in Space.  The brave crew of the Jupiter 2 are stranded on a strange planet, and initially the whole place seems to be a frozen wasteland.  But after a journey via their "chariot" (as they call their tank-like wheeled transport vehicle), they find the temperature is seesawing wildly -- at first it seems to be heading to cold temperatures that will eliminate all possibility of life, but unexpectedly the mercury begins to rise, and what was a crossing on solid ice turns into a treacherous sea voyage (the chariot, fortunately, has amphibious capabilities).

The explanation we're given is that the planet they're on has a very elliptical orbit, so it experiences huge temperature changes.  Unfortunately, the writers of the show apparently did not understand that there's a difference between a planet's rotation and its revolution, so they depict the excruciatingly hot temperatures when the planet is at its perigee as only lasting a minute or two, so all the Robinsons had to do was hide under a reflective shelter for a little bit to avoid getting cooked.

So good idea, lousy execution, which can be said of much of that series.

A more fundamentally startling change in my perception of what it'd look like on another planet occurred when I saw Star Wars for the first time, and hit the iconic scene where Luke is looking toward the horizon as sunset occurs on Tatooine -- and there are two suns in the sky.  Tatooine, it seems, orbits a binary star -- something I'd honestly never thought about before then.

Being a science nerd type, I wondered what the shape of a planet's orbit would be if it were moving around two centers of gravity, and found pretty quickly that my rudimentary knowledge of Newton's Laws and Kepler's Law were insufficient to figure it out.

Turns out I wasn't alone; physicists have been wrestling with the three-body problem for a couple of hundred years, and there is no general solution for it.  Three objects orbiting a common center of gravity results in a chaotic system, where the paths of each depend strongly on initial conditions (and some configurations are unstable and result in either collisions or one of the objects being ejected from the system).

It is known, however, that there are points in a three-body system called Lagrange points (after their discoverer, the French mathematician and astronomer Joseph-Louis Lagrange) which result in a stable configuration in which each of the orbiting bodies stays in the same locked position relative to the other, so the entire system seems to turn as one.  Some of the moons of Jupiter (the so-called Trojan moons) sit at the Lagrange points for that system, a pattern that seems to be stable indefinitely.  (Note that from the Earth perspective, an object at the L3 Lagrange point would never be visible -- leading conspiracy wackos to postulate that it could be a place for alien spacecraft to be hiding.)

[Image licensed under the Creative Commons Xander89, Lagrange points simple, CC BY 3.0]

Things only get worse when you add additional objects.  The only way to approximate the configuration of the orbits is to input the specific initial parameters and use computer modeling software to determine a solution; there is no general set of equations to predict what it will look like.

What brings this up is a paper this week in The Astrophysical Journal that went beyond the theoretical, and found actual data from binary star systems with planets to see what the various orbits looked like.  In "The Degree of Alignment between Circumbinary Disks and Their Binary Hosts," by a team led by Ian Czekala of the University of California - Berkeley, we read about new observations from the Atacama Large Millimeter/submillimeter Array (ALMA), which tells us that not only might objects orbiting a binary star exhibit chaotic paths, they might not all orbit in the same plane.

Because of the way planets form -- coalescence of dust and debris from a flat ring surrounding the host star -- planetary systems seem mostly to be aligned with each other.  In our own Solar System, the eight planets all orbit within seven degrees of the Earth's orbital plane (excluding, sadly, Pluto, which still hasn't recovered its planet status, and has an orbital tilt of just over seventeen degrees).

But apparently there are exceptions.  Some binary stars have planets that orbit in a highly tilted ellipse with respect to the orbit of the two stars around their own center of mass.  How this could happen -- whether the planets condensed from a ring that was already tilted for some reason, or that the three-body chaos warped the orbits after formation -- isn't known.  "We want to use existing and coming facilities like ALMA and the next generation Very Large Array to study disk structures at exquisite levels of precision," study lead author Czekala said, "and try to understand how warped or tilted disks affect the planet formation environment and how this might influence the population of planets that form within these disks."

Which is pretty cool.  While it won't solve the more general difficulty of the three-body problem (and the four-, five-, six-, etc. body problems), it will at least give some empirical data to go on with which to analyze other systems ALMA finds.

So we're homing in on Tatooine.  For what it's worth, it looks like the overall situation might be more similar to Star Wars than it is to Lost in Space.

Which is a good thing in a variety of respects.

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Any guesses as to what was the deadliest natural disaster in United States history?

I'd speculate that if a poll was taken on the street, the odds-on favorites would be Hurricane Katrina, Hurricane Camille, and the Great San Francisco Earthquake.  None of these are correct, though -- the answer is the 1900 Galveston hurricane, that killed an estimated nine thousand people and basically wiped the city of Galveston off the map.  (Galveston was on its way to becoming the busiest and fastest-growing city in Texas; the hurricane was instrumental in switching this hub to Houston, a move that was never undone.)

In the wonderful book Isaac's Storm, we read about Galveston Weather Bureau director Isaac Cline, who tried unsuccessfully to warn people about the approaching hurricane -- a failure which led to a massive overhaul of how weather information was distributed around the United States, and also spurred an effort toward more accurate forecasting.  But author Erik Larson doesn't make this simply about meteorology; it's a story about people, and brings into sharp focus how personalities can play a huge role in determining the outcome of natural events.

It's a gripping read, about a catastrophe that remarkably few people know about.  If you have any interest in weather, climate, or history, read Isaac's Storm -- 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!]