Skeptophilia (skep-to-fil-i-a) (n.) - the love of logical thought, skepticism, and thinking critically. Being an exploration of the applications of skeptical thinking to the world at large, with periodic excursions into linguistics, music, politics, cryptozoology, and why people keep seeing the face of Jesus on grilled cheese sandwiches.

Friday, July 21, 2023

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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