Not only is the universe stranger than we imagine, it is stranger than we can imagine.
—Sir Arthur Eddington.
In the first two columns in this series, we looked around the solar neighborhood and hunted for extrasolar planets. We found a few surprises along the way, but for the most part we encountered typical star systems, seeking understanding of the general properties of the stars and planets around us. This time . . . we get to have some real fun. In this, the third and final installment of the series, we'll explore some of the strangest systems known to science. Every neighborhood has a few real oddballs, right? But these particular space weirdos aren't fictional. As astronomers labor to twist photons into information, we would do well to remember the guidance of the good Sir Arthur.
Horseshoe Moons and Trojan Twins
Ursula Le Guin's classic novel The Dispossessed is set on the twin worlds of Urras and Anarres, which both orbit their common center of gravity, which itself orbits Tau Ceti. In a sort of interplanetary chauvinism, the inhabitants of each world regard the other planet as merely the moon of their own home. No one has yet found such a doubled world in reality (although to be fair our ability to detect such worlds is still extremely limited). But astronomers have recently found evidence for a different sort of doubled world, in which two planets share the same orbit around a star, yet remain separated from one another.
To understand these so-called Trojan planets, it helps to first look at a few examples from our own solar system. Predicting the orbital dynamics of three bodies is an infamously hard problem—in fact, theoretically insoluble in the general case. In many situations, eventually one of the three bodies (usually the smallest) will be flung by its larger neighbors out of the system into deep space. But there are certain special geometries which allow stable orbits to last through the vastness of astronomical time.
One such case is if the three bodies are located at the points of an equilateral triangle. These are the famous Lagrange equilibrium points, named for their 18th century French discoverer. The stable L4 and L5 points of the Earth-moon system have long been suggested as promising sites for orbital space colonies, but today they remain empty. Not so for Jupiter's! Just over a century ago, Max Wolf at Heidelberg Observatory discovered the first of many asteroids located in Jupiter's L4 and L5 Lagrange points. Wolf named his asteroid "Achilles," and the tradition of naming these asteroids for heros of the Trojan war was born. Today, the known Trojan asteroids are believed to rival the total population of the main belt.
There is also another, stranger kind of stable three-body orbit, in which the distance between the two smaller bodies does not remain fixed. Saturn's two small moons Janus and Epimetheus share two closely spaced orbits only 50 km apart. Every few years, their slightly different orbital velocities around Saturn bring the two moons close together, and a remarkable thing happens: they swap orbits. The inner moon moves to the outer orbit, while its gravity pulls the outer moon to the inner orbit, and then they begin to slowly move apart again. The whole process repeats from the opposite direction four years later. Seen from a vantage point riding along on one of the moons, the other would seem to oscillate to and fro along a curious horseshoe-shaped orbit.
So, what does this all have to do with Urras and Anares, or any worlds like them? Simply this: astronomers now believe that planets around other stars may actually come as either Trojan pairs or horseshoe-swapping sets. Way back in 2002, a pair of astronomers at UC Santa Cruz and NASA presented a tour-de-force paper showing that planets can remain stable for billions of years in such orbits, or even in a still-more-exotic arrangement where one moves on a circular orbit while the other moves on an overlapping highly elliptical yet perfectly synchronized orbit, unlike anything seen in the solar system. Further studies suggested that dynamical interactions in protoplanetary disks can realistically form such planets. They might even be fairly common! But detecting such worlds is challenging. Because the two planets move in sync, the radial velocity wobbles caused in the star are in phase, rendering the observed signal very similar to that of a single, larger planet. Extremely careful analysis using more sophisticated mathematical models can potentially spot the subtle differences. And some astronomers believe they just might have done that, first around the stars HD 128311 and HD 82943, and now also around HD 108874. That latter star could potentially have a habitable Earth-sized planet sharing an orbit with a Jupiter-mass planet. For now, these remain only possible detections, not certainties. A new technique promises to allow more sensitive tests for Trojan planets by combining radial velocity measurements with careful timing of eclipses, but only for those few systems which are aligned properly to transit. Will future observations confirm the presence of such doubled worlds? They might not be common, but I would bet there are at least a few out there. The great Richard Feynman's famous rule of thumb was that in physics, "anything not forbidden is mandatory," somewhere out there in the vast deep dark.
In my previous column, I mentioned that extrasolar planets can only be detected today if they're Neptune-mass or larger. And that's true. Well, mostly true. Except in that one case, with that one tiny extrasolar planet. Y'know, the one that's five times smaller than Pluto. Which, come to think of it, technically makes it an extrasolar dwarf planet.
So, if in general we can only detect planets at least 7 to 10 times larger than Earth, how in the world did anyone find a planet only 0.0004 times as massive as Earth? In fact, this tiny object, basically a large comet, is one of four known bodies orbiting the star PSR 1257+12, all of which are under 4.5 Earth masses. These diminutive bodies were detectable because their parent star is not an average sunlike star, unlike nearly all other known planet-hosting stars. Instead, as the name suggests it is a pulsar, a neutron star packing 1.4 solar masses into a volume scarcely larger than downtown San Francisco and spinning once every 6.219 milliseconds. The pulsar's magnetic field launches outward a beam of radiation which sweeps the cosmos like a lighthouse. The incredible precision of the spin period, which rivals the best atomic clocks ever built, provides an exquisite ruler with which we can measure tiny planets. Like the radial velocity method, planet detection via precision pulsar timing relies on the motion of the star induced by the planet's orbit, but now it is the position, not the velocity, that matters. Light travels about a foot in one nanosecond, so if a pulse arrives even one microsecond early (easily measurable by today's equipment) that corresponds to a stellar motion of a mere three hundred meters. Since pulsar periods are stable for many years (and a year is about 30 megaseconds), this means we can measure stellar wobbles as small as one millionth of a meter per second. Hence the ability to discover that excruciatingly tiny body around a dim star nearly a thousand light-years away.
In fact, the very first of all known extrasolar planets were discovered via this method around the same star, PSR B1257+12, in 1992 by the astronomer Alex Wolszczan of Penn State. Today three planets are known, plus the newly found dwarf planet. Yet because the star they orbit is an old, dim, dead one, these planets never gained as much fame as did the exoplanets around sunlike stars, found starting three years later. It's not even clear whether these pulsar planets are survivors from the star's main sequence lifetime, charred worlds that somehow endured the supernova explosion that marked the pulsar's birth, or if they instead formed after the cataclysm, coalescing out of the debris. Either way, they are dim and inhospitable worlds, but not cold ones, since they're constantly bathed in a searing stream of radiation and charged particles from the pulsar which probably keeps their surface temperatures a balmy few hundred Kelvin.
The question of how these planets formed is all the more fascinating, because despite the odd setting, the system around PSR B1257+12 is perhaps the most similar known planetary system to our own. The three planets have masses equal to 1/2 of Mercury, 4.3 Earths, and 3.9 Earths, and have orbits almost exactly half as large as those of Mercury, Venus, and Earth respectively. And now there turns out to be that dwarf planet, too: an object barely more massive than Ceres, the largest asteroid, and with a 2.6 AU orbit that would put it smack dab in the middle of our own asteroid belt. Not only are these masses and orbital sizes all surprisingly similar to our own solar system, the orbits are all extremely circular, far more so than the vast majority of eccentric extrasolar planets. Probably this is all just some cosmic coincidence, that such a solar-system-lookalike is found around the most un-sunlike star known to have planets. Or else, of course, there was a wise and ancient race of beings who used to live there, before some alien mad scientist blew up the sun.
A Beaming Black Hole
PSR B1257+12 is certainly a curiosity. But the consensus among astronomers is that the prize for oddest of all, the most exotic of known star systems, goes to SS 433. This stranger in the night sky is a binary system located about 18 thousand light-years away. But it is no ordinary binary: One of its two members is a supergiant star with a mass somewhere between 28 and 42 times that of the sun. The other is a black hole.
These two objects orbit each other separated by only 0.2 AU, circling (and eclipsing each other) once every 13 days. They're close enough that hot plasma is stripped away from the surface of the supergiant and falls into the twenty-solar-mass black hole's gaping maw. Along the way, a small portion of the infalling matter is deflected from the accretion disk by the black hole's rapidly spinning magnetic field. That fifteen-thousand-degree plasma is blasted outwards to form two narrow jets in opposite directions—each moving at fully 26% of the speed of light! These jets move so fast that, in addition to the tremendous redshift or blueshift resulting from the sheer velocities involved, they have a noticeable additional redshift due to relativistic time dilation slowing down time for all the atoms in the jets. These extraordinary outflows make SS 433 a member of the very rare class of objects called microquasars: medium-sized black holes within our own galaxy that mimic on an (astronomically) small scale the truly gigantic relativistic jets produced by million-solar-mass quasars billions of light-years away.
Because the black hole's spin axis and the orbital plane of the supergiant and black hole are misaligned, the directions of the two jets precess around in a 40-degree-wide circle every 162 days. One of the discoverers of SS 433 describes it as "looking for all the world like a berserk two-nozzle lawn sprinkler." The lawn being watered with relativistic plasma (ouch!) is the surrounding nebula, W50, which is almost certainly the remnant debris from the supernova ten thousand years ago in which a 40-solar-mass or larger star collapsed to form that black hole. As the precessing jet crashes into this surrounding material—at a quarter of the speed of light, mind you!—it creates a vast cosmic corkscrew at least 3/4 of a light-year long and glowing from radio waves to gamma rays. In fact, that "lawn sprinkler" is producing collisions so violent that nuclear fusion occurs within the jet, far outside the core of any star and blasting radiation across the galaxy.
The sheer violence of the cosmos can scarcely be comprehended, nor the scales on which these catastrophes take place. Yet there are subtle beauties to be found in the sky as well, such as twinned planets dancing in an orbital pas de deux, or a spiral helix image of DNA painted on a canvas a light-year across in a soothing gamma-ray glow. It is of course purely chance that a supernova remnant sort of resembles the shape of DNA, and yet that coincidence should remind us of a deeper truth: the atoms of which we are built were forged deep within stars. Once upon a time, the iron which makes our blood red was blasted out across the skies in a supernova remnant like W50. And perhaps someday, the heavy nuclei today being fashioned in the thundering plasma storm of a relativistic jet will make up DNA molecules in truth, on some new world orbiting a yet-unborn sun.
We have found a strange footprint on the shores of the unknown . . . And lo! It is our own.
—Sir Arthur Eddington
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