Vacations can be quite enjoyable. Visiting historic cities, lounging in the Sun on a tropical beach, or snuggling up at a cozy mountain resort. But while the destinations are great, traveling itself can be a chore. Crowds, cramped flights, delays. It would be great if there were some short cut to our destination. Now imagine the vacationers of a galactic empire. It’s great to visit the diamond shores of Exoticon 5, but nobody enjoys all that mucking about in hyperspace. So why not bring these worlds closer to home? That’s the idea behind a study recently published in MNRAS. It basically looks at how a super-advanced civilization might pack a bunch of planets into the habitable zone of a single star.
One of the downsides of habitable zones is that they tend to have a fairly narrow distance range. In our solar system, for example, Venus is too close to the Sun, and Mars is too distant for either of them to be solidly in the Sun’s habitable zone. If we become a super advanced species in the future, we might nudge the Venusian and Martian orbits closer to Earth, but that could raise some problems of its own. In particular, if the orbits are too similar, gravitational perturbations could make all three orbits unstable over the course of thousands of years, which would ruin the whole point of re-engineering our system.
Fortunately, there is a way to have two worlds share very similar orbits. We see this with two moons of Saturn, Epimetheus and Janus. Most of the time one of them has a slightly closer orbit to Saturn, which means it speeds along faster until it almost catches up to the other. The two moons then do a little gravitational dance, where the outer moon is pulled inward, and the inner moon outward. So the two moons never collide, even though they essentially share the same orbit. Relative to Epimetheus, Janus traces a horseshoe-shaped path, which is why these are called horseshoe orbits.
How multiple planetary horseshoe orbits might evolve over time. Credit: Raymond, et al
In principle, two Earth-like worlds orbiting a Sun-like star could have mutual horseshoe orbits, thus sharing a common habitable zone. There are examples of small bodies being captured into a horseshoe orbit with Earth, but they tend to be unstable. The orbits become more stable if they are of similar mass.
For this study, the team wanted to find out how many worlds could be packed into a similar orbit. They started with the assumption that all worlds would be Earth-mass, and they would orbit a Sun-like star at 1 au. They found that as you add more planets the orbits become a bit more variable, but it is possible to pack as many as 24 Earth worlds into stable horseshoe resonances. The orbits of these worlds would be stable for billions of years with the right set-up.
The team went further to study how such a system might look from light-years away. If the orbits of such a system were aligned to pass in front of their star from our vantage point, we could detect them as exoplanets using the transit method. Such an odd system could prove the existence of an advanced civilization.
We aren’t likely to discover such an unusual system, but it is a wild idea to contemplate, particularly if you try to imagine what our night sky would look like if it were filled with 23 other Earths. That would definitely be worth a vacation to a dark-sky location to take in the view.
Reference: Raymond, Sean N., et al. “Constellations of co-orbital planets: horseshoe dynamics, long-term stability, transit timing variations, and potential as SETI beacons.” Monthly Notices of the Royal Astronomical Society 521.2 (2023): 2002-2011.
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When Black Holes Merge, They’ll Ring Like a Bell
When two black holes collide, they don’t smash into each other the way two stars might. A black hole is an intensely curved region of space that can be described by only its mass, rotation, and electric charge, so two black holes release violent gravitational ripples as merge into a single black hole. The new black hole continues to emit gravitational waves until it settles down into a simple rotating black hole. That settling down period is known as the ring down, and its pattern holds clues to some of the deepest mysteries of gravitational physics.
Gravitational wave observatories such as the Laser Interferometry Gravitational-Wave Observatory (LIGO) have mostly focused on the inspiral period of black hole mergers. This is the period where the two black holes orbit ever closer to each other, creating a rhythmic stream of strong gravitational waves. From this astronomers can determine the mass and rotation of the original black holes, as well as the mass and rotation of the merged black hole. The pattern of gravitational waves we observe is governed by Einstein’s general relativity equations, and by matching observation to theory we learn about black holes.
General relativity describes gravity extremely well. Of all the gravitational tests we’ve done, they all agree with general relativity. But Einstein’s theory doesn’t play well with the other extremely accurate physical theory, quantum mechanics. Because of this, physicists have proposed modifications to general relativity that are more compatible with quantum theory. Under these modified theories, there are subtle differences in the way merged black holes ring down, but observing those differences hasn’t been possible. But a couple of new studies show how we might be able to observe them in the next LIGO run.
The modified Teukolsky equation. Credit: Li, Dongjun, et al
In the first work, the team focused on what is known as the Teukolsky Equation. First proposed by Saul Teukolsky, the equations are an efficient way of analyzing gravitational waves. The equations only apply to classical general relativity, so the team developed a way to modify the equations for modified general relativity models. Since the solutions to both the Teukolsky and modified Teukolsky equations don’t require a massive supercomputer to solve, the team can compare black hole ring downs in various gravitational models.
The second work looks at how this would be done with LIGO data. Rather than focusing on general differences, this work focuses on what is known as the no-hair theorem. General relativity predicts that no matter how two black holes merge, the final merged black hole must be described by only mass, rotation, and charge. It can’t have any “hair”, or remnant features of the collision. In some modified versions of general relativity, black holes can have certain features, which would violate the no-hair theorem. In this second work, the authors show how this could be used to test general relativity against certain modified theories.
LIGO has just begun its latest observation run, so it will be a while before there is enough data to test. But we may soon have a new observational test of Einstein’s old theory, and we might just prove it isn’t the final theory of gravity after all.
Reference: Li, Dongjun, et al. “Perturbations of spinning black holes beyond General Relativity: Modified Teukolsky equation.” Physical Review X 13.2 (2022): 021029.
Reference: Ma, Sizheng, Ling Sun, and Yanbei Chen. “Black hole spectroscopy by mode cleaning.” Physical Review Letters 130.2 (2023): 141401.
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There’s a Polar Cyclone on Uranus’ North Pole
Uranus takes 84 years to orbit the Sun, and so that last time that planet’s north polar region was pointed at Earth, radio telescope technology was in its infancy.
But now, scientists have been using radio telescopes like the Very Large Array (VLA) the past few years as Uranus has slowly revealing more and more of its north pole. VLA microwave observations from 2021 and 2022 show a giant cyclone swirling around this region, with a bright, compact spot centered at Uranus’ pole. Data also reveals patterns in temperature, zonal wind speed and trace gas variations consistent with a polar cyclone.
Uranus as seen by NASA’s Voyager 2. Credit: NASA/JPL
Scientists have long known that Uranus’ south pole has a swirling feature. When Voyager 2 flew past Uranus in 1986, it detected high wind speeds there. However, the way the planet was tilted did not allow Voyager to see the north pole.
But the VLA in New Mexico has now been studying Uranus the past several years, and observations collected in 2015, 2021, and 2022 were able to peer deep into Uranus’ atmosphere. The thermal emission data showed that circulating air at the north pole seems to be warmer and drier, which are the hallmarks of a strong cyclone.
“These observations tell us a lot more about the story of Uranus. It’s a much more dynamic world than you might think,” said Alex Akins of NASA’s Jet Propulsion Laboratory in Southern California, who is lead author of a new study published in Geophysical Letters. “It isn’t just a plain blue ball of gas. There’s a lot happening under the hood.”
The researchers said the cyclone on Uranus is similar to the polar cyclones observed by the Cassini mission at Saturn. With the new findings, cyclones (which rotate in the same direction their planet rotates) or anti-cyclones (which rotate in the opposite direction) have now been identified at the poles on every planet in our solar system that has an atmosphere. The researchers said this confirms a broad truth that planets with substantial atmospheres – whether the worlds are made of rock or gas – all show signs of swirling vortexes at the poles.
Uranus’ north pole is now in springtime. As it continues into summer, astronomers hope to see even more changes in its atmosphere.
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