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Bad Astronomy | Pluto’s atmosphere is beginning to freeze out

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The New Horizons mission to the outer solar system flew past Pluto in July 2015, skimming over its surface by just 12,500 kilometers after a journey of 5 billion kilometers and returning incredible images and data of the tiny, icy world.

The mission was conceived, designed, built, and launched in record time because scientists wanted to get to Pluto as rapidly as possible. Why? Because Pluto orbits the Sun on a decently elliptical path, and around the year 1989 it was its closest point to the Sun. After that, as it slowly pulled away on its 248-year orbit, it would get slowly colder. Pluto has an incredibly thin nitrogen atmosphere, just 1/100,000th as thick as Earth’s, but it’s there. As the temperature dropped, though, scientists worried it would freeze out by the time a probe got there.

As it turns out, they made it just in time. The atmosphere, such as it is, was still doing its thing when New Horizons zipped past. By 2018, however, there was evidence that the atmosphere was beginning to condense.

On August 15, 2018, as seen from Earth, Pluto passed directly in front of the star UCAC4 341-187633, which is only a little brighter than Pluto itself. An event like this is called an occultation, and these can be extremely useful to astronomers; sometimes giving the size and even the shape of the occluding object.

For an occultation, timing and location are everything. It makes a path across the Earth, like for a solar eclipse, and inside that path you see it while outside you don’t. For the 2018 Pluto occultation the southern limit was Belize and Guatemala in Central America, and the northern limit cut across the northern United States. The central line, where the star would pass right through Pluto’s middle, cut across Mexico, the Gulf Coast, and then up to the eastern Atlantic states.

Quit a few observers were able to measure the star’s brightness. If Pluto had no atmosphere the star brightness would drop very rapidly as the solid body of the world passed in front of it. But the atmosphere is there and actually reaches quite high off the surface, so the star dimmed gradually and rose again in brightness over time. This can tell scientists about the vertical structure of the atmosphere, which is extremely useful in characterizing it.

The plot displayed in the diagram here also shows a sharp uptick in brightness right in the middle of the event. Air bends light, just like how a spoon look bent in a glass of water. This is called refraction. When the star was exactly on the other side of Pluto, directly behind the center of Pluto, the air all around the edge of the world bent the starlight toward the observer, causing what’s called a central flash. If you had been looking through a telescope big enough, it would’ve looked like Pluto was surrounded by a bright ring of light. This is commonly seen in occultations where the occluding object has an atmosphere; it’s happened with Neptune’s moon Triton for example. This flash can also tell astronomers a lot about the atmosphere of the object.

So what were the results? When New Horizons passed Pluto in 2015 the atmosphere had actually been getting thicker, doubling in surface pressure every decade. The spacecraft measured a surface pressure of about 11.5 microbars (11.5 millionths of Earth sea level pressure). Given the trend, the 2018 measurement was expected to be more than 14 microbars.

It wasn’t: The occultation yielded a pressure of 11.4 microbars, almost exactly the same as what New Horizons saw. This indicates the increasing pressure trend has finally stopped.

Pluto’s atmosphere isn’t like Earth’s, where an equilibrium has been reached and the pressure stays relatively constant*. Pluto’s atmosphere is generated by nitrogen ice on the surface warming up and sublimating, turning into a gas. The rate at which this happens is steeply dependent on temperature. Pluto’s orbit takes it from as close as 4.5 out to 7.5 billion kilometers from the Sun, so it gets nearly three times as much sunlight when it’s at perihelion (closest to the Sun) then aphelion (when it’s farthest). This means that as it starts to move farther out the temperature drops enough that the nitrogen stops sublimating and the process reverses, with the nitrogen gas in the air starting to freeze back out.

But why now? After all, Pluto was closest to the Sun nearly 30 years before the occultation!

The reason is thermal inertia. Pluto was receiving the maximum amount of sunlight in 1989, but even as it (very slowly) moved farther away from the Sun it was still receiving sunlight, and heating up faster than it could radiate away that warmth (well, -230° C isn’t exactly warmth but you know what I mean). So it took a while before it could actually start cooling once again. That must have started only shortly after New Horizons got there.

Nice timing. I’ll note that an occultation in 2015, right before the New Horizons flyby, showed the atmosphere was not collapsing, and the results were released in 2020 (and published in early 2021). So it’s funny to me that as those observations were being analyzed, the 2018 occultation was already showing the air was collapsing! Sometimes science takes a while and results can cross paths.

If the 2018 occultation results showing the atmosphere starting to freeze are correct, then Pluto is finally starting to cool off after getting as close to the Sun as it can. Its orbit will take it farther out over the decades, and when it next reaches aphelion — around the year 2213 — it will be about as cold as it gets. The atmosphere will have mostly frozen out, and will likely stay that way for a century more before slowly starting to puff up again as meager sunlight warms the surface.

Hopefully we’ll have sent more probes (maybe even people) long before then.

*There are some similarities; both are mostly nitrogen, and both can create blue skies. Also, Earth’s air does freeze out a little; that’s how we get snow. But that happens in winter, which is due to Earth’s axial tilt and not so much its distance from the Sun.



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Bad Astronomy | NASA’s Lucy mission will visit at least 8 asteroids on a loop-de-loop trajectory

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In a little less than a month, NASA will launch an ambitious mission through the asteroid belt and out to Jupiter’s orbit. The spacecraft won’t be visiting Jupiter, though: It will fly by several asteroids that share an orbit with the giant planet, looking to investigate these fossils from the solar system’s early days.

This mission is pretty cool, but the flight it’ll take to get where it’s going is just nuts.

This mission is called Lucy, named after the skeleton of Australopithecus afarensis, a hominin dating to over 3 million years ago, an early ancestor to humans. The skeleton itself was named after the Beatles song “Lucy in the sky with Diamonds”, making this circuitous naming path come full circle.

That’s a recurring theme with this mission, as you’ll see in a moment.

Lucy’s main mission is to visit what are called Jupiter’s Trojan asteroids and investigate them with a suite of scientific instruments. Due to a quirk of orbital physics, when something like a planet orbits a star there are several points along and near that orbit where there are gravitational stable spots; if you place a small object there it will orbit the star stably as well. These are called Lagrange points, and are pretty useful for space missions (James Webb Space Telescope, due to launch in December, will orbit around the Sun near one of Earth’s Lagrange points).

Jupiter is a big planet, with powerful gravity, so its Lagrange points are very stable. Asteroids that wander in there will tend to stay there. One of these points, called L4, is 60° ahead of Jupiter in its orbit, and another called L5 is 60° behind. A lot of asteroids cluster in those points — the first few discovered were named after figures in the myth of the Trojan War, so they’re nicknamed the Trojan asteroids, and these two stable spots in particular the Trojan points. By tradition the one in the L4 point are named after Greek figures from the war and those in L5 after Trojans*.

It’s suspected that many of these asteroids sharing Jupiter’s orbit have been there for a very long time, even as long as the formation of the solar system. Sending a mission there is a Very Good Idea: It can visit lots of these asteroids more easily, since they congregate together. It’s like having lots of space missions all rolled into one.

The Lucy mission is set to launch no earlier than October 16, 2021. It has a launch window of 23 days; any launch in that time range will do the trick. But it cannot launch before the window opens or after it closed, because the planets literally have to be aligned for this to work.

And this brings me to the wonderful thing that is Lucy’s trajectory.

Getting to Jupiter’s orbit is hard. It takes a lot of boosting to lift something that far from the Sun, and Lucy is a decently hefty spacecraft. It’s solar powered (which itself is cool; it’s only been recently that solar cells have become efficient enough to power a spacecraft that far from the Sun) and when deployed will be 14 meters wide and has a mass (including on-board fuel) of 1,550 kilograms.

That’s a lot of machine to throw to Jupiter. So orbital engineers decided to get a little help. And hey, once they had the help, why not add to the mission just a little bit and maybe get more science for free?

The final path chosen for Lucy is, simply, jaw-droppingly cool. Strap in for this… and take a look at the “Where is Lucy” website for a fantastic animation that shows you where it will be and when. It’ll really help you understand this next part.

It launches on an Atlas V rocket from Florida. It’ll go into a high loop around the Earth, then some months later drop back down toward our planet and use a gravity assist to boost it into an even higher loop that’ll take it out a bit past the orbit of Mars. Then it falls back to the Earth a second time to get another boost. This will fling it through the main asteroid belt where it has its first science encounter with a 4-km asteroid called Donaldjohanson (named after one of the co-discoverers of the Lucy skeleton!) on April 20, 2025.

Lucy (the spacecraft) will then continue through the asteroid belt on a curving ellipse that peaks right at the orbit of Jupiter, timed perfectly such that it encounters Jupiter’s L4 asteroids. It will pass very close to four of them: Eurybates (68 km wide and which has a tiny moon!), Polymele (21 km), Leucus (~35 km), and Orus (50 km).

Then it will drop back down through the asteroid belt and back to Earth’s orbit, swing around and head back out to Jupiter’s orbit again. Enough time will have passed that this time when it gets there it will encounter an asteroid in Jupiter’s L5 point. And get this: It’s a binary asteroid! The two components are called Patroclus and Menoetius, each roughly 100 km in diameter. They orbit around each other at a distance of roughly 700 km every 4 days or so. I imagine the images we get from that system on approach will be amazing.

I think the coolest part of all this is that this pattern then repeats every six years: Lucy will drop back toward Earth then head out to Jupiter’s orbit, cycling between the L5 and L4 asteroids every time. As long as the spacecraft is healthy and NASA gives the thumbs-up to continue the mission, it could do this for a long time.

This whole thing is a brilliant bit of orbital planning and I am mightily impressed by it. It uses a minimum of fuel and gets a tremendous amount of science out of it. And we don’t have to wait very long!

But first things first: Launch on October 16. This is one I’ll be eagerly watching.

*I learned something researching this: So many Trojan asteroids have been discovered — over ten thousand — that names ran out rapidly, so the International Astronomical Union (the official Keeper of Space Names) allowed naming them after Olympic athletes. That’s very cool.

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Bad Astronomy | Cosmic web filaments have been seen glowing at large scales for the first time.

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For the first time, astronomers have obtained large-scale images of the cosmic web — the incredibly ancient scaffolding of dark matter and hydrogen gas out of which galaxies in the Universe were formed.

This material is so far away and so incredibly faint that it took one of the largest telescopes in the world coupled with one of the most powerful cameras to see it at all. But what they found in their images was the very framework of the Universe.

The Universe formed about 13.8 billion years in a sudden and colossal burst of expanding space and energy. In many ways it was like an explosion, though an explosion of space, not in space: It was the creation of space itself. It was crammed full of energy and matter, and the distribution wasn’t smooth. Some places had a teeny bit more matter in them than others. These over- and under-dense regions were incredibly small; a typical denser spot might be 1 part in 100,000 more dense than its neighbor. But that was enough to create all the structure we see in the Universe today.

These overdense regions had enough gravity to overcome the expansion of the Universe, and began to collapse. Dark matter — a still mysterious substance that doesn’t react with or emit light, but has mass and gravity — attracted material around it, and started forming long, thin, interconnecting filaments of material, like a web. “Normal” matter, the stuff we’re made of, was pulled toward these filaments, and collected on them. Matter flowed along the filaments due to gravity, piling up and forming galaxies, clusters of galaxies, and even immense superclusters, clusters of galaxy clusters, the largest scale structures in the known Universe.

All these from tiny fluctuations in the fabric of space!

Video of A Brief History of the Universe: Crash Course Astronomy #44

The problem is seeing this original structure, the original filaments that formed the cosmic web. They’d be loaded with hydrogen gas and glowing, but this all happened so long ago that it has taken over 13 billion years for the light from them to reach us. They’re faint. There’s been some success in detecting them, though.

Quasars, intensely bright galaxies blasting out radiation as their central supermassive black holes gobble down matter, can be used to find them, for example. As the quasar light passes through that primordial hydrogen gas, some of the light is absorbed in characteristics ways, and we can see that absorption in the quasar light. But that only shows you where that gas is in an extremely narrow spot on the sky, and even if you do this with hundreds of quasars the map you get is literally spotty.

Some of that gas has also been seen glowing (what we say is in emission), but only near where bright galaxies are lighting it up. Again, it’s a very localized detection in a special location. What astronomers needed was a map of this material in typical spots in the Universe, representative of the cosmos as a whole.

And that’s what they now have. A few years ago astronomers used the massive 8.2-meter Very Large Telescope (VLT) with the MUSE camera to look at the same spot in the sky Hubble observed to create the the Ultra-Deep Field, an area of the sky about the same size as a grain of sand held at arm’s length… but in which Hubble saw over 10,000 galaxies.

When they observed this field with VLT/MUSE they saw lots of hydrogen gas, so they were encouraged to take deeper observations. Much deeper: Over the course of 8 months they took a staggering 140 hours of usable images on that single spot on the sky. And these weren’t just images, either. They took spectra, breaking the light up into individual colors. Hot hydrogen gas in the early Universe glows at a characteristic color in the ultraviolet called Lyman-α (Lyman-alpha, or LyA for short). By the time this light reaches us billion of years later it’s redshifted to the near-infrared. By looking at the exact wavelength observed, the redshift and therefore the distance to that LyA gas can be determined.

And what they found were long filaments of glowing hydrogen gas, some of it over 13 billion light years away, structures forming when the cosmos was less than a billion years old!

They actually found clumps and filaments from 11.5 to 13+ billion light years away from Earth, some of them well over 10 million light years long and only a few hundred thousand light years wide. They found over 1,250 individual spots where LyA was emitted, some of which were grouped into 22 large overdense regions of LyA emission which had between 10 and 26 distinct clumps in them. Those clumps represent galaxies and clusters in the very earliest stages of forming, not long after the formation of the Universe itself.

It gets better. They also found lots of fuzzy LyA emission well outside those clumps, what’s called extended emission. Simulations of the way matter clumped together in the very early days of the Universe indicate this extended emission is caused by the birth of billions of dwarf galaxies, ones much smaller than our own Milky Way. These are called ultra-low luminosity emitters because they’re extremely faint, some only a few thousand times the brightness of our Sun. Given that the Milky Way is many billions of times more luminous than the Sun you can appreciate how faint these dwarf galaxies are, and how many of them there must be to light up that diffuse gas.

These galaxies are extremely young; we see the light from them when they were less than 300 million years old. Again, for comparison, the Milky Way is over 12 billion years old, so we are seeing a slice of the Universe when it was practically an infant.

On top of all this they found that of all their sources in the VLT/MUSE data, 30% were not seen in the Hubble Ultra Deep Field, meaning these are even fainter objects than Hubble could spot. That’s not hugely surprising, since VLT is far larger than Hubble and can collect more light. But it’s still quite the achievement.

As an astronomer I’m amazed that all of this was even possible to do, let alone find that it matches simulations of the way we think the early Universe would behave. That’s a critical point: Using just math, physics, and observations of the sky, we’ve been able to predict what the Universe was like when it was very young… and find that we’re right!

I hear people denigrating science all the time, poo-pooing results as mere guesses. But it is in fact and in truth the best method we have to understand objective reality, that which exists outside of us. It is a phenomenally successful method, and these new observations are more evidence of that

You may deny science if you like, but you’re going up against the Universe itself. You might want to think carefully on that position.

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Bad Astronomy | Comet near Jupiter is going to be ejected from the solar system

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In June 2019, the automated survey ATLAS (Asteroid Terrestrial-impact Last Alert System) found a new object moving against the background stars. Initially called 2019 LD2, it was thought to be an asteroid orbiting the Sun out near Jupiter. However, an amateur astronomer noticed it appeared to be fuzzy, not point-like, which means it was more like a comet: Icy material on the surface turning into a gas as it’s warmed by the Sun.

Checking archived images, astronomers determined it had been “active” for several months at least. The name of the object was then changed to P/2019 LD2, indicating its status as a periodic comet.

Images by other observatories confirmed this, including Hubble. When they looked at the comet in April 2020 they saw it sporting quite a grand tail, extending for about 600,000 kilometers, nearly twice the distance of the Moon from the Earth! Mind you, the nucleus — the solid part of the comet — is probably only about 4 kilometers across.

Calculations show that around that time it was losing about 80 kilograms of water ice per second. It was also shedding gases like carbon monoxide (about 50 kilos/second), carbon dioxide (7 kilos/second) and diatomic carbon (two carbon atoms bound together; at a rate of 40 grams per second).

That may sound like a lot, but it turns out it just started outgassing like this… and it won’t for very long. It’s status as a periodic comet is only temporary. Extremely temporary: Follow-up measurements to determine its orbit found it’s actually in a similar orbit as Jupiter, and there’s an excellent chance that, in the distant future, the mighty gravity of the giant planet will fling the comet out of the solar system entirely.

When that happens it will become an interstellar comet like 2I/Borisov or ‘Oumuamua, interstellar objects that both recently passed through our solar system (and which, I’ll note, are not alien spaceships).

That’s fitting, since it probably began life in the outer reaches of the solar system, too.

It’s likely that P/2019 LD2 started out as what’s called a Trans-Neptunian Object, an icy body orbiting the Sun in the Kuiper Belt out past Neptune. Over time, very gentle nudges by Neptune’s gravity urged it into a smaller orbit, closer to the Sun. Eventually it got close enough that Neptune could yank on it much harder, changing its orbit substantially, putting it in an orbit between that of Jupiter and Neptune (from about 800 million to 3 billion kilometers from the Sun). Objects on orbits like that are called Centaurs.

Centaurs are interesting. Over time, the gas giants tend to change their orbits still more. Generally, after a few million years in this part of the solar system, they get too close to one of the planets. Either they get dropped down into the inner solar system (and become what we call Jupiter Family Comets) or get thrown out of the solar system entirely. Because of that we call them transitional objects*.

What will be the fate of P/2019 LD2? And where did it originally come from?

Observations over time of an object can be used to determine its orbit, which can then be projected into the past and future. The problem is we can’t measure the orbit exactly; there’s always some uncertainty in it. The farther you try to predict its position in the future (or antedict its position in the past) the fuzzier it gets, the bigger the volume of space it might occupy. That makes this sort of prognostication dicey.

To get around this, astronomers did something clever: They simulated its orbit using what’s called a Monte Carlo technique. They take the physical characteristics of the orbit (the shape, the distance from the Sun, the tilt, and so on) and then change each one very slightly, creating a slightly different orbit. They then run that into the past and future and see what it does. They do this again and again, creating a virtual cohort of objects each with marginally different paths. This way, you get a more statistical idea of what the history and future of the object was and will be.

What they found for P/2019 LD2 is that it probably only entered Jupiter’s space about 2.5 years ago! Before that it was a standard-issue Centaur, but got nudged into its current orbit very recently.

And its future? They found it likely that it will only stay in its current orbit for 8 or 9 more years. After that it will likely drop down into the inner solar system, becoming a Jupiter Family Comet. This means is that it’s only making a pit stop near Jupiter.

Even that’s temporary. It has a 50% of being ejected from the solar system in 340,000 years, which rises to 95% in 4 million years.

It’s likely that, over the age of the solar system, billions of objects like this have been ejected. And there are billions of stars like the Sun… which is why astronomers think the galaxy is loaded with rogue interstellar iceballs like P/2019 LD2, and why it’s not so surprising that we see them passing through our solar system, too.

Will some alien scientists in the distant future see LD2 passing through their own system? What would they make of it? It’s fun, and oddly reassuring, to know that pieces of our neighborhood will be scattered among the stars, going from citizens of our solar system to citizens of the galaxy.

*Which is pretty cool it worked out that way, given that they’re named after mythical half-human/half-horse creatures.

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Bad Astronomy | Titan haze particles made in a lab and photographed in extreme detail

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Titan is the largest moon of Saturn, and the second largest moon in the solar system, about the same size as Mercury. Unique among moons, it has a thick atmosphere — despite the lower gravity, the surface pressure is 1.5 times Earth’s at sea level.

Its atmosphere is 95% nitrogen (Earth’s is 78%) and 5% methane. Normally that would be transparent, but Titan’s air is loaded with haze — tiny particles about a micron across (one-millionth of a meter; a human hair is roughly 50–100 microns wide). These particles are suspended in the atmosphere, making it opaque.

The haze particles are formed when ultraviolet light from the Sun and/or subatomic particles zipping around space slam into the nitrogen and methane, breaking it down into elements that then rearrange themselves into more complex molecules. Some of them are simple rings of carbon, and some are far more complex molecules called PAHs — polycyclic aromatic hydrocarbons. It’s not been clear how the simple ones link up to form the bigger ones, but now, for the first time, this process has been simulated in a lab and the results examined using a powerful type of microscope that reveals the basic atomic configurations of the molecules.

That’s amazing. Those are individual molecules you’re seeing in those images. The scalebar is 0.5 nanometers, half of a billionth of a meter. They’re not images like a photograph, though. It’s literally impossible to do this with visible light; the wavelength of light is hundreds of nanometers, too long to see structures this small. Instead, they used what’s called atomic force microscopy*.

This uses a technique analogous to the way phonographs work, by using a needle at the end of an arm that traces the grooves in a record. In this case though, a molecule at the tip of a microscopic needle runs along a molecule and can detect the change in the shape due to atomic forces holding the molecule together. It’s like running your fingers over an object to feel its shape.

The samples of molecules were created in a lab to simulate Titan’s atmosphere. Scientists filled a stainless steel vessel with a gaseous mixture that’s the same as Titan’s air and used an electric discharge (a spark maker, essentially) to simulate the UV and cosmic rays hitting the gas. It’s not exactly like Titan: They did this at room temperature, which is much warmer than Titan, but the reactions aren’t very sensitive to temperature. They also used a gas pressure of about 0.001 Earth’s, which, though very thin, is much higher than the top of Titan’s atmosphere where the reactions take place. However, the higher pressure allows the reaction rate to be much higher, so they aren’t waiting weeks to get a decent sample.

They found over a hundred different molecules, a dozen or so of which they could examine using their microscope. Many are simple carbon rings and more complex PAHs, as expected. But they also found that many of the PAHs had a nitrogen atom embedded in them, making what are called N-PAHs. These molecules were detected in Titan’s atmosphere by the Cassini mission, which orbited Saturn for 13 years and made over 100 passes of Titan during that time, examining its surface and atmosphere. The simulations in the lab confirm this result.

Moreover, the lab experiment created molecules made of many connected rings, up to seven of them, which will help atmospheric scientists understand how the more complex PAHs are made from simpler molecules.

This work is important for many reasons. Titan’s atmosphere is loaded with this stuff, collectively called tholins (Greek for “mud”, since they make molecules which color the environment yellow, orange, and reddish-brown), and they’re also seen on other worlds; Pluto’s reddish colored landscape is due to tholins.

Titan doesn’t have a water cycle like Earth, but it does have a methane cycle: Liquid methane in vast lakes at its north pole evaporates into the atmosphere, rains down on the hills nearby, then flows back into the lakes. Methane vapor may condense on the suspended tholins, helping it rain out, and then the tholins can coat the moon’s surface. That’s very interesting, because nitrogen and carbon molecules are important in prebiotic chemistry, making up amino acids, which in turn are the building blocks of proteins.

Earth’s early atmosphere was likely very similar to Titan’s, before the Great Oxygenation Event about 3 billion years ago which gave us the atmosphere, more or less, we have today. Studying Titan is like studying ancient Earth. Not to be too broad, but life evolved on Earth in that early atmosphere, so it’s not too silly to wonder if something similar is occurring on Titan. We certainly don’t know if life is brewing or thriving there, but it’s certainly within the realm of science to look into it.

Titan is an alien world over a billion kilometers from the Sun, and drier than any desert on our own planet. Yet there are aching similarities, ones we can study in the lab. NASA is already in the early stages of planning a mission to Titan called Dragonfly — a lander and quadcopter drone that will fly over the surface and examine regions likely to have or have had conditions conducive to life.

What will it find there? These lab results are an important step in figuring that out.

*Just typing those words makes me feel like a scientist in an old black-and-white sci-fi movie.

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Bad Astronomy | A sextuple star system where all six stars undergo eclipses

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This deserves a “whoa”: Astronomers have found a sextuple (six-) star system where, if you watch it for a few days, every star in it will at some point undergo an eclipse.

Whoa.

Multiple stars are just intrinsically cool: Unlike our Sun, sailing alone through space, multiples are where two or more stars orbit each other in a stable, gravitationally bound system. Half the stars in the galaxy are in multiple systems like that. Most are binaries (two stars orbiting each other) and some in trinaries (three stars). Fewer yet are in higher-order systems.

That’s the first thing that makes TYC 7037-89-1 special: It’s a sextuplet, a six-star system. It’s a little over 1,900 light years away, so a fair distance, but it’s bright enough to be detected by TESS, the Transiting Exoplanet Survey Satellite. TESS scans the sky measuring the brightnesses of stars to look for transiting exoplanets, which make mini-eclipses on their host stars, revealing their presence.

But it can find lots of other interesting things, too. TYC 7037-89-1 looks like one star in TESS data, but one that changes its brightness — a variable star. The astronomers who found it look in TESS data for stars that change brightness in a certain way, indicating that they’re multiple star systems.

Video of Binary and Multiple Stars: Crash Course Astronomy #34

What they looked for are eclipsing binaries: Stars that not only orbit each other, but also ones where we see their orbits nearly edge-on, so that the stars appear to pass in front of on another. When that happens the total light from the pair drops a little bit in a characteristic way. The astronomers set up automated software to look for such stars, and out of nearly half a million they found 100 that appeared to be three-star systems or more.

And that’s what brings up the second cool thing about TYC 7037-89-1: It’s not just six stars all orbiting every which way, but they’re arranged in binaries: One pair of stars orbits another pair of stars, and a third pair orbits them both!

The binary pairs are named A, B, and C in order of brightness, and each star in them is given the number 1 or 2 (again in order of brightness). The two inner binaries are then A (made up of stars A1 and A2) and C (C1 and C2), orbited farther out by the binary B (B1 and B2). A and C are separated by about 600 million kilometers (very roughly the distance of Jupiter from the Sun), taking about 4 years to go around each other — this was determined using archival data from other telescopes, including WASP and ASAS-SN. B orbits them both at a distance of about 38 billion km, taking 2,000 years to complete one period.

And that now brings up the coolest thing about this system: All three pairs of stars are eclipsing binaries! We see all three binary orbits nearly edge-on. A1 and A2 undergo mutual eclipses (A1 eclipses A2, then half an orbit later A2 eclipses A1) every 1.57 days, so they’re very close together. C1 and C2 orbit each other every 1.31 days, and B1 and B2 take 8.2 days.

Because each star in any given pair eclipses the other, by measuring how long the eclipse takes as well as other parameters (including taking spectra) we can learn important things like how big the stars are, how hot they are, and more. And this yields another surprise: All three binaries are very similar. They’re triplets!

In each, the bigger star is about 1.5 times the diameter of the Sun, slightly hotter, and about 1.25 times the Sun’s mass. Also in each, the smaller stars are about the same as each other, too: about 0.6 times the Sun’s mass and 0.6 times its diameter. They vary a little, but the point is they’re pretty close, which is peculiar.

This sort of system is just ridiculously unlikely. Models of how stars form show that sextuples are far more often made up of two trinary systems orbiting each other, not three binaries. So that’s rare enough, but to have all three binaries be seen edge-on seems impossible.

… “seems.” In fact it’s likely they formed from a swirling disk of material, each star collapsing out of it. Because of that it’s actually likely that the three orbital planes of the binaries are the same. Therefore if we see one edge-on, we see all of them edge on, or nearly so. That makes it not as unlikely as you might think that all three are eclipsing.

I’ll also note the orbits of the binaries around each other are not edge-on. We see the orbit of A and C around each other from an angle of roughly 40°, even as we see the individual stars in the binaries edge-on. The inclination of the orbit of B around them both isn’t well constrained by the observations, though.

Hopefully longer-term study of this system will yield more information about how they formed. We don’t really know much about multiple systems like this one, so understanding under what conditions they form would be pretty interesting.

I know, this is headache-inducing. So many orbits, angles, stars… Sometimes nature is complex, and it’s hard to keep up. If it helps, I describe a similar fictional system that played a key role in the first season of Star Trek: Picard. And more systems a bit like TYC 7037-89-1 are known; for example CzeV1640 is a quadruple system with two pairs of eclipsing binaries. Nature is complex, but sometimes frugal, reusing the same idea over and again.

But oh my, would I like a ship like Enterprise right now! To be able to see such a thing up close for myself, watch as these six stars — six! — dance around each other…

Strange new worlds indeed.

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