# Can a star orbit around multiple planets or a planet with massive moons?

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Note: I'm not talking about a star orbiting around a single or lonely planet :)

I know a star orbiting a planet is almost impossible because if a planet is more massive than a star, that "planet" would probably be a star. But how about if the planet is not lonely like:

1. A multiple planetary system which consists of free floating planets only but the planets are so massive so that their total mass is larger than a star

2. A planet itself is not more massive than a star but it has so many massive moons

Is it possible for a very small star to orbit around planets in such situations?

There's theoretical ways to do it but it's so unlikely as to probably not exist. Assuming you want a system where the planets are in stable orbits around each other. The basic difficulty is the 3 body problem or n body problem. More on it here and here.

For example, a massive planet could (in theory) have a single super-massive moon of similar mass to the planet, though that would probably be considered a 2 planet system. It's not possible for one planet to have two super-massive moons because that wouldn't be stable. In general, planets are many times the mass of all their moons combined, similarly stars are many times the mass of all their planets combined and when that stops being true the system is no longer stable. It's very difficult to generate sufficient mass by adding moons to a system, or by adding enough planets to get more massive than the sun, unless the planets crash into each other but past a certain mass, they'd stop being planets and become a kind of star when enough planets combined.

If we look at gravitational systems, like our solar System, something like 99.7% of the mass is in the sun, so the sun dominates and everything orbits the sun. A few of the larger objects have moons, and curiously, but only cause they're in relatively calm orbits far away from other planets, a few asteroids orbit each other, but the system is very structured around the sun with much smaller stable orbital zones around the planets.

Pluto also has a rather curious orbital system of it's own, likely caused by an impact, where Pluto and Charon are comparatively large and 4 tiny moons orbit around them.

Source:

But these kind of structured systems are only possible if you have a significant mass differential. When you have 3 or more bodies of similar mass and similar distance you get a high degree of mathematical chaos and instability. There are creative mathematical tricks to make it work, but none of them are stable or likely.

This is what 3 similar mass bodies look like, and in such a system, with constant changes, the most likely scenario is that one of the bodies eventually gets ejected. (source, N-body problem above)

There are star systems with several stars, but they are either unstable or contain significant differences in mass. The gravitational structure applies for large planets the same as it does for stars, and there's an article about that here.

You can create stability by having 2 objects orbiting each other and a 3rd massive object quite distant. (Alpha Centauri is that kind of set-up though Proxima Centauri is quite a bit smaller, but it's the same system).

You can even create the heirarchy where you have 2 objects orbiting each other and then 2 more, orbiting each other, but distant so the 2 co-orbitals orbit around each other, and if you do this enough times, you can kind of create enough planetary mass where a more massive star could distantly orbit the entire thing, but it gets very structured and very distant. It's not what I'd consider a normal orbit.

You could also cheat and have several planets in a wildly unstable general proximity orbit and have a star some distance away orbiting the chaotic mess in the middle, but it wouldn't be stable for long.

Don't forget that there are brown dwarfs between planets and stars in mass. The heaviest planets are around a dozen Jupiter masses, and the lightest stars are a little over 80 Jupiter masses (from the Wikipedia article on brown dwarfs). This means you would need some kind of system of seven super-Jupiters for one red dwarf to orbit around. As userLTK made clear in the accepted answer, there's really no plausible stable configuration where this works.

I can propose an implausible stable configuration, though. In an empty universe, or deep intergalactic space, two objects can be very far apart and still stably orbit each other. So, imagine two super-Jupiters orbiting their common center of gravity. Far away from them, a third super-Jupiter orbits the pair. More accurately, it orbits the common center of gravity of the 3-planet system.

Because the system is so isolated, we can, unless I am mistaken, repeat the process indefinitely. You can always move far enough from a cluster so that it is nearly a point from your perspective, and then orbit the center of gravity between you and that point. Using that process, you build a system with seven or more super-Jupiters and one red dwarf orbiting very far away.

If such a system existed, I would consider it evidence of intentional stellar engineering. Such a chain of perfect orbits is just too big a coincidence.

## PLANETPLANET

### Building the ultimate Solar System part 6: a system with multiple stars

In this post we will take the Ultimate Solar System to the next level. The key ingredient that we will add is multiplicity. There will be many stars in this system, not just one or two.

A quick recap. In part 5 we came up with two different ways of packing planets into our star’s habitable zone, where our worlds could host life. Ultimate Solar System 1 only included Earth-like (rocky) planets (in a funky orbital configuration). In Ultimate Solar System 2, half of the Earth-like worlds were not planets but moons of giant planets.

I couldn’t choose between these two ultimate Solar Systems so I chose them both. I put them in a binary star system:

The ultimate Solar System. It consists of two of our chosen stars orbiting each other at a distance of about 100 Astronomical Units (1 Astronomical Unit = the Earth-Sun distance). Each star hosts one of our ultimate Solar Systems. Original post here.

Looking back, I don’t think the Ultimate Solar System was ambitious enough. Sure, we crammed 60 possibly life-bearing planets into one system. But we can do better!

In this post I will focus on one aspect of the system we can build on: increasing the number of stars in the system. I was inspired by a recent post in which I generated a system in which a planet had five Suns in the sky (it was a 3-part series: see here, here and here). What we are going to do is to build a system containing many stars, each of which has its own habitable zone packed full of planets. This gets us into murky philosophical waters because how broadly should we define a “planetary system”? We are going to skip that discussion, jump in and build a new Multiple-Star Ultimate Solar System (if you have an opinion about this, feel free to leave a comment).

A lot of stars have companion stars. A binary system is simply two stars that orbit each other. Our original Ultimate Solar System formed a binary system with the two stars separated by about 100 Astronomical Units. There are lots of known triple, quadruple, quintuple, and even sextuple star systems. For example, this is what the Castor 6-star system looks like:

Credit: Caetano Julio/NASA JPL.

Star systems follow a standard blueprint that keeps their orbits stable. They are organized in a hierarchical setup. What that means is that each set of orbits is on a different size scale. The sizes of stars’ orbits do not go 1-2-3, they go 1-10-100. Any one star is only really close to one other star. After that, other stars are much farther away.

Here is a cartoon of a hierarchical 8-star system:

A hierarchical system of 8 stars. The circles show the paths of the stars’ orbits. This image is not to scale. The separation between the close binaries (the distance between a and b, or between c and d, etc) should be about 10 times smaller than the separation between pairs of close binaries (the distance between a+b and c+d, or e+f and g+h). Technical note: This system has a hierarchy of 3 (see here).

This system is hierarchical because each close pair of stars (stars a and b, b and c, etc) is much closer to each other than any other stars (or pairs of stars). The separation between stars a and b is much smaller than the separation between stars a+b and c+d, which is much smaller than the separation between stars a+b+c+d and e+f+g+h. Let’s say that the separation between the closest binaries is 0.1 Astronomical Units, the separation between each pair of close binaries is 1 Astronomical Unit, and the separation between clumps of 4 stars is 10 Astronomical Units.

A hierarchical setup can double the number of stars for every additional level of hierarchy. For example, let’s start from the 8-star system in the image above. We can take two 8-star systems and put them in orbit around each other. We need to ensure that the new orbit is very wide, about ten times larger than next level down. In our setup, the two 8-star systems would need to be about 100 Astronomical Units apart for the whole system to be stable.

This system is simply two of the 8-star hierarchical systems placed in orbit around each other. This system has a hierarchy level of 4. Not to scale.

Let’s keep going. We can indeed take two 16-star hierarchical systems and place them in orbit around each other. Now the size of the largest orbit is 1000 Astronomical Units, and there are 32 stars in the system.

How far can we go with this? How big of a hierarchical star system can we reasonably build? (Is it really turtles all the way down?) If a system becomes too big then it feels gravitational kicks from the Galaxy itself, from other stars and giant gas clouds. These extra kicks start changing the stars’ orbits when the stars are about 1000 Astronomical Units apart. Orbits larger than about 100,000 Astronomical Units are really at the borderline, and can be broken at almost any time.

Now let’s introduce planets into hierarchical star systems. For now we won’t worry about planets themselves but rather just the stars’ habitable zones.

In contrast with our previous thinking (from part 1 of this series), now the type of star really does matter. Smaller, lower-mass stars are fainter so their habitable zones are more compact than the habitable zones of larger, more massive stars. In our quest to increase the number of stars in a given system, it makes sense to choose low-mass stars, sometimes called red dwarfs.

In the hierarchical star systems above, the closest binary stars were 0.1 Astronomical Units apart. Let’s switch out the two stars in those close binaries for one stars and its habitable zone. For this to fit, we need stars whose habitable zones are about 0.1 Astronomical Units away. That is 10 times closer than the Sun’s habitable zone, which means that the stars we want are 100 times fainter than the Sun. We want M dwarfs. (The kind that are a little cooler than Kepler-186). M dwarfs are much more common in the Galaxy than Sun-like stars, so it makes some sense to use them to build a star system.

After the switch, here is what the 8-star hierarchical system looks like:

A hierarchical 4-star system. The green rings, which replaced the close binary stars in the 8-star hierarchical system, represent each star’s habitable zone.

Instead of two stars orbiting each other, the closest binary stars are now M dwarfs orbited by habitable zones. These habitable zones are stable and can each host planets. We will come back to that.

The next step is simply to add another level of hierarchy. Let’s put two systems — each with 4 stars with stable habitable zones — in orbit around each other:

Now we’re up to 8 stars, each with a stable habitable zone that can host planets. Let’s go ahead and add one more layer, and double the number of stars one last time. Here is what we get :

A hierarchical system containing 16 stars closer than 1000 Astronomical Units. The small green circles represent each star’s habitable zone (there are 16 of them if you look closely, but any two are smushed together because of the whole hierarchical thing).

We have reached the limit. We cannot add another layer of hierarchy without treading into dangerous waters, with Galactic gravitational perturbations playing the role of the crocodile.

We have the infrastructure for our multiple-star ultimate Solar System. It contains 16 M dwarf stars. Each star’s habitable zone is well-separated from any other stars and is dynamically stable. Even with so many stars in the system, the light from the other stars does not have an appreciable effect on the habitable zone, since the closest star is 10 times farther away and 100 times fainter.

Let’s fill these habitable zones with planets! We know (from part 3 and part 4 of this series) how to pack as many planets as possible into the habitable zone while keeping their orbits stable. As we saw in part 5, we can fit about thirty life-bearing worlds into the habitable zone (24 in Ultimate Solar System 1 and 36 in Ultimate Solar System 2).

It is tempting simply to choose 16 copies of Ultimate Solar System 2. This would give 576 habitable worlds in the system, versus only 384 if we chose 16 copies of Ultimate Solar System 1. However, M dwarf stars don’t have as many gas giant planets as Sun-like stars. So Ultimate Solar System 1 is a more reasonable choice than Ultimate Solar System 2 for most of the stars in this system. And as long as there are a few Ultimate Solar System 2’s, the number of planets is still higher than 400. Not too shabby.

Here is what our 16-star Ultimate Solar System looks like:

The 16-star Ultimate Solar System. Each green circle is one star’s habitable zone. The 16 M dwarf stars are arranged in a hierarchical configuration. Each star’s habitable zone is far enough from other stars to be stable, and contains 24-36 planets (see here) for a total of up to 576 possibly life-bearing worlds in the system.

We did it! This is a big step up from the original Ultimate Solar System, from 60 habitable worlds to 400 or more! Some might say that it’s even “Ultimater”!

SUMMARY: We built a giant (1000 Astronomical Unit-wide) hierarchical system with 16 stars and stable habitable zones. By packing planets into those habitable zones we created a system containing more than 400 (and up to 576) potentially habitable worlds. Boom!

Imagine the stories to be told in a system like this. Astronomical battles pitting one world against another. Rivalries between planets orbiting different stars. Alliances among creatures on moons, Trojan planets, or binary planets. Hostile Takeovers of planets and moons. Imagine a lovable band of swarthy vagabonds exploring different parts of the system, chasing adventure while fleeing their pasts (I’m thinking of Firefly I love that show). How long would it take for the inhabitants of one planet to discover the planets orbiting other stars? What would the sky look like on these worlds?

Speaking of storytelling, I have a confession. This post is really just a setup for a story that I will tell in the next post. Something not-so-cheery is brewing in the 16-star Ultimate Solar System… Read on to learn what happens when a good planetary system goes bad.

## Second-chance Star Systems

On September 16th, a team of astronomers led by Andrew Vanderburg (University of Wisconsin-Madison) announced in Nature that they had discovered the first intact exoplanet orbiting close to a white dwarf.

“We weren’t expecting to find a planet that was intact,” Vanderburg says. “We were expecting to find more planets that were being destroyed. This one seems to have made it past the most dangerous parts of its evolution.”

The planet, WD 1856b, is 80 light-years from Earth in the constellation Draco. It is seven times bigger than its host, and orbits so closely that it transits every 1.4 days. But how did it escape the chaos of its star’s death throes unscathed? And how might studies of these kinds of worlds enrich the growing field of exoplanet research? Vanderburg and his colleagues estimate the planet must have originated at least 50 times farther out than it is now, taking a winding and incredibly unlikely path right to the doorstep of its host and then somehow securing a stable orbit.

“We know that planets migrate inward sometimes because of hot Jupiters,” says Thea Kozakis (Technical University of Denmark), team member of a companion study published in the September 20th Astrophysical Journal Letters.

“When we first found these worlds,” Kozakis says, “we had no idea how that could happen, because gas giants just can’t form that close to the host star. Over time, we realized that they had formed further away, then moved.”

## Four massive planets have been discovered orbiting a distant young star

Researchers have identified a young star with four Jupiter and Saturn-sized planets in orbit around it, the first time that so many massive planets have been detected in such a young system. The system has also set a new record for the most extreme range of orbits yet observed: the outermost planet is more than a thousand times further from the star than the innermost one, which raises interesting questions about how such a system might have formed.

The star is just two million years old – a ‘toddler’ in astronomical terms – and is surrounded by a huge disk of dust and ice. This disk, known as a protoplanetary disk, is where the planets, moons, asteroids and other astronomical objects in stellar systems form.

The star was already known to be remarkable because it contains the first so-called hot-Jupiter – a massive planet orbiting very close to its parent star – to have been discovered around such a young star. Although hot-Jupiters were the first type of exoplanet to be discovered, their existence has long puzzled astronomers because they are often thought to be too close to their parent stars to have formed in situ.

Now, a team of researchers led by the University of Cambridge, England, have used the Atacama Large Millimeter/submillimeter Array (ALMA) to search for planetary ‘siblings’ to this infant hot-Jupiter. Their image revealed three distinct gaps in the disk, which, according to their theoretical modelling, were most likely caused by three additional gas giant planets also orbiting the young star. Their results are reported in The Astrophysical Journal Letters .

The star, CI Tau, is located about 500 light years away in a highly-productive stellar ‘nursery’ region of the galaxy. Its four planets differ greatly in their orbits: the closest (the hot-Jupiter) is within the equivalent of the orbit of Mercury, while the farthest orbits at a distance more than three times greater than that of Neptune. The two outer planets are about the mass of Saturn, while the two inner planets are respectively around one and 10 times the mass of Jupiter.

As hot-Jupiters are enormous planets orbiting close to their host star, they are easier to spot. Image credit: NASA/ESA/G. Bacon (STScI)/N. Madhusudhan (UC)

The discovery raises many questions for astronomers. Around one percent of stars host hot-Jupiters, but most of the known hot-Jupiters are hundreds of times older than CI Tau. “It is currently impossible to say whether the extreme planetary architecture seen in CI Tau is common in hot-Jupiter systems because the way that these sibling planets were detected – through their effect on the protoplanetary disk – would not work in older systems which no longer have a protoplanetary disk,” says Professor Cathie Clarke from Cambridge’s Institute of Astronomy.

According to the researchers, it is also unclear whether the sibling planets played a role in driving the innermost planet into its ultra-close orbit, and whether this is a mechanism that works in making hot-Jupiters in general. And a further mystery is how the outer two planets formed at all.

“Planet formation models tend to focus on being able to make the types of planets that have been observed already, so new discoveries don’t necessarily fit the models,” says Clarke. “Saturn mass planets are supposed to form by first accumulating a solid core and then pulling in a layer of gas on top, but these processes are supposed to be very slow at large distances from the star. Most models will struggle to make planets of this mass at this distance.”

The task ahead will be to study this puzzling system at multiple wavelengths to get more clues about the properties of the disk and its planets. In the meantime, ALMA – the first telescope with the capability of imaging planets in the making – will likely throw out further surprises in other systems, re-shaping our picture of how planetary systems form.

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## A six-planet system dances in time to the tune of gravity

Astronomers have found a remarkable solar system, a system of planets orbiting a nearby star. For one thing, there are at least six planets found there. For another, the outer five planets are orbiting the star in synch, moving like dancers to the tune of gravity!

The star is called TOI-178, and it's a hair over 200 light years from Earth. TOI stands for TESS Object of Interest, a star with candidate planets detected by the Transiting Exoplanet Survey Satellite (making TOI an abbreviation with an embedded acronym that's not important but for some reason those crack me up).

TESS looks for regular, periodic dips in starlight indicating we're seeing that planet pass directly in front of its star, making a mini-eclipse, what we call a transit. That only happens when we see the orbit edge-on. But from that the period (the planet's "year") and the size of the planet can be found — a bigger planet blocks more light.

When astronomers analyzed the TESS observations of TOI-178, they found there are six planets orbiting the star, and the outer five planets all have periods that are simple multiples of each other!

The planets are called TOI-178b through TOI-178g (the first planet discovered is given the star's name plus a lower case b). The periods of the planets, in order out from the star and in Earth days, are b = 1.91, c = 3.24, d = 6.56, e = 9.96, f = 15.23, and g = 20.71.

Take a look at those numbers: It takes planet d very close to twice as long to orbit the star as planet c, so c goes around the star twice in the time it takes d to go around once. The period of planet e is three times that of c, so c goes around three times for every time e goes around once. Planet f goes around twice for every three times planet e does, and, finally, planet g goes around 3 times for every four times planet f does.

When one planet has a period that's a simple multiple (a number that can be expressed as a fraction with two integers, like 2/3 or 5/4) we say they are in resonance. In this case, it's a resonance chain, with all the outer five planets moving in simple multiple periods.

This animation of the TOI-178 system plays a tone whenever one of the five outer planets makes completes one-half or a full orbit, with a different tone for each planet. Because the orbital periods of the planets are simple multiples of each other, the patterns repeat regularly. Credit: ESO/L. Calçada

We know of a few systems like this TOI-178 brings the number to 5. In one sense they come about naturally and easily. Planets form from a disk of gas and dust around the star, and as they interact with that disk their orbits change. They tend to slowly move closer to the star. But as that happens they can move into a resonance pattern, and their gravitational interactions tend to reinforce that pattern. If one planet moves a little too fast the planet outside it pulls it back a little, and vice-versa.

On the other hand, when you have five planets in a chain like this, it can be a delicate thing if one planet is off by even a little bit it can throw off the entire dance, and the planets' periods will change, disrupting the resonance. This tells us something about how they formed: It must have been a relatively gentle process, allowing them to settle into these orbits. If there had been another big planet yanking on them it would have disrupted the chain. The star is roughly 7 billion years old, so this system has been stable for a very long time.

I'll note that these planets are pretty close in to their star, which is what we call a K-type star, smaller and cooler than the Sun. Still, they're very close and all cooked by it.

Size comparison between Earth (left) and Neptune (right). Credit: NASA / jcpag2012 at wikimedia

Transits tell us the sizes of the planets, too: In order from the star, the planets' size relative to Earth are b = 1.18, c = 1.71, d = 2.64, e = 2.17, f = 2.38, g = 2.91. They're all bigger than Earth, but smaller than Neptune, so we call them super-Earths on the low end and mini-Neptunes on the bigger end. But they're all mixed up. In our solar system, the smaller planets orbit closest in, and the giants farther out. That's not the case here.

Odd. But there's more. The astronomers followed up on the discovery with other telescopes to measure the reflex velocity of the star, which tells us how massive the planets are (as they orbit the star they tug on it, making it go around in a complex pattern the more massive the planet the harder it tugs).

If you calculate the density of the planets (the mass divided by volume) it's even more mixed up. In terms of Earth's density (about 5.5 grams per cubic centimeter, or 5.5 times as dense as water), in order, the planets of TOI-178 are b = 0.91, c = 0.9, d = 0.15, e = 0.39, f = 0.58, g = 0.19. So the inner two are a little less dense than Earth, but d is much less, with e is much denser than d, and f denser still, and then g is way lower. They're all over the place!

Artwork depicting the six-planet system orbiting the star TOI-178. Credit: ESO/L. Calçada/spaceengine.org

The density is important because it tells you what kind of planet it is. Gas giants have densities up to 0.2 Earths or so, and rocky/metal planets closer to 1. Here we see they're mixed up in their order from the star, completely unlike our own solar system. That's hard to explain, and is telling us something important about how these planets formed. We just don't know exactly what yet.

I'm delighted that we're finding all these systems so different than ours. I was going to call them "odd" first, but I wonder. If this one is only 200 light years away, it implies systems like this are common it seems like long odds one would be so close if they were incredibly rare.

Maybe we're the weird system. I think that would be delightful, too. Maybe we just seem normal because we're what we're used to and that's what we base our opinion on.

If there's a moral lesson there, why, maybe we should listen to the Universe more.

## Three Massive Planets Have Been Discovered in This Bizarre Star System, Baffling Scientists

In an intriguing new discovery, astronomers have identified three huge gas planets orbiting a young star, according to a study published in the Astrophysical Journal Letters. The star already hosted one gas giant&mdashtaking the total number of large worlds orbiting it to four.

Not only is this the first time that so many massive planets have been detected around such a young star, but the system has also set a new record for having the most extreme range of known orbits, with the outermost planet more than a thousand times further from the star. Together, these findings raise questions about how such systems formed.

The star, known as CI Tau, is "just" two million years old, meaning it's still close to the beginning of its life cycle. Like other young stars, it is surrounded by a vast disk of dust and ice&mdashknown as a protoplanetary disc&mdashin which planets, moons, asteroids and other astronomical objects form.

The CI Tau system&mdashlocated about 500 light-years away in a highly productive "stellar nursey" region of the galaxy&mdashis already notable for containing the first "hot Jupiter" around such a young star. Hot Jupiters are a class of gas giant exoplanets which orbit extremely close to their host star, which usually means they have incredibly high surface temperatures. The existence of these planets has long puzzled astronomers because they are thought to be too close to their stars to have formed in their current positions.

Using the Atacama Large Millimeter/submillimeter Array (ALMA), the team identified three distinct gaps in the protoplanetary disc, which theoretical modeling suggests were caused by three additional gas giant planets orbiting the star, in addition to the already known hot Jupiter.

These four planets have a range of masses&mdashfrom the mass of Saturn to 10 times that of Jupiter&mdashand have vastly different orbits. The closest is the hot Jupiter, which is nearer to the star than Mercury is to our Sun. Meanwhile, the farthest two planets orbit at a distance more than three times greater than that of Neptune.

These properties of the system make it particularly puzzling and interesting to astronomers, especially given the fact that that the one percent of stars that host hot Jupiters tend to be hundreds of times older than CI Tau.

"It is currently impossible to say whether the extreme planetary architecture seen in CI Tau is common in hot Jupiter systems because the way that these sibling planets were detected&mdashthrough their effect on the protoplanetary disc&mdashwould not work in older systems which no longer have a protoplanetary disc," Cathie Clarke from the University of Cambridge's Institute of Astronomy, the study's first author, said in a statement.

It is also unclear whether the three newly detected worlds helped to push the hot Jupiter into its very close orbit and if this process is common among such planets. Furthermore, the researchers have not been able to explain how the outer planets formed.

"Planet formation models tend to focus on being able to make the types of planets that have been observed already, so new discoveries don't necessarily fit the models," said Clarke. "Saturn mass planets are supposed to form by first accumulating a solid core and then pulling in a layer of gas on top, but these processes are supposed to be very slow at large distances from the star. Most models will struggle to make planets of this mass at this distance."

The next steps for the researchers will be to further investigate this puzzling star system at multiple wavelengths using ALMA in an attempt to unravel more of its secrets.

## 5 planets found in unusual rhythmic dance around a star 200 light-years away

Astronomers have discovered a planetary system including six planets and it’s not quite like anything they’ve seen before. The system could challenge the theories scientists have about how planets form and change over time.

Our solar system is just one of many planetary systems, and so far, no two systems are alike. The systems vary in the number and type of planets they contain.

About 200 light-years away from us is the star TOI-178, found in the Sculptor constellation. The research team initially thought there were only two stars orbiting the planet.

A closer look revealed something else entirely.

“Through further observations we realized that there were not two planets orbiting the star at roughly the same distance from it, but rather multiple planets in a very special configuration,” said lead study author Adrien Leleu, a CHEOPS fellow at the University of Bern, Switzerland, in a statement. (CHEOPS stands for the European Space Agency’s CHaracterizing ExOPlanet Satellite.)

The study published Monday in the journal Astronomy & Astrophysics.

Five of the six planets are essentially locked in a rare rhythmic orbit around the stars, creating a resonance. This means that some of the planets are actually aligned every few orbits and that there are discernible patterns as the planets complete their orbits.

This happens around Jupiter in our solar system as three of its moons, Io, Europa and Ganymede, orbit the gas giant. Io is the closest of the three moons. For every four orbits it completes around Jupiter, Europa completes two and Ganymede completes one. This creates a 4:2:1 pattern.

The resonant orbits of the TOI-178 system are more complicated. It’s one of the longest resonant chains found in a planetary system, the researchers said.

The five outer planets of the system follow this pattern: 18:9:6:4:3.

That means for every 18 orbits of the second closest planet to the star, the third planet completes 9.

A chain of resonance can reveal information about how a planetary system forms and evolves and what it was like in the past.

“The orbits in this system are very well ordered, which tells us that this system has evolved quite gently since its birth,” said study coauthor Yann Alibert, an affiliated professor of astrophysics at the University of Bern, in a statement.

Planetary systems can be volatile places in their early days and disruptions caused by the gravitational influence of large planets can disrupt and kick out others. Other times, impacts between planets or other objects can disrupt systems.

This system has been preserved, hence the resonant orbits. However, the densities of the planets are not well ordered, the researchers said.

“It appears there is a planet as dense as the Earth right next to a very fluffy planet with half the density of Neptune, followed by a planet with the density of Neptune. It is not what we are used to,” said study coauthor Nathan Hara, a postdoctoral researcher and CHEOPS fellow at the Université de Genève, in a statement.

The planets in our solar system are arranged with more dense, rocky planets closest to the sun, while the lower density gaseous planets are farther away.

“This contrast between the rhythmic harmony of the orbital motion and the disorderly densities certainly challenges our understanding of the formation and evolution of planetary systems,” Leleu said.

Multiple telescopes were used to study the system, including the CHEOPS satellite and multiple ground-based telescopes at the European Southern Observatory in Chile.

Exoplanets are difficult to observe directly, but the scientists used two methods to observe them. These methods include radial velocity, or observing starlight for telltale wobbles as planets move around a star in orbit, and transiting, or dips in starlight as planets pass in front of stars.

Both techniques revealed that the planets are much closer and in quicker orbits around their star than Earth is to the sun. For example, the closest planet to the star completes a full orbit in a couple of Earth days the farthest takes about 10 times that.

The planets range in type, including rocky and larger than Earth, known as super-Earths, as well as gaseous planets smaller than those in our solar system, called mini-Neptunes.

While the planets are between one to three times the size of Earth, their masses are 1.5 to 30 times that of Earth.

None of the planets are considered to be in the habitable zone of the star, or the perfect distance from the host star where these planets could support liquid water, or life, on their surfaces.

However, more observations of this system could reveal more planets orbiting the star that are in that zone. Future telescopes will be able to directly image some of these exoplanets and peer into their atmospheres, revealing more of the TOI-178 system’s secrets.

## Ask Ethan: Can Two Planets Share The Same Orbit?

Despite the dangers an occasional comet or asteroid strike might bring, our Solar System is actually wonderfully stable place, with all eight planets expected to remain in their orbits, stably, for as long as the Sun lives. But are all solar systems this way? After sifting through our questions and suggestions for Ask Ethan this week, I selected this outstanding question by Dee Hurley:

Is it possible to have a solar system with two planets sharing the same orbit?

It's a really good question, and our own Solar System offers some clues to the answer.

Image credit: Wikimedia Commons user WP.

According to the International Astronomical Union (IAU), there are three things an orbiting body needs to do in order to be a planet:

1. It needs to be in hydrostatic equilibrium, or have enough gravity to pull it into a spherical shape. (Plus whatever rotational effects distort it.)
2. It needs to orbit the Sun and not any other body (like another planet).
3. And it needs to clear its orbit of any planetesimals or planetary competitors.

This last definition, strictly speaking, rules out two planets sharing the same orbit, since the orbit wouldn't be cleared if there were two of them.

Image credit: NASA/Ames/JPL-Caltech.

But why worry about technical definitions? Let's worry, instead, about whether it would be possible to have two Earth-like planets that share the same orbit around their star. The big worry, of course, is gravitation, which can ruin a dual orbit in one of two ways: either a gravitational interaction can "kick" one of the planets very hard, either sending it into the sun or out of the solar system, or the mutual gravitational attraction of the two planets can cause them to merge, resulting in a spectacular collision.

Image credit: NASA/JPL-Caltech.

This latter case is, in fact, something that happened to Earth when the Solar System was only a few tens of millions of years old! The collision resulted in the formation of our Moon, and very likely caused a major resurfacing event on our planet.

Two planets don't do a great job of occupying the same exact orbit, because there's no such thing as true stability in these cases. The best you can do is hope for a quasi-stable orbit, meaning that while, technically, on infinitely long timescales, everything is unstable, you can obtain configurations that last billions of years before one of these two "bad" things occurs. And for that, I want to introduce you to a concept: Lagrange points.

Image credit: NASA and the WMAP science team, via . [+] http://map.gsfc.nasa.gov/mission/observatory_l2.html.

If you only considered two masses -- the Sun and a single planet -- there are five points (known as Lagrange points) around each one where the gravitational effects of the Sun and the planet cancel out, and all three bodies move in a stable orbit forever. Unfortunately, only two of these Lagrange points, L4 and L5, are stable anything that starts out at the other three (L1, L2, or L3) will unstably move away, and wind up colliding with the planet or getting ejected.

But L4 and L5 are the points around which asteroids collect. The gas giant worlds all have thousands, but even Earth has one: the asteroid 3753 Cruithne, which is presently in a quasi-stable orbit with our world!

Although this asteroid in particular isn't stable on billion-year timescales, it is definitely possible for two planets to share an orbit just like this. It's also possible to have a binary planet, which would be a lot like the Earth/Moon system (or the Pluto/Charon system), except with no clear "winner" as to who's the planet and who's the moon. If you had a system where two planets were comparable in mass/size, and only separated by a short distance, you could have what's known as either a binary or double planet system. Recent studies indicate that this is, in fact, possible.

But there's one more way to do it, and this is something you might not have thought was stable: you can have two planets in two separate orbits, one interior to the other, where the orbits swap periodically as the inner world overtakes the outer world. You might think this is crazy, but our Solar System has an example where this happens: two of Saturn's Moons, Epimetheus and Janus

Every four years, whichever moon is interior (closer to Saturn) comes to overtake the exterior one, and their mutual gravitational pull causes the inner moon to move outward, while the outer moon moves inward, and they swap.

Image credit: Emily Lakdawalla, 2006, via . [+] http://www.planetary.org/blogs/emily-lakdawalla/2006/janus-epimetheus-swap.html/.

Over the past 25 years, we've observe these two moons dance quite a bit, and as far as we can tell, this configuration is stable over the lifetime of our Solar System. In other words, it's totally conceivable that we'd have a planetary system somewhere in our galaxy with two planets (rather than moons) that do exactly this!

Image credit: NASA / JPL / David Seal.

The unfortunate news, at least for now, is that out of the thousands of discovered planets around other stars, we don't have any binary planet candidates yet. (You may have heard of one a few years ago, but it was retracted.) Of course, our technology hasn't progressed to the point where we've discovered moons around exoplanets yet, either, and yet we fully expect them to be there.

The reality is that these orbit-sharing circumstances are expected to be rare, but not so exceedingly rare that we don't expect to see it ever. Give us a better planet-finding telescope, a million stars and about 10 years, and I'd be willing to bet we'd find examples of all three cases of planet-sharing orbits. The laws of gravity and our simulations tell us they ought to be there. The only step left is to find them.

## Gas Giants Bounce Around — and Collide — in Alien Solar Systems

Gas giants around other stars often travel along highly-elliptical orbits, contrary to common thought, and massive collisions and interactions between gas giants may be to blame, a new study finds. The Cosmic Companion talks to lead researcher Renata Frelikh of UC Santa Cruz.

M ore than 4,000 worlds are now known to orbit stars other than our sun, and a fraction of these are giant worlds, like Jupiter and Saturn, orbiting close to their parent star. Basic laws of physics (as well as common intuition) would indicate that such a world should have a largely-circular orbits, due to the forces acting on the bodies.

Observations of large exoplanets near their stars, however, reveal just the opposite — that many of these worlds are tracing out highly-elliptical orbits as they race around the stellar companion.

“A giant planet is not as easily scattered into an eccentric orbit as a smaller planet, but if there are multiple giant planets close to the host star, their gravitational interactions are more likely scatter them into eccentric orbits,” Renata Frelikh, a graduate student in astronomy and astrophysics at UC Santa Cruz, stated in a press release from UC Santa Cruz.

Get Together or Go Rogue

A new series of simulations show that massive planets which formed close to stars can interact with each other, radically altering each other’s orbits. During a giants-impact phase of planetary evolution, massive planets collide, building up even larger worlds. Our own Moon was likely formed as our budding solar system passed through this stage of development billions of years ago.

“Exoplanetary systems host giant planets on substantially noncircular, close-in orbits. We propose that these eccentricities arise in a phase of giant impacts, analogous to the final stage of solar system assembly that formed Earth’s Moon,” researchers describe in an article published in Astrophysical Journal Letters.

Some gravitational interactions between massive worlds are capable of sending planets out of their solar system, to soar free among the stars as rogue planets.

As inertia rises with mass, it should be harder to alter the orbit of a more massive world than it would be to act on a smaller world. So, large worlds close to their local stars should tend to trace out near-circular orbits.

Smaller planets should, therefore, be more susceptible to this gravitational scattering than larger worlds. But, astronomers have detected giant worlds tracing out highly-elliptical paths around their parent star(s). These patterns are far different than that seen among our own family of worlds, where the inner solar system is filled with small planets, traveling along highly-circular orbits.

“Gas giant planets with orbital periods less than 400 days occur around about 5% of stars… We call these planets warm Jupiters,” Frelikh explains.

Exoplanets are usually found using one of two techniques. In systems where exoplanets travel in front of their star as seen from Earth, light from that star appears to dim as the planet passes between its sun and our home world. A regular pattern of dimming and brightening, unrelated to stellar processes, can reveal the presence of an exoplanet. The radial velocity method looks at the tiny gravitational tug a planet has on it’s parent star as a tell-tale sign of an alien world.

Using either of these methods, it is easier to find massive worlds close to their Sun. Highly-elliptical orbits also assist astronomers in finding gravitational pulls from undiscovered worlds, but this method works best for exoplanets close to their parent stars.

“It becomes difficult to detect planets this way beyond the distance of about Jupiter from the Sun. Planets smaller than Neptune are actually thought to be the most common type of exoplanet, but, especially at larger distances from the star, they become a lot harder to detect. The most massive planets would initially appear to be more common to us, and this is why when working with observational data sets it is crucial to consider the observational biases,” Frelikh tells The Cosmic Companion.

Practically A Planetary Mosh Pit

The team created a computer model based on a system containing 10 worlds. The total mass of each planet, as well as the total mass of the solar system, was altered each time a different simulation was conducted. Each simulation was run for 20 million (simulated!) years.

The planets modeled during this virtual investigation were much like Jupiter or Saturn, holding on to vast quantities of gases. Smaller worlds orbiting close to their stars can lose their atmospheres to space due to pressure coming from the nearby star. However, massive planets like the ones modeled in this study are able to retain their atmospheric cover.

“They will not lose a substantial amount of their atmospheres over their lifetimes because they are massive enough and far enough away from their host stars. For Jupiter-sized planets, atmospheric escape can become significant when they are extremely close to their host stars (closer than the orbit of Mercury from our Sun),” Frelikh describes for our readers.

Simulations showed planets interacting with each other and colliding, often forming larger bodies which continued to orbit near their parent star.

The largest planets produced in the simulations were produced at distances from the star between one and eight times greater than the distance between the Earth and Sun.

The final results of the study showed the systems with the greatest amount of total mass produced the largest worlds near the central star, and those planets had the greatest eccentricities seen in the virtual model.

This finding helps to answer mysteries of exoplanets, and could help researchers better model climates of distant worlds, some of which may be home to life.

## Two exoplanet families redefine what planetary systems can look like

Astronomers expect dense planets to lie close to a star and fluffy planets farther away. But TOI-178’s six worlds, shown in this artist’s illustration, are all jumbled up.

February 5, 2021 at 6:00 am

Two tightly packed families of exoplanets are pushing the boundaries of what a planetary system can look like. New studies of the makeup of worlds orbiting two different stars show a wide range of planetary possibilities, all of them different from our solar system.

“When we study multiplanet systems, there’s simply more information kept in these systems” than any single planet by itself, says geophysicist Caroline Dorn of the University of Zurich. Studying the planets together “tells us about the diversity within a system that we can’t get from looking at individual planets.”

Dorn and colleagues studied an old favorite planetary system called TRAPPIST-1, which hosts seven Earth-sized planets orbiting a small dim star about 40 light-years away. Another team studied a recently identified system called TOI-178, which has at least six planets — three already known and three newly found — circling a bright, hot star roughly 200 light-years away.

Both systems offer planetary scientists an advantage over the more than 3,000 other exoplanet families spotted to date: All seven planets in TRAPPIST-1 and all six in TOI-178 have well-known masses and radii. That means planetary scientists can figure out their densities, a clue to the planets’ composition (SN: 5/11/18).

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The two systems also offer another advantage: The planets are packed in so close to their stars that most are engaged in a delicate orbital dance called a resonance chain. Every time an outer planet completes an orbit around its star, some of its closer-in sibling planets complete multiple orbits.

Resonance chains are fragile arrangements, and knocking a planet even slightly out of its orbit can destroy them. That means the TRAPPIST-1 and TOI-178 systems must have formed slowly and gently, says astronomer Adrien Leleu of the University of Geneva.

“We don’t think there could have been giant impacts, or strong interactions where one planet ejected another planet,” Leleu says. That gentle evolution gives astronomers a unique opportunity to use TRAPPIST-1 and TOI-178 as testbeds for planetary theory.

In a pair of papers, two teams describe these systems in unprecedented detail. Both buck the trend astronomers expected from theories of how planetary systems form.

In the TOI-178 system, the planets’ densities are all jumbled up, Leleu and colleagues report January 25 in Astronomy & Astrophysics.

“In the most vanilla scenario, we expect that planets farther from the star…would have larger components of hydrogen and helium gas than the planets closer in,” says astrophysicist Leslie Rogers of the University of Chicago, who was not involved in either study. The closer to the star, the denser a planet should be. That’s because farther-out planets probably formed where it’s cold, and there was more low-density material like frozen water, rather than rock, to begin with. Plus, starlight can strip atmospheres from close-in planets more easily than far-out ones, leaving the inner planets with thinner atmospheres — or no atmospheres at all (SN: 7/1/20).

TOI-178 flouts that trend entirely. The innermost planets seem to be rocky, with densities similar to Earth’s. The third one is “very fluffy,” Leleu says, with a density like Jupiter’s, but in a much smaller planet. The next planet out has a density like Neptune’s, about one-third Earth’s density. Then, there’s one with about 60 percent Earth’s density, still fluffy enough to float if you could put it in a tub of water, and the final planet is Jupiter-like.

“The orbits seem to point out that there was no strong evolution from [the system’s] formation,” Leleu says. “But the compositions are not what we would have expected from a gentle formation in the disk.”

TRAPPIST-1’s planet septet, on the other hand, has an eerie self-similarity. Each world is roughly the same size as Earth, between 0.76 and 1.13 times Earth’s radius, astrophysicist Eric Agol of the University of Washington in Seattle and colleagues reported in 2017 (SN: 2/22/17). Plus, at least three of them appear to be in the star’s habitable zone, the region where temperatures might be right for liquid water.

Now, Agol, Dorn and colleagues have made the most precise measurements of the TRAPPIST-1 masses yet. All seven worlds are almost identical to each other but slightly less dense than Earth, the team reports in the February Planetary Science Journal. That means the planets could be rocky yet have a lower proportion of heavy elements such as iron compared with Earth. Or it could mean they have more oxygen bound to the iron in their rocks, “basically rusting it,” Agol says.

TRAPPIST-1’s seven planets seem to have similar compositions to each other, but different from Earth. They could have an Earthlike makeup but with a smaller iron-rich core (center), or have no core at all (left). They could also have deep oceans (right), but the inner three planets are probably too hot for that much water to last. JPL-Caltech/NASA

TRAPPIST-1’s seven planets seem to have similar compositions to each other, but different from Earth. They could have an Earthlike makeup but with a smaller iron-rich core (center), or have no core at all (left). They could also have deep oceans (right), but the inner three planets are probably too hot for that much water to last. JPL-Caltech/NASA

Oxidized iron wouldn’t form a planetary core, which could be bad news for life, Rogers says. No core might mean no magnetic field to protect the planets from the star’s damaging flares (SN: 3/5/18).

However, it’s not clear how to form coreless planets. “There are propositions for how to form such planets, but we don’t actually have one candidate in the solar system where we see this,” Dorn says. The analogs in the solar system are all asteroid-sized bodies much less massive than Earth.

Astronomers may soon get a better handle on the compositions of TRAPPIST-1’s planets. The James Webb Space Telescope, set to launch in October, will probe the planets’ atmospheres (if they have any) for signs of chemical elements that would reveal in more detail what they’re made of.

The TRAPPIST-1 planets’ similarities to each other are not as surprising as the differences among TOI-178’s planets, Rogers says. But they’re still unexpected. If all the planets have identical compositions, then any formation model needs to explain that, she says.

While these systems challenge astronomers’ views of what sorts of planets are possible, Dorn says, it will take discovering more multiplanet systems to tell how weird they truly are.