Weird Planets




Artist rendering of an “eyeball world,” where one side of a tidally locked planet is always hot on the sun-facing side and the back side is frozen cold.  Definitely a tough environment, but  might some of the the planets be habitable at the edges?  Or might winds carry sufficient heat from the front to the back?  (NASA/JPL-Caltech)

The very first planet detected outside our solar system powerfully made clear that our prior understanding of what planets and solar systems could be like was sorely mistaken.

51 Pegasi was a Jupiter-like massive gas planet, but it was burning hot rather than freezing cold because it orbited close to its host star — circling in 4.23 days.  Given the understandings of the time, its existence was essentially impossible. 

Yet there it was, introducing us to what would become a large and growing menagerie of weird planets.

Hot Jupiters, water worlds, Tatooine planets orbiting binary stars, diamond worlds (later downgraded to carbon worlds), seven-planet solar systems with planets that all orbit closer than Mercury orbits our sun.  And this is really only a brief peak at what’s out there — almost 4,000 exoplanets confirmed but billions upon billions more to find and hopefully characterize.

I thought it might be useful — and fun — to take a look at some of the unusual planets found to learn what they tell us about planet formation, solar systems and the cosmos.


Artist’s conception of a hot Jupiter, CoRoT-2a. The first planet discovered beyond our solar system was a hot Jupiter similar to this, and this surprised astronomers and led to the view that many hot Jupiters may exist. That hypothesis has been revised as the Kepler Space Telescope found very few distant hot Jupiters and now astronomers estimate that only about 1 percent of planets are hot Jupiters. (NASA/Ames/JPL-Caltech)


Let’s start with the seven Trappist-1 planets.  The first three were detected two decades ago, circling a”ultra-cool” red dwarf star a close-by 40 light years away.  Observations via the Hubble Space Telescope led astronomers conclude that two of the planets did not have hydrogen-helium envelopes around them, which means the probability increased that the planets are rocky (rather than gaseous) and could potentially hold water on their surfaces.

Then in 2016 a Belgian team, using  the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile, found three more planets, and the solar system got named Trappist-1.  The detection of an additional outer planet was announced the next year, and in total three of the seven planets were deemed to be within the host star’s habitable zone — where liquid water could conceivably be present.

So, we have a most interesting 7-planet solar system quite close to us, and not surprisingly it has become the focus of much observation and analysis.

But consider this:  all seven of those planets orbits Trappist-1 at a distance much smaller than from our sun to the first planet, Mercury. The furthest out planets orbits the star in 19 days, while Mercury orbits in 88 days.



The Trappist-1 solar system, with the transit data used to detect the presence of seven planets, each one blocking the light curve at different locations. (NASA/JPL-Caltech)


Given this proximity, then, why are the Trappist-1 planets so interesting, especially in terms of habitability?  Because Trappist-1 puts out but .05 percent as much energy as our sun, and the furthest out planet (though very close to the star by the standards of our solar system) is nonetheless likely to be frozen.

So Trappist-1 a mini-system, with seven tidally-locked (never-rotating) planets that happen to orbit in resonance to each other.  Just because it is so different from our system doesn’t mean it isn’t fascinating, instructive, and even possibly the home of planets that could potentially support life.

And since red dwarf stars are the most common type of star in the Milky way (by lot), red dwarf solar system research is an especially hot field.

So there are mini planets and systems and massive planets in what used to be considered the impossibly wrong place.  And then there are planets with highly eccentric orbits — very different from the largely circular orbits of planets in our system.

The eccentricity of HD20782b superimposed onto our circular-orbiting inner solar system planets. (Stephen Kane)

The most extreme eccentric orbit found so far is HD 20782, measured at an eccentricity of .96. This means that the planet moves in a nearly flattened ellipse, traveling a long path far from its star and then making a fast and furious slingshot around the star at its closest approach. 

Many exoplanets have eccentricities far greater than what’s found in our solar system planets but nothing like this most unusual traveler, which has a path seemingly more like a comet than a planet.

Researchers have concluded that the eccentricity of a planet tends to relate to the number of planets in the system, with many-planeted systems having far more regularly orbiting planets.  (Ours and the Trappist-1 system are examples.)

Unusual planets come in many other categories, such as the chemical makeup of their atmospheres, surfaces and cores.  Most of the mass of stars, planets and living things consists of hydrogen and helium, with oxygen, carbon, iron and nitrogen trailing far behind.

Solid elements are exceptionally rare in the overall scheme of the solar system. Despite being predominant on Earth, they constitute less than 1 percent of the total elements in the solar system, primarily because the amount of gas in the sun and gas giants is so great.  What is generally considered the most important of these precious solid elements is iron, which is inferred to be in the core of almost all terrestrial planet.

The amount of iron or carbon or sulfur or magnesium on or around a planet generally depends on the amount of these “metals” present in the host star, and then in molecular protoplanetary disc remains of the star’s formation.  And this is where some of the outliers, the apparent oddities, come in.

A super-Earth, planet 55 Cancri e, was reported to be the first known planet to have huge layers of diamond, due in part to the high carbon-to-oxygen ratio of its host star. That conclusion has been disputed,  but the planet is nonetheless unusual.  Above is an artist’s concept of the diamond hypothesis. (Haven Giguere/Yale University)

The planet 55 Cancri e, for instance, was dubbed a “diamond planet” in 2012 because the amount of carbon relative to oxygen in the star appeared to be quite high.  Based on this measurement, a team hypothesized that the surface presence of abundant carbon likely created a graphite surface on the scalding super-Earth, with a layer of diamond beneath it created by the great pressures.

“This is our first glimpse of a rocky world with a fundamentally different chemistry from Earth,” lead researcher Nikku Madhusudhan of Yale University said in a statement at the time. “The surface of this planet is likely covered in graphite and diamond rather than water and granite.”

As tends to happen in this early phase of exoplanet characterization, subsequent measurements cast some doubt on the diamond hypothesis.  And in 2016, researchers came up with a different scenario — 55 Cancri e was likely covered in lava.  But because of heavy cloud and dust cover over the planet, a subsequent group raised doubts about the lava explanation. 

But despite all this back and forth, there is a growing consensus that 55 Cancri e has an atmosphere, which is pretty remarkable given its that its “cold” side has temperatures that average of 2,400 to 2,600 degrees Fahrenheit (1,300 to 1,400 Celsius), and the hot side averages 4,200 degrees Fahrenheit (2,300 Celsius). The difference between the hot and cold sides would need to be more extreme if there were no atmosphere.


Could super-Earth HD 219134 b be a sapphire planet? (Thibaut Roger/University of Zurich)

And then there’s another super-earth, HD 219134, that late last year was described as a planet potentially featuring vast collections of different precious stones.

To back up for a second, researchers study the formation of planets using theoretical models and compare their results with data from observations. It is known that during their formation, stars such as the sun were surrounded by a disc of gas and dust in which planets were born. Rocky planets like the Earth were formed out of the solid bodies left over when the protoplanetary gas disc cooled and dispersed.

Unlike the Earth however, HD 219134 most likely does not have a massive core of iron — a conclusion flowing from measurements of its density.  Instead, through modeling of formation scenarios for a scalding super-Earth close to its host star, the researchers conclude the planet is likely to be rich in calcium and aluminum, along with magnesium and silicon.

This chemical composition would allow the existence of large quantities of aluminum oxides. On Earth, crystalline aluminum oxide forms the mineral corundum. If the aluminum oxide contains traces of iron, titanium, cobalt or chromium, it will form the noble varieties of corundum, gemstones like the blue sapphire and the red ruby.

“Perhaps it shimmers red to blue like rubies and sapphires, because these gemstones are aluminum oxides which are common on the exoplanet,” said Caroline Dorn, astrophysicist at the Institute for Computational Science of the University of Zurich.



A variation on the “eyeball planet” is a water world where the star-facing side is able to maintain a liquid-water ocean, while the rest of the surface is ice. (eburacum45/DeviantArt)


Super-Earths, which are defined as having a size between that of Earth and Neptune, are also inferred to be the most likely to be water worlds.

At a Goldschmidt Conference in Boston last year, a study was presented that suggests that some super-Earth exoplanets are likely extremely wet with water – much more so than Earth. Astronomers found more specifically that exoplanets which are between two and four times the size of Earth are likely to have water as a dominant component.  Most are thought to be rocky and to have atmospheres, and now it seems that many have ocean, as well.

The new findings are based on data from the Kepler Space Telescope and the Gaia mission, which show that many of the already known planets of this type (out of more than 4,000 exoplanets confirmed so far) could contain as much as 50 percent water. That upper limit is an enormous amount, compared to 0.02 percent of the water content of Earth.

This potentially wide distribution of water worlds is perhaps not so surprising given conditions in our solar system, where Earth is wet, Venus and Mars were once wet, Neptune and Uranus are ice giants and moons such as Europa and Enceladus as global oceans beneath their crusts of ice.


Might this be the strangest planet of all? (NASA)


As is apparent with the planetary types described so far, whether a planet is typical or atypical is very much up in the air.  What is atypical this year may be found to be common in the days ahead.

The Kepler mission concluded that small, terrestrial planets are likely more common than gas giants, but our technology doesn’t let us identify and characterize many of those smaller, Earth-sized planets.

Many of the planets discovered so far are quite close to their host stars and thus are scalding hot. Planets orbiting red dwarf stars are an exception, but if you’re looking for habitable planets — and many astronomers are — then red dwarf planets come with other problems in terms of habitability.  They are usually tidally locked and they start their days bathed in very high-energy radiation that could stertilize the surface for all time.

A prime goal of the Kepler mission had been to find a planet close enough in character to Earth to be considered a twin.  While they have some terrestrial candidates that could be habitable, no twin was found.  This may be a function of lacking the necessary technology, or it’s certainly possible (if unlikely) that no Earth twins are out there.  Or at least none with quite our collection of conditions favorable to habitability and life. 

With this in mind, my own current candidate for an especially unusual planet is, well, our own.   Planet-hunting over the past almost quarter-century leads to that conclusion — for now, at least.

And it may be that solar systems like ours are highly unusual, too.  Pretty surprising, given that not long ago it was considered the norm.



How Planet 9 Would Make Ours a More Typical Solar System



The six most distant known objects in the solar system with orbits exclusively beyond Neptune (magenta) all mysteriously line up in a single direction. The new report shows a planet with 10 times the mass of the earth in a distant eccentric orbit anti-aligned with the other six objects (orange) is required to maintain this configuration. Image: Caltech/R. Hurt (IPAC)"
The six most distant known objects in our solar system with orbits (magenta) exclusively beyond Neptune all mysteriously line up in a single direction. A new report identifies the potential presence of a distant solar system planet — with 10 times the mass of the Earth and in a distant and eccentric orbit (orange) — as the reason why.  (JPL/Caltech; R. Hurt)

There’s been a ton of justifiable excitement these days about the possible discovery of a ninth planet in our solar system — an object ten time the mass  of Earth and 200 times further from the sun.  Especially in the context of the recent demotion of Pluto from a planet to a dwarf planet, the announcement of a potential replacement seems almost karmic, stage managed, in its take-and-give.  This is especially so since the astronomer probably most responsible for the diminished position of Pluto is also the one who now asserts the very far away presence of a different Planet 9 — planetary astronomer Michael Brown of the California Institute of Technology.

The validity of the possible detection of a Planet 9 has set off hot debates — with NASA officials, for instance, making clear that the agency sees the “discovery” as an exciting but early step towards establishing the existence of possible new planet.  We are all drawn to discovery and controversy, so the presence, or non-presence, of the planet has been the focus of attention.

But another most intriguing aspect of the finding has been largely ignored — the way  that such a Planet 9 would make our solar system surprisingly more similar to the many more eccentric exoplanet solar systems now known to be out there.  Our solar system would also suddenly have a range of planets sized more like the galactic norm.

What’s more, there’s reason to consider that a Planet 9 could have been spun off another solar system rather than having been ejected from the inner solar system, as proposed by Brown and colleague Konstantin Batygin.

In other words, Planet 9 may be an “exoplanet” in origin.  And if not, a finding that it was ejected long ago from our inner solar system would answer some questions about why our system seems to be so different from many of the other exoplanetary systems discovered so far.

Mike Brown and Konstanytin Batyglin of Caltech
Astronomers Mike Brown and Konstantin Batygin of Caltech.  They took research by Scott Shepard of the Carnegie Institution for  Science and Chad Trujillo of the Gemini Observatory in Hawaii regarding the unusual paths of objects orbiting beyond Pluto and carried it further to conclude there is a Planet 9 in the distant solar neighborhood.  (Lance Hayshida/Caltech)

“Our Planet 9 has a very eccentric orbit like planets in many other solar systems, and it’s a size of planet not found in our solar system but is the most common in other solar systems,”  said Brown.  “Seems odd to say, but it would make our solar system more normal.”

More specifically, here are the reasons why:

  • The most commonly sized exoplanet detected so far is larger than Earth and smaller than the next largest planet in our solar system, Neptune.  Since the difference in size is substantial — Neptune’s diameter is 4 times greater than Earth’s and its mass is 17 times greater — that leaves a lot of exoplanets of a size category different from anything in our solar system.  This absence has been a puzzle and would be reduced if Planet 9, some 10 times more massive than Earth, was determined to be real.
  • Many, if not most, solar systems identified so far are home to planets with very eccentric orbits.  In our solar system, the eight planets orbit on a generally singular plane, and most orbits are more circular than not.  The proposed Planet 9 would orbit on a very different plane — thirty degrees off the rest of the solar system’s planetary plane — and it circles the Earth in a most peculiar 10,000 to 20,000-year orbit.
  • Astronomers theorize that planets are ejected from their solar systems all the time, and roam through space without an orbit.  But in theory, they can easily move into a solar system where a sun and other planets pull it into an orbit around them.  This kind of planet capture has been successfully modeled many times, and has even once been identified.


Artist rendering of possible Planet 9, described in a recent edition of the Astronomical Journal. The authors estimate that the planet comes as close to the sun as 100-200 astronomical units (the distance from the Earth to the Sun) and travels as far away as 1200 AUs. (Caltech/R. Hunt)
Artist rendering of possible Planet 9, described in a recent edition of the Astronomical Journal. The authors estimate that the planet comes as close to the sun as 100-200 astronomical units (the distance from the Earth to the Sun) and travels as far away as 1200 AUs. (Caltech/R. Hunt)

The question of whether the object identified came from the inner solar system (as deemed likely  by Brown) or from elsewhere is a complicated one with a special interest for exoplanet researchers.

As reported by Brown and Batygin, the best theory to explain the faraway presence of Planet 9 is that it was ejected long ago from the region around Jupiter to Neptune.  Such solar system ejections are understood to happen all the time, and it would be a logical explanation given the relative closeness of our solar system planets.  The planet could have gotten knocked off course by coming too close to Jupiter, with its strong gravitational pull.

As theorized by the two authors, the planet could have then come to an orbital rest after being slowed down by gases.  But that wouldn’t occur until it was well past the solar system we know:  each orbit around the sun would take an estimated 15,000 years.

But Hagai Perets, an astrophysicist formerly at the Harvard-Smithsonian Center for Astrophysics and now at the Israel Institute of Technology, says it is equally or perhaps more plausible that Planet 9 (if it exists) came from another solar system entirely.  Having studied “roaming planets” kicked out of their solar systems, he says he is convinced that it could happen.

“Solar systems,” he said, “throw around their planets like we toss balls.

“We know there are planets with very wide orbits, thousands of astronomical units {the distance from the sun to Earth} from their suns.  We need a mechanism to explain this phenomenon, since the planets could not be formed in that region.

Hagai Perets, an astrophysicist at Technion- Israel Institute of Technology. He has studied rogue planets kicked out of their solar systems, and argues that the possible Planet 9 could have arrived from somewhere other than our solar system.
Hagai Perets, an astrophysicist at Technion- Israel Institute of Technology. He has studied rogue planets kicked out of their solar systems, and argues that the possible Planet 9 could have arrived from somewhere other than our solar system.

“That’s where stellar clusters come in, because most stars are formed in these clusters.  With so much activity going on as stars and solar systems are formed, it makes sense that there would be a great scattering of planets in their early epochs, and some of those planets would be ejected completely.

“They become free-floating, rogue planet,” he said.  “We have observational evidence that they exist, as well as our theoretical models.”

Brown agrees that Planet 9 could have come from another solar system, but he believes that an ejection from our inner solar system is the most plausible explanation.

Proposed orbit for a Planet 9 -- eccentric and distant from the sun, like many exoplanets and their host stars. For more information about planets that orbit far, far from their host stars, check out this recent discovery:
Proposed orbit for a Planet 9 — eccentric and distant from the sun, like many exoplanets and their host stars. For more information about planets that orbit far, far from their host stars, check out this recent discovery:

The potential discovery of a Planet 9 was made the way that Neptune was first identified — by detecting its gravitational effects on other objects.  (In the case of Neptune, that meant the effects on Uranus.)  This indirect process of discovery is not dissimilar from the first, and still widely used, method of finding exoplanets — by detecting through radial velocity the gravitational “wobble” that exoplanets cause in their host stars.

Brown and Batygin found evidence for the planet’s existence in the peculiar orbits of objects well beyond Neptune detailed in a previously published study by Scott Shepard of the Carnegie Institution for Science.  The authors analyzed six of the objects and found that they moved in their elliptical orbits while pointing in the same direction and while tilted at similar 30 degrees angles.

“It’s almost like having six hands on a clock all moving at different rates, and when you happen to look up, they’re all in exactly the same place,” Brown said in a statement. “Basically it shouldn’t happen randomly. So we thought something else must be shaping these orbits.”

Brown has a long history of studying the vast Kuiper Belt well beyond Neptune and its untold objects large and small.  It was in the course of his research of these “trans-Neptune objects” that he came to the conclusion that Pluto didn’t meet the accepted standards for what defines a planet.  It was just too small and its presence has little or no effect on surrounding objects.  Having reached that conclusion, he became a leader in the effort to have the planet demoted.

So having been involved in the undoing of the original Planet 9, he is now convinced there is another — very different — Planet 9.  Brown specifically calls it “Planet 9” rather than the long-discussed “Planet X” because, he said, there have been so many false claims made about a possible “Planet X.”

As he explained it:  “We wanted to highlight the strong science behind the finding.”

Now that stronger evidence for the distant world has been discovered, Brown thinks that within five years the planet can be directly imaged by astronomers — or perhaps will be discounted as unable to be confirmed.

Ironically, the naming of a “Planet 9” has already hit some headwinds — well before its existence is confirmed or rejected.  As with the change of the name of Pluto from a “planet” to a “dwarf planet,” there is interesting science behind the objections.

Alan Stern, principal investigator for the New Horizons mission to Pluto, has dismissed the name “Planet 9” due to his firm belief that there are many objects orbiting out beyond Pluto that are potentially planet size.

“I think the number of planets in our own solar system is going to explode, and that this is going to be one of the important lessons of 21st century astronomy.  I think people will get over worrying about their names pretty quickly.”

The reported Planet 9 inhabits the icy realm of the Kuiper Belt. (NASA)
The reported Planet 9 inhabits the icy realm of the Kuiper Belt. (NASA)

Stern pointed to research suggesting the early presence in our solar system of large planets that were later ejected to places unknown.  Some of those planets likely stuck around in far-off orbits like the proposed Planet X (or Planet 9.)

If this turns out to be the case,  Stern said, their existence would confirm his (and others’) long-held belief “that the majority of the planets in our solar system orbit far beyond the classical ones we grew up with.”

In addition to being compelling science, such detections would also support the view that the primary difference between planets in our solar system and exoplanets beyond is simply where they orbit.  Consequently, just as the study of our solar system informs the exploration and characterizing of exoplanets and their systems, so too does the science of exoplanets help better understand our solar system.

Together, they also tell us that our understanding of the vast menagerie of planets out there remains quite limited, with far less known than unknown.