The entire universe in blog form

Jan. 28 2015 7:30 AM

AMAZING Halo Display

Over the weekend I posted an amazing and lovely picture of a sun halo, taken in Sweden. That prompted Kalle Centergren, a self-proclaimed science nerd to send me a photo he took of a halo he saw skiing in Austria.

That’s the photo above, showing an incredible array of optical effects: The 22° halo, parhelia, tangent arcsparhelic circle, a supralateral arc, a Parry arc, a circumzenithal arc, possibly a 46° halo, and probably others I’m not even sure of. That is one of the craziest and most vivid displays I’ve ever seen.


But it gets better: Centergren took video, including some using a drone:

Seriously, right? I expect there was a lot of small snow crystals in the air, suspended over the slopes, creating all those ridiculously beautiful effects.

I went skiing once and tore my ACL. Centergren goes skiing and gets some of the most incredible halo footage I’ve ever seen. Seems fair.

Also, this made me laugh: I noticed that in the drone footage there’s a lot of what looks like detector noise; white specks that flash all over the field. I'm used to seeing this in astronomical data taken right off the telescope; they're caused by cosmic rays, little particles zipping though space that slam into the detector and deposit their energy behind. They’re a massive pain when you’re trying to observe faint objects; you have to go through a lot of careful processing to get rid of them.

So of course that was my first thought when I saw the footage; it's what I'm used to. But that's not at all what those flecks are: They're actually snow crystals blowing around in the air, the very same crystals causing the halo and other optical effects! They appear bright because they're near the camera, and flashing in the sunlight as the camera looks toward the Sun. 

There's an old expression in skepticism: When you hear hoof beats, think horses before you think zebras. In this case, my brain is trained to listen for zebras!

It goes to show that your brain can be easily fooled, even when nothing is really trying to fool you. It just happens. That's always worth keeping in mind.

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Jan. 27 2015 11:41 AM

Astronomers Find Ancient Earth-Sized Planets in Our Galactic Backyard

Astronomers have announced what may be the most interesting exoplanet discovery yet made: five planets, all smaller than Earth, orbiting a very ancient star. And I do mean ancient: Its age is estimated to be more than 11 billion years old, far older than the Sun. These are old, old planets!

There’s a lot going on here, as far as the science goes. Let me explain. No, there is too much. Let me sum up. Here are the bullet points:

  • The planets were found using the Kepler space telescope, which uses the transit method: If a star has planets, and we see those orbits edge-on, the planets pass in front of their star as seen from Earth. This blocks a bit of the light, and we can measure that. The amount of light blocked (compared with known properties of the star like its size) tells us how big the planet is. The length of time it takes the planet to transit the star also gives us its orbital period, orbital size, and an estimate of its temperature.
  • The star is called Kepler-444. It’s a bit cooler, more orange, and smaller than the Sun (a K0 dwarf, if you want the details), and is about 117 light-years from Earth. That’s relatively close! Amazingly, it’s actually a triple-star system: There’s a pair of cool red M dwarfs orbiting each other, and the pair in turn orbits the K star. The binary is about 10 billion kilometers from the K star, about twice the distance Neptune is from the Sun.
  • The five planets orbit the primary K star, and are called Kepler-444b up to Kepler-444f. All five are smaller than Earth, and get bigger in order with their distance from the star: Kepler-444b has a diameter of 0.403 Earth, Kepler-444c is 0.497 Earth, d is 0.530, e is 0.546, and f is the biggest at 0.741 our home planet’s size.
  • The planets are not in any way Earthlike! The system is very compact; all five planets are quite close to their parent star—even the most distant one, planet f, is closer to its star than Mercury is to the Sun—and therefore pretty hot. They all orbit the star in fewer than 10 days. They’re pretty well cooked.
  • The system is very old. This was determined using a method called astroseismology, a bit like using earthquakes to observe the Earth’s interior. In this case, the surface of the star vibrates, like standing waves in a bathtub or the way a drumhead vibrates. The character of these waves depends on a lot of the physical properties of the star: its density, mass, surface gravity, size, and age. Very careful observations taken over many weeks were used to get the astroseismological results, and the age was found to be about 11.2 billion years, give or take a billion years. (I’ll note that in part this work was funded through the Pale Blue Dot project, which lets people adopt a star for a small fee that goes toward astroseismology research. Previously, the smallest exoplanet found used research funded through this group, too! And someone named Brian Finley had adopted Kepler-444, so congrats to him, too.)
  • Assuming the planets formed along with the star—a reasonable assumption—these planets have been around a long, long time. The Universe itself is 13.8 billion years old, and the Milky Way galaxy somewhat younger. These stars and planets formed when the Universe itself was young. Put it this way: When the Sun and Earth formed, these planets were already older than the Sun and Earth are now.

So what does all this mean?

Quite a bit, actually. For one, until now we weren’t sure just how old planets could be. We’ve found some Earth-sized planets older than us, but none this old.

Initially, the Universe was almost all hydrogen and helium, with the heavier stuff coming later. The iron and nickel in the Earth, for example, were formed in supernovae, massive stars that exploded billions of years ago. As the Universe ages, it gets more and more of these elements as more of these big stars explode.

When Kepler-444 formed, there were relatively fewer of these heavy elements, and spectra of the star confirm a paucity of elements like iron. We’ve discovered enough exoplanets now that we see an interesting relationship between heavy elements and planets: Gas giants (like Jupiter and Saturn) tend to form around stars that have more heavy elements; these elements aid in the formation of larger planets. But when you look at smaller, more Earth-sized planets, that relationship goes away. Smaller planets form around stars that have lots of heavy elements, and they also form around stars that have relatively few.

The Kepler-444 system supports this. A gas giant planet would’ve been seen, so it looks like these five planets are all it has (or the biggest it has), and each is small and presumably rocky.

But what of life?

Let me remind you, these planets are flippin’ hot. The coolest most likely has a surface temperature way above the boiling point of water. I wouldn’t think there could be life there.

But don’t be so specific. Take a step back and realize that what this means is that Earth-sized planets could form around Sunlike stars even 11 billion years ago! That may have profound implications for life.

You may have heard of the Fermi paradox: If life is easy to get started on planets, then where are the aliens? We do know that life formed on Earth not too long after the planet’s crust had cooled enough to support it. Let’s say it takes 4 billion years for those protozoa to evolve and build spaceships. It turns out that, even with the vast distances between stars and limiting your ships to far less than the speed of light, you can colonize the entire galaxy in just a few million years. That’s far less than the age of the galaxy.

Perhaps you see the problem. If planets like Earth formed 11 billion years ago, and happened to form at the right distance for more clement conditions on the surface, life could have arisen long enough ago and started building spaceships long before the Earth even formed! They’d have planted their flags on every Earth-sized habitable planet in the Milky Way by now.

Where are they?

How long have aliens been around? Assuming any exist in the first place ...

Photo by Pictures/Phil Plait

We don’t know. There are too many “maybes.” Maybe Earth is special in some way that made life easier to form here. Maybe you need iron and nickel to build spaceships (but even then there are planets a billion or two years older than us that would’ve had plenty of such elements). Maybe evolution doesn’t always work its way to intelligence. Maybe every civilization advanced enough to manipulate its environment did so to its own detriment (cough cough). Maybe they blew themselves up. Maybe they’re out there but so advanced we don’t even recognize them.

Maybe we’re just the first.

That’s always been an idea in my back pocket to explain the Fermi paradox. Someone has to be the first. But that’s a bit tougher to swallow when you see rocky planets that are more than twice as old as our own home planet. Eleven billion years is a long time.

Clearly, we just don’t have all the information yet. We’re just getting started here! We’ve discovered thousands of planets orbiting other stars, but there are probably billions of them out there. Billions! We have a lot more data to collect, a lot more information to analyze, and a lot more thinking to do before we can solve this particular mystery.

But we’re working on it. Kepler-444 and its five, small, melted, ancient worlds are just one small piece of a puzzle that is vast and deep. And they’re a good start.

Jan. 27 2015 7:00 AM

Rosetta Catches a Comet’s Snowflakes on Its Tongue

I’ve seen a lot of stuff when it comes to space science and astronomy, and sure, I’m easily excited about it all … but still, it takes a lot to get me to boggle at something.

So this is me, boggling: This photo below shows grains of comet dust caught on the fly by the Rosetta spacecraft!


Yeah. That is very, very cool.

To be more accurate, they’re the remains of comet dust caught on the fly by the Rosetta spacecraft. OK, let me explain.

Comets are essentially dust, gravel, and rocks packed together by various types of ice. Generally speaking, we’re talking water, carbon dioxide, ammonia, carbon monoxide, and other things that are usually gaseous on Earth, but which are frozen in the depths of space.

Lots of comets orbit the Sun on long, elliptical paths, taking them out into the black, then back in closer to the Sun. As they near the Sun, the ice turns into a gas and blows off, and the other junk making up the comet are blown into space as well.

The Rosetta spacecraft is currently following along a comet, called 67P/Churyumov–Gerasimenko. It orbits the Sun once every 6.5 years, going out as far as Jupiter’s orbit (it’s called a Jupiter-family comet, in fact, a member of many comets with similar orbits), dropping down to just outside Earth’s orbit. As I write this, the comet is about 370 million kilometers from the Sun, a bit more than twice Earth’s distance, and still outside the orbit of Mars.

Still, that’s close enough that it’s already becoming active, and we see streams of gas flowing out of it. That means dust particles are coming off too. The thing is, “dust” is a somewhat generic term for tiny flakes of stuff that can have wildly different compositions. Rosetta is in the unique position to find out what 67/P’s dust is made of. So engineers and scientists gave it a shot.

That shot is COSIMA, the Cometary Secondary Ion Mass Analyser. Have you ever been in a snowfall and caught snowflakes on your tongue? That’s COSIMA, except it has a plate exposed to space instead of a tongue, and instead of snowflakes it’s catching, well, comet snowflakes.

When 67/P was still more than 450 million km from the Sun, and just 30 km from the comet, Rosetta caught several flakes of material from the comet. They impacted the plate at speeds of just 1–10 meters per second, roughly bicycling speed. The photo above shows two such specimens (the scale bars represent 500 and 300 microns, where a human hair is roughly 100 microns wide).

When they hit the plate they fragmented. If there had been lots of ice in them, they would have been held together better and wouldn’t have shattered, so right away this tells us the flakes were dry (not like Earth snowflakes at all). They also have a high sodium content, which matches lots of other interplanetary dust particles, particularly meteoroids that burn up in our atmosphere during meteor showers. We know those come from comets, so that checks out! This means we’ve actually found a sample of the parent material of meteor showers. Cool.

But what’s also interesting is what this means for the surface of the comet. These particles were emitted when the comet “turned on” again, getting close enough to the Sun to become active. Scientists think these grains were actually left over from the last time 67/P came ‘round the Sun. As the comet began to head away from the Sun, the flow of gas outward weakened, and wasn’t strong enough to lift dust away. That material then sat on the surface, and was lifted off as the outflow became strong once again a few months ago.

That outer mantle of older dust will be shed, and then more stuff deeper down will start to get flung away. When this happens the dust content may change, possibly showing us other types of material as well. Rosetta will be around for that; it will follow the comet for many more months as it gets to its closest point to the Sun (called perihelion). The comet should become more active, and we’ll get to investigate what lies beneath.

That to me is incredibly exciting. We know a lot about comets, but the devil’s in the details, and every comet is different. Heck, even a single comet changes a lot over the course of a single orbit, so by monitoring 67/P for several months, we’ll learn a lot about these weird beasts. And that’s the whole point.

Jan. 26 2015 7:00 AM

Comet Close-Ups Reveal an Alien World

After a 10-year voyage, the Rosetta spacecraft entered orbit around the comet 67P/Churyumov–Gerasimenko in August 2014, the first time such an achievement had ever been made. Most of the images made public at the time (and since) were from the wide-angle NAVCAM instrument, and only a handful of the higher-resolution OSIRIS camera pictures were released. 

But now the first results from the observations have been published, and quite a few close-up images from OSIRIS have been revealed … and they’re spectacular. The comet is a bizarre, alien place, where our notions of up and down get stymied, and where our “common sense” (from having grown up on a vast, heavy-gravity, be-atmosphered planet) is likely to betray us.

Here are some of those pictures that caught my eye.

Jan. 25 2015 7:45 AM

His First Halo

Swedish astrophotographer Göran Strand took that picture above. It shows a halo around the Sun, replete with parhelia, over Lake Storsjön. That’s his son in the photo.

Besides being extremely beautiful and poignant, it’s also just an astonishing shot. Strand used a 14mm wide-angle lens, which he needed because the full halo is 44° across, a quarter of the way around the horizon. A lens like that really compresses distance, so I’d guess his son was standing right in front of him.


Halos likes this are relatively common. They’re due to flat hexagonal ice crystals in the air. As sunlight enters a crystal it gets bent, refracting as it enters and as it leaves the faces of the crystal. The light gets bent by a total of about 22°, so crystals that distance from the Sun along your line of sight bend the sunlight toward you. Crystals closer to the Sun bend the light away from you so it’s darker inside the halo. Depending on the exact orientation of the crystal, some of the light gets bent more than 22°, so ice outside that 22° bright ring also bend light toward you, though not as strongly, so the halo fades away outside the optimal 22° angle.

When I say “degrees”, I mean an angular distance on the sky, where 90° is the angle between the horizon and the zenith, straight overhead. Your outstretched fist is roughly 10° (I have big hands, so for me it’s more).

The light forms a circle because that shape defines a constant distance from a point; for a halo that point is the Sun (the center of the circle), and the ice crystals 22° away from it fall along the circle of that size.

As it happens, red and blue light get bent by different amounts as they enter and leave the ice crystals; red light is bent a wee bit less, so the inner edge of the halo is redder and the outer part bluer. You can see that in Strand’s photo.

Parhelia—also called sundogs—are also caused by the hexagonal crystals. As the crystals fall through the air they align flat, face down to the ground. When the line between the Sun and crystal is parallel to the horizon, a lot more light gets bent toward you, so you get really bright spots on the halo on opposite sides.

It looks like he has a sun pillar in there, too: a vertical shaft of light caused when the flat crystals are slightly tipped, reflecting light toward the observer. That can only happen with crystals seen directly above or below the Sun.

I’ve seen halos and parhelia I-don’t-know-how-many times. Dozens certainly. Hundreds? Maybe. I look up a lot. If you haven’t ever seen this incredible and lovely display in the sky, then you know what to do.

And check out more of Strand’s photography. I’ve featured a lot of his work on this blog, and you’ll see why. 

Jan. 24 2015 7:30 AM

Watch the Moon Get Squashed From Orbit

Sometimes what you see depends on where you are. And when you look around from orbit, things can look really, really strange.

Don’t believe me? Then watch this Vine video posted by ISS astronaut Terry Virts, an animation showing the Moon setting as seen from space:


See what I mean? The caption says,

#moonset during high-beta a while back. It gets squished, turns red, and disappears- pretty cool.

There’s a lot packed into those few words. First, when the Moon sets as seen from the space station, it’s more a reflection of the space station’s movement around the Earth than the Moon’s. For us on the ground, the Moon sets once per day, but from the station it sets 16 times per day! The Moon appears to move around the sky faster, and it moves through its own diameter in just under eight seconds. From the ground that takes closer to two minutes. The video is sped up, but not as much as you might think.

Why does it get squished? That’s an atmospheric effect. The air acts like a lens, bending the light from the Moon; as the Moon sets, the bottom is seen through thicker air, which bends the light more. In effect, the bottom of the Moon looks like it sets slower than the top, so the Moon gets squished.

The redness is due to another atmospheric effect: scattering. Light from the Moon comes in from space and encounters our air. Photons, particles of light, hit the molecules of nitrogen and oxygen and scatter, a bit like two pool balls hitting each other. Blue light is way more sensitive to this than red. When the Moon is on the horizon, we’re looking through a lot more air, so there’s a lot more scattering. The blue light gets sent off in random directions, away from our eyes, while the red light gets through, making the Moon look red. It’s more complicated than this (haze, pollution, smoke, and other factors can amplify the effect), but that’s the gist of it.

You can see this photos, too: Appropriately enough, here’s one Virts posted right after the moonset video:

Moon set
Moon set into a rainbow atmosphere.

Photo by NASA

Finally, what’s all this about the “high-beta a”? In this case, for Twitter, Virts was being brief: He means the beta angle, which has to do with the orientation of the space station as it orbits the Earth. Here’s a diagram via NASA:

ISS angles
In this part of the year, the ISS sees sunlight almost if not all the time.

Photo by NASA

The angle between the space station’s orbital tilt and the direction to the Sun is the beta angle. In this drawing, the tilt is such that the station is nearly face-on to the Sun. That’s a high angle. If it were more edge-on to the Sun (as it would be three months later, when the Earth has moved around its orbit ¼ of the way) then it would be low beta angle.

This affects the orientation of the Moon. This gets complicated pretty quickly, but in general, if his video were taken during low beta angle, the Moon would have looked more like it was facing into its own movement, in this case moving more right-to-left (or maybe left-to-right, depending on orbital angles). At high beta angle, the movement is closer to perpendicular to that.

Another way to look at it: See that line dividing night and day on the Moon? That’s called the terminator. Speaking very roughly, seen from Earth the Moon tends to move across the sky in a direction perpendicular to that line. But because the space station was at high beta angle, the Moon’s movement is nearly parallel with it.

I know, this can be headache inducing. I had to draw myself some diagrams to make sure I understood this myself. The details get pretty maddening, like the Moon’s orbital tilt with respect to the Sun, the season, the latitude of the observer, and more. That’s why I’m saying my descriptions are very general! To make them specific would take a lot of words. A lot.

Anyway, the point is, there are more things in heaven and Earth than are dreamt of in your philosophy … unless you understand the physics. And play the angles.

Jan. 23 2015 7:00 AM

Crash Course Astronomy Episode 2: Naked-Eye Observing

Astronomy is a funny science. There’s all the technical, physical stuff: orbits, planets, galaxies, stars, and all that. You can spend a lifetime—multiple lifetimes—learning that.

But there’s also going out and doing it. Looking up, observing the skies. And the easiest way to do that is without telescopes, binoculars, cameras, or any other equipment: Just stand (or sit or squat or lie down) under the stars and watch them.


And I mean really watch them. What do you see?

A lot, actually. There’s an amazing amount you can learn about the Universe just by paying attention to what’s going on over your head … and that’s what Episode 2 of Crash Course Astronomy is all about: naked-eye observing.

This week is a great time to go out; the Moon is new, Venus and Mercury still grace the western horizon after sunset, Jupiter is on the rise in the east, and lots of bright, pretty, colorful stars are yours for the viewing.

All you have to do is go outside and look up. Go!

P.S. Don’t forget to watch Episode 1, too.  

Jan. 22 2015 7:00 AM

GOP Senators Momentarily Pull Heads Out of Sand, Then Ram Them Back In

Ah, your government inaction. Yesterday, the Senate voted on whether reality was real. And they voted the wrong way.

The senators were debating the bill to give a go-ahead to the Keystone XL pipeline. As is usual with a bill of this size, there were several amendments attached to it. One, by Sen. Sheldon Whitehouse (D–Rhode Island), was quite simple. It stated:

To express the sense of the Senate that climate change is real and not a hoax.

That’s it. That’s the whole amendment. The Senate voted, and it went 98-1 in favor (the one nay vote: Sen. Roger Wicker (R-Mississippi)). So the Senate voted that climate change is real, and not a hoax.

This may surprise some folks, given the GOP’s majority, and how so many of them are climate change deniers. But it didn’t surprise me at all, because I’m familiar with how deniers weasel their way around an argument … and sure enough, I was proven right.

Sen. James Inhofe (R-Oklahoma) is the most vocal science denier in the Senate, literally having written the book on it. You’d expect him to vote nay on the amendment, right?

Ah, here’s where I knew what he was going to do. He voted yea. How? Because as the deniers like to say, “The climate’s always changing.” It’s a cheat, a cop-out. Yes, the climate is always changing, but we know that in the past few decades that change is due almost if not entirely to human activity. The amendment left out that last bit, giving deniers the wiggle room they needed.

And, as I expected, that’s exactly what happened. Inhofe, for example, was able to vote for the amendment because while the climate changes, he thinks that the idea of humans causing it is a hoax. He tweeted as much:

Knowing this, he even went so far as to co-sponsor the amendment!

This really shows just how out of touch Inhofe and his compatriots are with reality. Even his yea vote was nothing more than a craven political ploy. It was simply a dodge.

And his ridiculousness just got worse when he took to the floor to give his speech; he said it was arrogant to think humans can change the environment. Um, Mr. Inhofe, I refer you to the ozone hole, which was caused by humans, and then actually put on the mend by humans, too.

Anyway, things got weirder with another amendment, sponsored by Sen. Brian Schatz (D-Hawaii), saying that the climate is changing and that humans are “significantly responsible.” That amendment, as I expected, was shot down, though interestingly only by a 50 to 49 vote. It needed 60 votes to pass, so 50 votes for it were not enough.*

One particularly brain-twisting bit about this came from Sen. Lisa Murkowski (R-Alaska):

Republican Lisa Murkowski of Alaska urged her colleagues to vote down the amendment for one specific reason: the amendment says that human activity “significantly” contributes to climate change. That word was a matter of “degrees,” she said on the floor.

That’s really ironic, given that Murkowski’s understanding of global warming is pretty screwed up. She claims volcanoes spew out more emissions than cars. That’s exactly wrong, for two reasons: 1) Humans generate far more CO2 than volcanoes do annually, and b) volcanoes actually emit a lot of sulfur dioxide, which is an aerosol that acts to cool the climate. Despite aerosol emission, the planet’s getting hotter. Why? Humans.

Anyway, that amendment was Inhofe’s (and the other deniers’) out: They could vote that change is real (essentially with their fingers crossed behind their backs) knowing that they could also vote on the other amendment saying humans aren’t causing it.

I’ll note four GOP senators voted yea on this: Kelly Ayotte (New Hampshire), Lamar Alexander (Tennessee), Mark Kirk (Illinois), and (to my surprise) Lindsey Graham (South Carolina). I will have to keep my eye on them; I don’t know if this means headway is being made into the atmosphere of GOP climate change denial or not.

This vote was largely for show anyway, since the resolutions aren’t binding in any way. And President Obama has said he’ll veto the pipeline bill in any case.

So in the end, this vote only shows us what we already knew: Climate change deniers in the Senate are still willing to ignore the overwhelming evidence of science, and are more willing to score cheap political points than take any real action. The real question now is, will they be able to come up with the number of votes needed to override the president’s veto?

*Update, Jan. 22, 2015, at 19:30 UTC: This paragraph has been updated to clarify that the amendment needed 60 votes to pass. 

Jan. 21 2015 12:49 PM

Milky Way Moonset

Michael Shainblum is a photographer whose favorite target is the Milky Way (though he took one of the most amazing photos of 2014, lightning hitting the Burj Khalifa, the tallest building in the world).

He sent me a note recently that he caught an unusual scene in Big Sur, California, and, well, take a look:


How cool is that? The Milky Way is almost exactly vertical, plunging down into the Pfeiffer Beach Keyhole Rock, a natural arch carved by erosion. But what’s that glow in the hole? That’s the crescent Moon, setting into the horizon but blocked by the rock itself. The Moon’s path across the sky against the background stars passes fairly close to the center of our spiral galaxy, which we see edge-on because we’re inside it.

This shot is a mosaic of five panels, going from nearly to the zenith down to the rocks at the foot of the tripod supporting the camera. I suspect the subtle illumination of the arch is from the rocks, water, and beach that were lit by the Moon; their back-reflection would then light up the side of the arch facing the camera.

I’ve seen a lot of photos of the Milky Way on the sky, so sometimes you really need to pick your foreground—and the timing—just right to get a photo that really stands out.

You can see more of Shainbum's work on his Facebook page, too.

Jan. 21 2015 7:30 AM

Two New Planets in the Solar System? Not So Fast, Folks.

A team of astronomers made something of a news splash late last week when they announced they have indirect evidence that there could be one or more massive planets orbiting in the solar system well beyond Neptune.

I read their journal paper, and their argument is certainly interesting (I’ll explain it in a sec). But let me be clear here: Their evidence of any possible planets out past Neptune is indirect (they don’t have photos or anything like that), it’s based on a small number of objects, and we do have evidence that there aren’t really big (like gas giant–sized) planets past Neptune. And it pains me to even have to bring this up, but of course this has nothing to do with Nibiru crackpottery, either.


Bottom line: To me, this is an interesting and potentially promising line of research, but right now it is quite inconclusive about the existence of planet-sized bodies past Neptune.

How this works isn’t that hard to understand in principle. (Note: After writing this but before posting it, I found that the AstroBites blog also discusses this topic, with more technical info.) In our solar system we have the Sun at the center, and it pretty much runs everything. It has 98 percent of the mass of the solar system, so its gravity is in charge of how everything else moves. BUT, there are also planets that have gravity as well. Their gravity is weak compared with the Sun’s but is strong enough that, given time, the planets can affect the orbits of other objects.

Out past Neptune is a region occupied by objects that are similar to asteroids but made of ice instead of metal and rock (making them more like comets, really). There are various names for them, but in general they’re called trans-Neptunian objects, or TNOs. Some are on circular orbits, some more elliptical, some have orbits tipped to the plane of the solar system, some don’t.

A handful, about a dozen discovered so far, have really weird orbits. They are highly elongated, and tipped significantly to the plane of the solar system. The authors of the study call them Extreme TNOs.

Their orbits are difficult to explain from what we know about the solar system now. However, the authors note that there is a comet called 96/P Machholz 1 that also has an odd orbit. It goes around the Sun backward (retrograde) relative to the planets, and the shape and orientation of its orbit change over time. This is due to the influence of Jupiter; the comet’s orbit takes it out as far from the Sun as Jupiter’s orbit, so the huge planet pokes and prods the comet over time. This changes the comet’s orbit, making it undergo all sorts of peculiar behavior.

The authors then speculate that the weird TNOs may be explainable in a similar way. The TNOs fall into four groups according to distance, implying a series of planets at distances ranging from 40 to 150 billion kilometers from the Sun. (For comparison, Neptune is about 4.5 billion km out.) They don’t give specifics about the possible masses these planets would need, except to say they would need “at least several Earth masses” to affect the TNOs.

Again, the evidence they present is interesting, maybe even compelling, but it by no means is proof. They only look at the orbital characteristics of about a dozen extreme TNOs, and it’s hard to extrapolate safely from that. It seems clear something odd is going on, but the mechanism behind it isn’t clear. Planets? Maybe. But it could be something else.

I’ll note that a similar study was done with long period comets, which also found weird orbital characteristics that could be explained by a planet or planets past Neptune affecting their orbits. Unfortunately, this too relied on small number statistics and is interesting but not conclusive.

If these planets exist they can’t be too much bigger than Earth. Otherwise they’d have been seen by now; the NASA infrared survey observatory WISE has shown that no more Jupiter- or Saturn-sized planets can exist in our solar system, even way far out.

Personally, I’d love to have direct evidence of such planets. When I worked with Hubble, I spent some time trying to figure out ways of finding such planets! There’s no real theoretical reason they don’t exist, and we see evidence of planets orbiting other stars at great distances. So why not?

In the end, this research is perhaps motivation to keep looking. Even big planets would be terribly faint and difficult to detect at 150 billion km, so it may be quite a while before we have a confidently complete survey of the solar system. And even if they don’t exist, I’m glad people are still thinking about things like this. It’s best in science not to get too complacent with the “current understanding.” Nature is tricky and a lot more clever than we are.