What is the biggest known planet in the Universe?

What is the biggest known planet in the Universe?
 VLT NACO image, taken in the Ks-band, of GQ Lupi. The feeble point of light to the right of the star is the newly found cold companion. It is 250 times fainter than the star itself and it located 0.73 arcsecond west. At the distance of GQ Lupi, this corresponds to a distance of roughly 100 astronomical units. North is up and East is to the left.   Credit:    ESO

VLT NACO image, taken in the Ks-band, of GQ Lupi. The feeble point of light to the right of the star is the newly found cold companion. It is 250 times fainter than the star itself and it located 0.73 arcsecond west. At the distance of GQ Lupi, this corresponds to a distance of roughly 100 astronomical units. North is up and East is to the left. Credit: ESO

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Our list of known planets and exoplanets unfortunately doesn’t extend much beyond our own Milky Way galaxy - to spot a planet, you need to be able to measure the light from an individual star and monitor it over time. You’re looking either for tiny flickers in the amount of light you receive, as a planet happens to pass in front of the star you’re watching, or you’re looking for there to be a little Doppler shift in the color of the star’s light, as the planets tug it slightly off center as they orbit.  Known by the names of the transit method and the Doppler shift method respectively, both of these require really careful observations over a significant amount of time, without the light from the star mixing with the light from other stars. This limits us pretty well to the stars within or surrounding our Milky Way.

Because the measurements required to spot planets must be so precise, generally the telescopes we send out to do these measurements only look at a small patch of the sky. So while I can give you our current high scoring planets, there’s no guarantee these will remain the all-time bests, if we point our telescopes in a new direction.

There is one fundamental limitation to how massive a planet can get - if you pack too much material into a planet, it will start to fuse elements in its core, and it formally becomes a star instead of a planet. This transition happens when the object is somewhere in the range of 13 to 80 times the mass of Jupiter, and is the point at which we typically start calling objects a brown dwarf star, orbiting another star, instead of a planet. The list of biggest planets can also change if we get better measurements. It's possible to learn that what we thought was a planet should really be called a brown dwarf, which then bumps that object off the list of biggest planets, and onto the list of known brown dwarfs.

 This artist's conception illustrates what a "Y dwarf" might look like. Y dwarfs are the coldest star-like bodies known, with temperatures that can be even cooler than the human body. Image credit:  NASA/JPL-Caltech

This artist's conception illustrates what a "Y dwarf" might look like. Y dwarfs are the coldest star-like bodies known, with temperatures that can be even cooler than the human body. Image credit: NASA/JPL-Caltech

However, you can still have very large, fluffy planets, well before they get to this boundary of being a star. Most of the ones we know about are Jupiter like in style - massive, gaseous planets, orbiting distant stars. The easiest to find are hot Jupters - exoplanets which are not only bigger than Jupiter, they’re much closer into their star than Jupiter is to our Sun. Currently, the majority of the biggest, fluffiest planets are about twice the radius of Jupiter. Considering that you could stack 22 and a half Earths edge to edge to match the width of Jupiter, you’re looking at a planet so large, you could line up 45 Earths behind it, and not see any of them. These planets have the very pronounceable names of ROXs 42Bb, which is estimated to be about 2.5 times the size of Jupiter, or Kepler-13 Ab, which sits around 2.2 times the size of Jupiter.

There are some larger ones, but these have preliminary estimates of their size, and may yet turn out to be brown dwarfs. The current record holder is a planet orbiting a star known as GQ Lupi, and estimates place it at somewhere around 4 times larger than Jupiter. This particular object is so large that our theoretical models of how it has formed are not particularly happy, and so the estimates on its size and mass are both pretty hazy. It is likely to remain a planet, but if it turns out that its mass is on the high end of our current estimates, it could wind up on a brown dwarf list. (This object is also extremely young, and will change and compress as it evolves.)

 Artist's impression of the simultaneous stellar eclipse and planetary transit events on Kepler-1647.  Credits: Lynette Cook

Artist's impression of the simultaneous stellar eclipse and planetary transit events on Kepler-1647. Credits: Lynette Cook

These big fluffy planets are orbiting your default solar system - one with a single star, around which all the planets orbit.  If you have two stars (which isn’t that uncommon), it seems to be much harder to build very large planets. The largest planet known to circle two stars at once was only confirmed in 2016, and is almost identical to Jupiter in size. At “only” 22.5 Earths in size, it orbits its parent star once every three years.


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If we can't build a magnetic bubble for a spacecraft, how about a magnetic tunnel?

If it is impractical to provide an artificial magnetosphere on the ship which would travel to Mars (due to cosmic ray cascades in the material of the ship), what about generating the magnetic fields externally and projecting them into space at a series of waypoints? Or would the distance involved (225 million miles) be too great?
 Our planet's magnetic field changes shape constantly due to strong winds from the sun. Image credit:  NASA's Scientific Visualization Studio

Our planet's magnetic field changes shape constantly due to strong winds from the sun. Image credit: NASA's Scientific Visualization Studio

A little while ago we covered some of the main radiation based difficulties of sending people to Mars, and while the solar wind is generally not so troublesome, cosmic rays, which we are shielded from here on Earth, are both more dangerous and much harder to redirect or stop.

Generally we want the outer walls of our spacecraft to be pretty durable, both for airtightness, protection against space junk, and to help protect against the solar wind, which can be stopped by a pretty reasonable amount of shielding. However, as you build up your shield, cosmic rays will start to play a nastier role. While you certainly don’t want a cosmic ray to be able to pass straight through your spacecraft and hit your astronaut unhindered (they’re very energetic particles, the sort that bodies deal very badly with), when a cosmic ray hits a dense object like a wall, it doesn’t just bounce back the way it came from.

 Standard spacecraft shielding, integrated into hull design, is strong protection from most solar radiation, but defeats this purpose with high-energy cosmic rays it simply splits into deadly showers of secondary particles. Image credit: NASA

Standard spacecraft shielding, integrated into hull design, is strong protection from most solar radiation, but defeats this purpose with high-energy cosmic rays it simply splits into deadly showers of secondary particles. Image credit: NASA

It creates a radiation cascade instead; what was one particle is now two, four, sixteen, and beyond, very rapidly, as the particle interacts with the dense material of the spacecraft wall. Sixteen slightly lower energy particles is mathematically worse than one high energy one, and a serious point of concern once we get out of the Earth’s magnetic shielding. So a very reasonable response is to ask if we can bring along our own magnetic shielding, to prevent the high energy cosmic rays from hitting the wall of the spacecraft in the first place. Theoretically, this should reduce the amount of radiation inside the spacecraft cabin, since it would reduce the number of cosmic rays that can make it all the way to the spacecraft shield. The main reason this is impractical right now is simply a logistical one - we don’t have a good way to build a generator for a sufficiently strong magnetic field which is also lightweight enough not to be hard to launch.

Setting up waystations would be an interesting way of approaching the same challenge. If there were a fixed orbital path between the Earth and Mars, and we could build a magnetic tube between the two planets, you could do away with the need to have an onboard magnetic bubble. Because you’re not trying to launch them on the spacecraft, you wouldn’t need to worry about the weight as much, but the magnetic field you’d have to generate would need to be much larger, to guarantee that the spacecraft (within errors) would definitely travel safely through the buffered region. The distances involved here are vast, and so setting up a series of waypoints would almost definitely be unfavorable, at least from an energy consumption perspective. There’s also the question of fueling those waypoints. Are they solar powered? Fission powered? What happens if their solar panels break down or they run out of energy? They’d also have to be able to correct their own orbits in order to be in the right places for the protection of the traversing spacecraft, and at this point we’re looking at a giant electromagnet with rockets, which is a great sounding device to have, but practically speaking, it’s a more powerful version of what we’d like to have on the spacecraft in the first place, and if we can get by with one device instead of several hundred, one is probably better.


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Why do we always think of North as up?

Hi! It might be a dumb question but it’s been in my mind for a while. We are convinced that North is up and South is down because that’s the way maps have been for many many years, but we don’t really know which way is actually up, it could be east or northwest, etc, right? Because there isn’t a real orientation/position in space, there’s no fixed up or down, but... doesn’t the way the Earth rotate determine in a way which way is up? How do those two things related to each other? Or is there no connection at all? Thank you!
 "The Blue Marble" is a famous photograph of the Earth taken on December 7, 1972, by the crew of the Apollo 17 spacecraft en route to the Moon at a distance of about 29,000 kilometres (18,000 mi). It shows Africa, Antarctica, and the Arabian Peninsula. In this version, it has been flipped upside down, with South at the top of the image. Image credit:  NASA

"The Blue Marble" is a famous photograph of the Earth taken on December 7, 1972, by the crew of the Apollo 17 spacecraft en route to the Moon at a distance of about 29,000 kilometres (18,000 mi). It shows Africa, Antarctica, and the Arabian Peninsula. In this version, it has been flipped upside down, with South at the top of the image. Image credit: NASA

You’re right that the way we draw our maps with North pointing up and South pointing down is largely arbitrary, and indeed there are a number of maps with the Southerly direction at the top rather than at the bottom, and they’re good fun to look at However, there are good reasons to say that a Northerly or Southerly direction should be “up”, and these reasons extend beyond just the rotation of the Earth.

The rotation of the Earth is a good starting place, though - the rotation axis of the Earth goes more or less through the North and South magnetic poles of the Earth. The magnetic North & South poles wander a little, so some years they’re closer to the rotation axis than others. Fixing the rotation of the Earth as a cardinal direction makes good sense, and is what we’ve done - East and West point 90 degrees from North and South.

There’s one more reason to put North as up, and it’s a physics convention. Most of the time, when we’re talking about rotation, we say that the direction of the rotation axis is actually just in one direction, rather than having to indicate both North and South. If we do this, it allows us to encode both the axis of rotation, and the direction of rotation at the same time. The way we determine which of North or South should be “the direction”, we use what’s called the “right hand rule”. You curl your fingers in the direction of rotation, and your thumb points in the direction of the rotation axis. In the Earth’s case, we rotate towards the East, so your thumb will point in the direction of North.

 A drawing of the solar system shows Pluto's tilted orbit. Pluto's orbital path angles 17 degrees above the line, or plane, where the eight planets orbit. Credits:  NASA

A drawing of the solar system shows Pluto's tilted orbit. Pluto's orbital path angles 17 degrees above the line, or plane, where the eight planets orbit. Credits: NASA

However, if you’re thinking of orientations beyond just the Earth’s own rotation, while it’s true that there’s no way to set an entirely objective zero point from which to measure other positions, and a sphere doesn’t have much intrinsic orientation to it, we can still do relative positions pretty well. And on the scale of our solar system, we have a pretty solid alignment going on. All the major planets in our solar system trace oval paths around the Sun as they go about their respective years. Not only do they orbit around the Sun in the same direction, they all tend to point their rotation axes in the same direction (notable exceptions here are Venus and Uranus). On top of all that, the ovals are almost perfectly aligned in a flat plane. If we take our same physics convention and use the rotation of the planets around the Sun to tell us which direction we’re going to point up, our Planet Earth based North is more or less pointing in the right direction. Our planet’s spin is not perfectly aligned with the “up” out of the solar system, but tilted by 23 degrees, a feature of our planet responsible for our seasons. This tilt is why many globes are set at an angle - they’re mimicking the tilt of our planet relative to the “up” defined by our solar system.

So the North is up convention is partially mapmakers, partially the spin of our Earth, and partially physics notation, but there are definite ties between all of them.


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Where is the Moon's water?

If there is water on the Moon, will it be on the surface or will it be within the ground?
 This is a spectacular high-Sun view of the Mare Tranquillitatis pit crater revealing boulders on an otherwise smooth floor. This image from LRO's NAC is 400 meters (1,312 feet) wide, north is up. Image credit: NASA/Goddard/Arizona State University

This is a spectacular high-Sun view of the Mare Tranquillitatis pit crater revealing boulders on an otherwise smooth floor. This image from LRO's NAC is 400 meters (1,312 feet) wide, north is up. Image credit: NASA/Goddard/Arizona State University

There is water on the moon! As we’ve outlined before, it’s somewhat tricky to keep water on the surface of the moon, because the combination of heat and particles from the Sun, a lack of an atmosphere, and no magnetic field means that it’s pretty hard to keep water that’s exposed to sunlight from evaporating away into space.

What that means is that if you want to have water persist anywhere on the Moon, it has to be sheltered from the Sun somehow. An easy way for this to happen is at the poles, where some craters are deep enough that the Sun’s rays never reach into the bottom of the crater. Places like this are called “cold traps”, because it can trap material in a solid form that would otherwise escape if it weren’t so cold.

Near the south pole of the Moon in particular, we found frost in some of these deep dark places. This frost makes the surface more reflective than it would be if there were only rock sitting around in those craters - so the coldest places also wind up being more reflective if you’re bouncing light off of ice. This particular study is careful to note that we’re not seeing frozen pond-style pools of water, but more like the frost that builds on the outer edges of leaves in fall.

But craters aren’t the only places that water ice could hide - we have a sneaking suspicion that the Moon also has tunnels woven under its surface. The Moon had a surprisingly long era of volcanic activity in its younger years, and where there are lava flows, you can wind up with lava tunnels. We are pretty sure that the moon has these. We see them most easily as they collapse, because then you get a snake-like pattern of collapsed ground, twisting its way across the surface as a series of giant potholes.

 These images from NASA's LRO spacecraft show all of the known mare pits and highland pits. Each image is 222 meters (about 728 feet) wide. Image credits: NASA/GSFC/Arizona State University

These images from NASA's LRO spacecraft show all of the known mare pits and highland pits. Each image is 222 meters (about 728 feet) wide. Image credits: NASA/GSFC/Arizona State University

Every so often, there’s a more isolated cave-in, giving us a glimpse into a sublunar cavern - a deep shadow cast into the depths catches the eye and the imagination. If water had accumulated in these hidden tunnels, they would also be relatively protected from evaporation. However, it’s one thing to have a plausible place for water, and another to find it for sure in those places. Lava tunnels are an extremely appealing place for water, though - because they’re also an appealing place to put a human base on the Moon. While we don’t have to worry about humans evaporating, any shelter from intense heat and cold helps us as well. So if there were also water down there, they’d be a great place to put an inhabited base.

You can definitely also wind up with watery molecules bound up in the rocks themselves. A recent study suggests that instead of having lots of water ice hanging around, the Moon may have a lot of hydroxyl, which is one hydrogen and one oxygen bound to each other, rather than the two hydrogens and one oxygen that make up your standard water molecule. Hydroxyl binds easily to other things, so it can wind up binding itself to minerals in the earth - you can extract it and create water, but it’s more energy intensive than just having water lying around.

So the true answer is that there’s going to be a mixture of places where water will be found - on the surface in sheltered places, possibly in underground tunnels, and some not-quite water bound up in minerals!


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