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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Why don't planets hit each other?

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Do all planets rotate around their stars?

Do all planets rotate as they go around their stars? Do they all rotate in the same direction (e.g. clockwise or anticlockwise?) Or does it just depend on what started them rotating in the first place?
image

Originally posted @ Medium!

Before we can expand our thinking out to “all planets”, the easiest wayto start looking at planets and how they rotate is to look at our ownsolar system, which we can investigate in far more detail (and far moreeasily) than anywhere else in the Universe. What we see in our own solarsystem is that all of the major planets are rotating around their own internal axis.

We’re wellacquainted with the rotation of the Earth, even if we haven’t thoughtabout it in this way — the Earth’s rotation is what creates our days. Asa result, we all know how long it takes for the Earth to rotateonce — 24 hours. But 24 hours isn’t the rule within our solar system; infact, of all the other planets, only Mars rotates at a similar speed.Mars completes one rotation in 24 hours and 40 minutes; nearly identicalto our home planet.

Venus,our planetary sister gone horribly awry, rotates much slower than theearth — one rotation takes 243 days and 26 minutes; this makes it theslowest rotator in our solar system. Mercury comes in second slowest,and rotates once in 58 days, 15 hours, and 30 minutes. The gas giantsall seem to have similar rotation speeds, all of which are faster thanany of the inner, rocky worlds. Jupiter rotates once every 9 hours and55 minutes. Saturn recently had its rotation speed re-measuredto be 10 hours and 36 minutes. Uranus rotates once every 17 hours and14 minutes, whereas Neptune rotates once every 16 hours and 6 minutes.

Mosteverything in our solar system rotates in the same direction — the samedirection as the Earth. If we had a bird’s eye view of our solarsystem, where we’d flown into space “up” via the North Pole, and lookedback down, most of the planets would be rotating counterclockwise — orfrom the West towards the East.

You can usually remember which way the earth is rotating by thinking about thetime zones; the further east you go, the earlier it is — they’re pushedtowards daylight sooner than the west.

Sowhy does almost everything rotate in the same way? A hint of the answerlies in the Sun. The sun also rotates, and it rotates in the samedirection as the majority of the planets — counterclockwise from ourbird’s eye view. Since the Sun is also following our consistent rotationpattern, we’re going to have to make our way back to the formation ofthe solar system in order to make sense of this.

Our sun formed out of a cloud of gas and dust — enormous molecular clouds arethe only places where, under the persistent pull of gravity over time,material gets dense enough to begin to collapse and form a star (orten). However, during the collapse of the cloud of dust and gas, anyinitial motion in the cloud becomes quite important.

It’svery unusual for objects in the universe to be completely still inrelation to each other (in fact, they would have to be at absolute zeroto prevent any motion), so there’s a little bit of stirring about withinthe cloud that is always present. As the cloud collapses, the averagedirection of motion of the cloud is kept — and as gravity pullseverything closer to the center, the conservation of angular momentumcauses that average motion to speed up. You can do this experiment athome if you have a chair that spins. If you start yourself turning alittle bit with your arms and legs extended, and then pull your legs andarms in quickly, you’ll find yourself spinning much more rapidly thanyou were to start out. Pulling your arms in plays the same role thatgravity is playing in the collapse of the early solar system.

Because therotation speed is so much faster than it was initially, the gas formsinto a flat disk that’s all rotating in the same direction. If you’veever played with a ball of bread dough or silly putty, and spun it onyour finger, you’ll find it flattens out into a thin disk fairlyquickly. The star at the center is therefore being formed in anenvironment where everything is already rotating — as the star collapseseven more, its rotation will increase even more. All the proto-planetsare also forming out of gas which is rotating, so it makes sense thatthey too will absorb the rotation of the disk they formed in. So inprinciple, we expect most planets to form with a rotation that’sconsistent with the star at the center of their solar system.

However,“most everything” isn’t everything — we’ve got two notable exceptionswithin our solar system to the counterclockwise rotation rule: Venus andUranus. Uranus rotates 90 degrees off from everything else. If youconsider the plane of all the planets’ orbits around the sun as a flatsurface, most planets spin as though they were a coin spun on its edge,flicked counterclockwise. Uranus, on the other hand, spins like a beadrolled along the ground, instead of spinning vertically. Venus is evenodder- it spins clockwise. As far as we can tell, this means Venus issomehow upside down.

Whathappened to these planets? The picture I painted above indicates thatthey should have formed with a rotation aligned with every other planetin our solar system. In fact, they probably did form with a rotationthat matched. However, the very early solar system was a violent place,filled with many more proto-planets which were inclined to collide withother objects. Some of these collisions were likely with the earlyversions of the planets which exist today. A particularly bad collisionor series of collisions could cause such an energetic punch to the otherplanet that the planet was effectively tipped over onto its side (inthe case of Uranus) or upside down completely (in the case of Venus.)

Toexpand to the Universe outside of our own solar system; physics is thesame everywhere in the Universe, so solar systems should form in thesame way everywhere. However, we wouldn’t expect every planet to rotatein the same direction as its parent star, since — like our own solarsystem — the planets may have had a very collision-intensive early life.

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If you plot the planets in log, they look evenly spaced?

I was plotting the distances of the planets from the sun today for a decorating project, and I noticed that doing so put the inner planets within only 3-6in. from the sun, so I tried seeing what they would look like on a log scale. When I plotted it out, I noticed that, if you include the asteroid belt, things are very evenly distributed. Is this indicative of anything or just coincidence? What does this tell us about the formation of our system or the nature of the sun’s gravity?



This is a very good question, and detailed enough about the formation of our solar system (which is not my field of astrophysics) that I went and bothered one of my planetary science friends, Alex Parker, to see if this is a coincidence or not, and he immediately recognized this as the Titius-Bode relation.

The Titius-Bode relation is an empirical statement (meaning only that there was an observation that two properties appeared to be correlated with each other) which found that the planets appeared to have a very regular pattern in how far apart they are, as you’ve noticed by taking the log of each of their distances.

However, unless an observed relationship can be explained somehow using physics, it’s difficult to assign any meaning to that relationship. In the case of the Titius-Bode rule, for many years it was accepted as the way that solar systems formed without a physical explanation behind that rule. But it was very convincing, since it predicted the locations of all of the known planets at that time (Mercury, Venus, Earth, Mars, Jupiter, and Saturn). Even Ceres, which is now classified as a dwarf planet, but at the time was considered a planet, seemed to work with it.

However, it’s now largely viewed by the planetary science community as a simple coincidence. Ceres is now known to be one of the larger bodies among the asteroid belt, which is full of a number of smaller (but similar sized) objects, and the predictions failed to explain Neptune’s orbit. There have been a few attempts to “fix” the Titius-Bode relation so that it would work for all the planets, but the evidence for any kind of physical mechanism that would force this relation to be true is so poor that the planetary science journal Icarus does not accept these papers as scientific works.

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Why is it called an exoplanet?

Dear Astroquizzical: why is it called an EXOplanet? What’s the opposite of an exoplanet?

We haven’t had a naming question in a while!

An exoplanet is also called an “extrasolar planet” - both terms simply mean a planet which is in orbit around a star which is not ours.

The ‘exo’ part comes from the same root as an “exoskeleton”, “exothermic” or “exotic”. The first tells us that an animal’s skeleton is an ‘outer’ skeleton such as those of spiders and insects, not an interior skeleton like mammals have. If you’re a chemistry person, you’ll recognize “exothermic” as a sign that a chemical reaction produces more heat than it consumes. Put another way, it’s dumping heat “outside” of the reaction. “Exotic” simply means that it comes “from outside” where you’re from.

The opposite of “exo-“ is “endo-“, which means “internal” instead of “external”. While we don’t tend to use the word “endoskeleton” to mean an internal skeleton, we do use “endothermic” to mean something that must suck energy out of its environment.

An exoplanet is simply an “external” planet, in that it isn’t in our solar system. So the opposite of that would be something internal to our solar system, which also is not a planet! Any of the many moons, comets, and asteroids in our solar system (or Pluto) would qualify.

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