What is the size of a rogue planet?

We just measured the mass of a rogue planet for the first time, and it is similar to Saturn’s

[1]

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KMT-2024-BLG-0792 / OGLE-2024-BLG-0516 is a recent microlensing event detected by both ground and space-based telescopes that made it possible to identify and measure the mass of a rogue planet located near the outer edge of the central bulge of our galaxy. When we observe such an event across multiple instruments, we can estimate the mass of the microlensed object because of the time delay in observing the object at different locations in space and on Earth.

This extraordinary discovery indicates that rogue planets with masses below that of Jupiter exist and may be more widespread than previously believed. We already observed directly, with the James Webb Telescope, many double Jupiter-size planets in the Orion Nebula in 2023. At first, we thought they were ejected from newly formed planetary systems, but other theories of their formation have since been proposed. They might have formed via the fragmentation of nebulae in which stars form when they collide.

Nevertheless, a large population of such planets is likely to form in star systems and be ejected during the turbulent early history of those systems. Planets form via collision with planetoids and other planets within the protoplanetary disk, and a large number of them are expected to be ejected. When a large gas giant planet is banished from a protoplanetary disk, in about 90% of cases, it loses its moons, and some can become rogue planets, as well. Let’s not forget that planets more massive than Jupiter can have moons as massive as Earth and even larger.

We now think that we previously underestimated the number of transneptunian objects in our system. There could be up to 10,000, some as large as Mars and even half the mass of Earth. We can only detect them for 1% of their orbit, which can take tens of thousands of years, and our technological civilization is too young to take advantage of the short window for their possible detection until we get better instruments that could see these objects even when they are far away. They can also be ejected sometimes and become rogue planets.

This is why the upper estimate of the number of rogue planets is as large as 100,000 for each star

[2]

 . Some can have subsurface oceans heated by the tides of a moon and by radioactivity. There is even speculation that some may have liquid water on the surface if they have a specific atmospheric composition that prevents heat from escaping. This is even though they float freely in space, without a star to provide heat, and it means that they can be a fascinating destination for the search for extraterrestrial life or stopovers in our quest to reach other star systems.

This also means that rogue planets can be any size, but we cannot see them yet if they are too small—they don’t produce their own light.

The question was: What is the size of a rogue planet?

Footnotes

[1] 

https://www.science.org/doi/10.1126/science.adv9266

[2] 

Nomads of the Galaxy

Can a "planet" be too large to be classified as a planet?

 This is Jupiter:

It’s a big ball of hydrogen.

This is the Sun:

It’s also a big ball of hydrogen.

Basically, if an object is primarily composed of hydrogen, it will start undergoing fusion once it reaches a certain mass. We call those objects “stars”. However, the line between a star and a planet is not clear. There are small star-like objects known as brown dwarfs which are either too small to undergo fusion or, if they do, are too cold to emit strongly in visible light. However, if they don’t orbit another star, they’re not planets by definition. Astronomers tend to call them “brown dwarfs” because, well, they’re small, brownish balls of hydrogen.

Just like Jupiter.

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Edit: For those of you complaining that Jupiter isn’t a star, you’re missing the point. Yes, stars fuse hydrogen and Jupiter doesn’t. And, yes, I agree that defining “star” based on nuclear fusion is the best possible definition.

However, that doesn’t affect my answer.

Jupiter is a ball of hydrogen.

If it was about 13 to 15 times more massive, it would be able to fuse deuterium and we’d call it a brown dwarf. However, it would STILL be a ball of hydrogen.

If it was about 80 times more massive, it would fuse hydrogen and we’d call it a star. However, it would STILL be a ball of hydrogen.

In other words: if Jupiter was bigger, it would have enough mass to produce a fusion reaction and we’d call it a star.

The mass of the object is the only thing that makes a difference, so please stop being pedantic and saying I don’t understand the difference between planets and stars.

Mercury is the innermost planet of the Solar System.

 In particular, it is located 58 million km from our star, just over a third of the distance that separates the Earth from the Sun. At this distance corresponds a revolution period, the Mercurian year, of 88 days.

Like all other celestial objects, Mercury also rotates on its axis. Unlike other planets like Earth or Mars, Mercury rotates on itself very slowly, completing a full rotation every 59 days.

In the title of this post, I wrote that a day on Mercury lasts more than a year, but if math is objective, 59 is smaller than 88, so in theory, the year is the longer period of time between the two.

However, this depends on the definition we want to give to the word "day". If by day we mean the period it takes for a planet to complete one rotation on its axis, then a year on Mercury lasts more than a day. This definition is known as a sidereal day.

However, we could consider the day to be the time that elapses between two consecutive culminations of the Sun (that is, when the Sun passes the meridian and reaches its maximum height in the sky). This day is known as the solar day.

The solar day is longer than the sidereal day, since due to the revolution around the Sun the planet must rotate a little more to bring the Sun back to culmination compared to the previous day.

The difference between the two days on Earth is minimal, only 4 minutes. However, on Mercury, while the sidereal day lasts 59 Earth days, the solar day lasts a whopping 176 Earth days! This means that between two consecutive passages of the Sun in the same position in the Mercurian sky, a full 176 days pass!

Depending on the definition of day we choose, here is how on Mercury a day can last longer than a year.

Credit: NASA.

Why is Pluto no longer considered a planet?

 The reason that Pluto is no longer a planet is not because of its size. In fact, it passes the test for "size" (mass).

The definition of 'planet' was made more strict. It is as following:

1. Massive enough to be round. Very massive bodies have so much gravity that they crush down any irregular edges towards their centre, and so become ball-like.

2. The primary object orbiting the Sun. For instance, the Moon orbit's the Sun, but it does so by orbiting the Earth. The Earth is the primary object orbiting the Sun.

3. Has cleared it's own orbit. Planets clear their orbits of debris/asteroids (by attracting them with their gravity).

Pluto passes 1 and 2, but has not passed 3. It has not cleared it's orbit of debris.

Objects like Pluto are called Dwarf Planets.

What is the biggest planet/star in the universe?

 The biggest planet currently known in the universe is planet TrES-4. It is located in the constellation Hercules. It is 70% larger than Jupiter in diameter, but has only 80% of Jupiter's mass. As this planet orbits very closely around its sun, the gases of the planet expand due to the intense heat and cause the density of the planet to become similar to a marshmallow.

The largest known star in the observable universe is UY Scuti. It is located in the Constellation of Scutum.

It has a

• diameter of 1,500,000,000 miles,
• radius of 750,000,000 miles,
• circumference of 4,712,388,980 miles.

If earth is geoid and every moon and planet are not a perfect sphere, why do NASA images show an exact circle?

 Okay, so let's take a look at the most famous picture of Earth that NASA ever took.

This photo has been compressed by Quora for display purposes, but my original image says it's 3000 x 3002 pixels. Let's say there are roughly 20–25 pixels of blackness on each side, that gives us an image size of roughly 2950 x 2960 for the Earth. Let's divide 7,901 miles by 2950. That gives us a visual scale of roughly 2.7 miles per pixel. That would make Mount Everest about 2 pixels tall, should be right there at the upper right. That scale would make the equatorial bulge about 5 pixels on either side!

Do you see the problem now? This photo from NASA shows the Earth as a nearly perfect sphere because it is! The variations from perfectly spherical are far too tiny to see at this scale. You're trying to see lumps and bumps less than a couple of pixels in size. You're looking for ten pixels of total bulge at the equator.

You can't see such tiny variations! You could easily measure them in Photoshop, but you can't see them on this screen.

This photo was taken from 18,300 miles away. From that distance, the Earth appears to be a nearly perfect sphere.

Can an asteroid crack a planet?

 A planet may look very solid and hard from our human perspective, but in the large scale it can be compared to a ball of slushy water suspended in midair. An asteroid, sufficiently large in comparison, can not just crack the planet, but actually smash it into it with such force that the released kinetic energy turns both bodies into smaller droplets, just like hitting the floating ball of water with a baseball bat.

What afterwards happens to the droplets depends on a lot of things, but most likely the majority of the fragments will reattach, melt together and form a new planet. The remaining fragments might create a ring around the new planet or even form one or several moons, such as this computer simulation below shows.

This is in fact what is now believed to be the origin of our own Moon: a smaller planet called Theia smashed into the original Proto-Earth some 4.5 billion years ago, breaking it into glowing droplets that later formed our current Earth and also the Moon.

If that would happen today, the Earth would pretty much turn inside out and after the carnage, not a single shred of our civilization or anything resembling our home planet would remain. The oceans would evaporate and vent out into space, forming a spectacular, sparkling ice ring, and most likely eventually falling back as thundering, dirty icebergs crashing down onto the planet with devastating force.

The new planet would probably end up having some kind of atmosphere, initially perhaps resembling the atmosphere of Saturn’s largest moon Titan, consisting mainly of nitrogen and methane. The new Earth would not look like a blue marble from space, but sickly brownish yellow, the atmosphere more or less opaque and filled with smoke and murky clouds, only occasionally revealing a bit of a scorched and black surface, with rivers of lava faintly glowing in the night.

The new planet would initially be fully exposed to the Sun’s deadly radiation, but eventually the heavier material, mainly iron, would settle back into the planet’s core and start rotating, creating a new magnetic field that would shield the surface from the solar wind.

As the comets consisting of water and soil from the old Earth’s oceans would come crashing down with regular intervals, they would bring back water to the planet. Carbon dioxide would be released, again reinitiating the atmospheric greenhouse effect. New continents would begin to form and the new water would eventually collect in basins to form new oceans and lakes.

Evaporation would create new clouds, rain would start to clear out the atmosphere from airborn dust so it becomes transparent once more and the sky would again start to turn blue, at least where not shrouded by the deadly smoke and gas from the remaining lava rivers and erupting volcanoes.

After a billion or so years, when the new heavy bombardment has ceased and the planet has been resolidified, life might eventually emerge again in oceans, as microbes, algae and possibly more advanced shapes conquering land once again. But whatever it would be like, it would not have any resemblance to what exists today.

There would be absolutely no remains of our cities and infrastructure, not even fossils from the original biosphere. The only physical evidence that we ever existed would be the handful of slowly receding probes leaving the Solar System.

Should we be worried? I’d say no. The risk for such a cataclysmic event is very, very, VERY low. There are no known asteroids or planets in the Solar System big enough and in an excentric enough orbit for anything like that to happen. Rogue interstellar planets might of course exist, but the probability of one finding its way to our home and hit the Earth is so astronomically slim that it is almost non-existant. Furthermore there would not be a single thing we could do to prevent it, so, well, why worry.

Why is Pluto no longer considered a planet?

 The reason that Pluto is no longer a planet is not because of its size. In fact, it passes the test for "size" (mass).

The definition of 'planet' was made more strict. It is as following:

1. Massive enough to be round. Very massive bodies have so much gravity that they crush down any irregular edges towards their centre, and so become ball-like.

2. The primary object orbiting the Sun. For instance, the Moon orbit's the Sun, but it does so by orbiting the Earth. The Earth is the primary object orbiting the Sun.

3. Has cleared it's own orbit. Planets clear their orbits of debris/asteroids (by attracting them with their gravity).

Pluto passes 1 and 2, but has not passed 3. It has not cleared it's orbit of debris.

Objects like Pluto are called Dwarf Planets.

What would it be like to fall into the surface of Saturn?

 What would it be like to fall into the surface of Saturn?

Saturn has a very low density and its gravity is not that much different than Earth’s, so I imagine you would initially free fall similar to how you would on Earth. The Saturnian atmosphere is closer to pure hydrogen/helium than other planet so your early visibility would be pretty good.

The drawback is you would not have a lot of sunlight to work with, so hopefully your excellent space suit would have powerful headlights.

When you started your descent the temperature may be as low as -250 C, but as you zip through the clear air to the first cloud layer the temperature climbs to -130 C. You are not falling that fast, your terminal velocity is around 200 km/h and will gradually slow as you reach thicker and thicker air, but so far you have fallen about 100 km and start passing through your first light cloud layer of ammonia crystals.

Saturn is a very windy place with the equatorial wind reaching speeds of 1,800 km/h, slowing somewhat towards the poles, but either way you are getting some lateral buffeting.

By the time you’ve passed through the second cloud layer of ammonia, hydrosulfides and ice crystals, the temperature is getting warmer, in the -70 C range and you have fallen for 170 km.

Another 130 km further and it seems more Earth-like, zero C or more and clouds of water, some of it liquid so you flip on your windshield wipers.

You have now fallen close to 400 km, and it’s taken over 2 hours, temperatures are climbing from freezing to as much as 80 C. Pressure is increasing rapidly now and you are seeing more and more liquid compounds, an hour or 2 more and the hydrogen is beginning to liquefy under the pressure. Helium is becoming more of the mix and temperatures continue to rise. Friction and static electricity generate lightning that arcs in from the troposphere.

As the liquid hydrogen thickens you continue to slow your descent until you have to fire up your propulsion system to proceed further. Eventually you see pressure liquefied helium as well and then a new barrier - metallic hydrogen which is much denser than water and crackles with magnetic fields it generates.

Other than increasing heat (up to 11,700 C) and pressure (>1000 bars) in this sea of metallic hydrogen you have a long way to descend until you finally reach a rocky core of compressed heavier elements like carbon, oxygen, silicon, and iron in a sphere possibly twice the diameter of the Earth.

Now the hard part….how in the hell are you going to get back out ?

How could Jupiter be shielding us from space debris when it is revolving in a huge orbit?

 Jupiter not only shields the earth from space debris, it also ensures that the asteroid belt does not fling asteroids into the sun or form an accretion disc and develop into a new planet. This is accomplished due to Jupiter’s mass and extraordinary gravitational effects. Space debris that could pose a danger to our planet may come in three forms. Man-made debris such as left over rocket stages, and a group of asteroids known as Hildas and Trojans.

Every planet has a gravitational pull on space debris. Any celestial body with substantial mass has an area in space between itself and another massive body known as a Lagrangian point, L-point, where the gravitational pull of both bodies settles to an average and anything stuck in this area, known as a Trojan hereon, is locked into orbit. Trojans share an orbit with planets but follow an oscillating movement along the orbit, called libration. 6 of the 8 planets in our system, including earth, have Trojans in their L-points. Earth and Venus have one each while Mars has seven. Jupiter has more than 6000 documented Trojans.

This represents data collected between 2005–2017 on the movement of both Trojans (Green) and Hildas (purple). The effects of Jupiter’s gravitational pull is evident.

Jupiter’s Trojans are classified into The Greek camp and The Trojan camp, in the leading and trailing edge of Jupiter’s orbit respectively. Jupiter’s gravitational pull is 2.5 times that of earth, and it essentially shepherds asteroids along its orbit. Hildas observe an elliptical orbit which slows their movement near aphelion. This causes them to congregate near Jupiter's L-point and create the appearance of a triangular orbit.

Jupiter is doing us a solid, no doubt, but its not our last line of defense. Not by a long shot. In case anything escapes Jupiter’s gravitational pull and heads straight for earth, our moon will be waiting.

In 1969, object J002E3, third stage of the Apollo 12 Saturn V rocket, was improperly injected into a heliocentric orbit and instead went into an unstable high earth orbit that switches between heliocentric and geocentric orbits in a 40 year cycle. It was spotted in 2003. In its approach to Earth, it was noted just how essential a part the moon plays in protecting us from debris as well.

5-year Update, April 2023:

Thanks to all the upvotes, questions, contributions and suggestions to this topic. I’m glad its peaking interest. I have updated some more info regarding the Asteroids, specifically the Hildian asteroids explaining their orbit.

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