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I know that asteroids are huge chunks of rock, orbiting a solar system. Do asteroids have a gravitational field and do they gravitationally attract each other to form planets?

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  • $\begingroup$ @DavidHammen made an important point in a comment on the popular answer: Asteroids do have gravity and billions of years ago that gravity played an important role in the creation of our solar system. However, that gravity no longer appears to play an important role in the universe. Ancient asteroids are now planets and moons or sucked into black holes. $\endgroup$ – Dave G Dec 28 '15 at 1:03
  • $\begingroup$ There's now a space probe Hayabusa 2 orbiting around the 1km-wide asteroid Ryugu. That's proof that it has gravity :) $\endgroup$ – Barmar Aug 22 '18 at 14:34
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By definition, gravity is a result of mass. Any body with a non-zero mass (even atoms) will have a gravitational field associated with it. The higher the mass the stronger will be the field. This is basic of classical mechanics. Until we reach quantum scale where the gravitational force is dominated by other 3 forces and the gravitational field becomes irrelevent.

When it comes to graitational field of asteroids, it exists, but is very weak. However over a course of few million years these small asteroids combine together to form large masses of bodies that we now call planets. That is one of the prominent theory of Solar system formation, where the gravity of small dust particles from first our generation disintegrated star over a course of time accumulated to give us what we now know as our Solar system. Think of it like this, every planet that you see now would once have been an asteroid at some point during its evolution.

Another proof to support this is the presence of numerous binary asteroids that orbit each other around a common center of mass, which requires gravitational attraction.

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    $\begingroup$ Why so many upvotes for a fundamentally wrong answer? Yes, asteroids gravitate, but then again, so do microscopic grains of dust. The issue with this answer is that asteroid-like objects no longer form to combine larger bodies, and they more or less haven't done so for 4.5 billion years or so. With regard to binary asteroids, the consensus view is that they are formerly larger asteroids that have broken into two objects (or more) thanks to collisions and/or the YORP effect. $\endgroup$ – David Hammen Dec 26 '15 at 6:36
  • $\begingroup$ I don't get your point, at no point I have said that asteroids are currently forming planets. It's not possible, due to the presence of heavier planets. But being an asteroid would be one the stages of planet formation. wrt binary asteroids I mean their presence tells that they have gravity which is causing them to have an orbit around each other. I have not implied that gravity is responsble for creating them. $\endgroup$ – Astroynamicist Dec 26 '15 at 18:27
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Sure. Any mass has its gravitational field. However, its size is proportional to the mass, so as most asteroids have little mass, they have little gravitational field, and therefore pull only very slightly at each other, resulting in not enough effect to get them to lump together.

Typically, their difference in momentum/speed is too large to be removed by the little pull of the gravitation between them.

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You asked two questions.

Do asteroids have a gravitational field.

Of course. Even a microscopic grain of dust has a gravitational field.

Do they gravitationally attract each other to form planets?

Not any more. During the formation of the solar system, asteroid-like and comet-like objects collided to build larger objects, which in turn collided to form even larger objects, and so on, eventually building the cores of giant planets and later, the terrestrial planets. But that stage ended long ago, shortly after the solar system formed.

Asteroids do of course gravitationally attract other objects, but this attraction is so weak due to the small masses of asteroids that it is easily overwhelmed by other perturbing forces. The vast majority of the asteroids lie between Mars and Jupiter, and Jupiter is the primary culprit in explaining why no planet exists in that gap.

When two astronomical bodies collide, one of the outcomes is a purely inelastic collision that makes two bodies form a single body. This only happens with a rather mild collision. A more energetic collision will result in some mass being expelled. An even more energetic collision will result in lots of mass being expelled; the colliding bodies become many smaller bodies. With a few exceptions, the latter is what is what is happening amongst the asteroids today, and for the last four-plus billion years or so.

Jupiter is such a huge perturbing body that collisions in the asteroid belt are generally very energetic. Instead of forming ever larger bodies, the asteroid belt is gradually being broken up into smaller and smaller bodies. Some of these collisional bodies are ejected from the solar system thanks to interactions with Jupiter. The smallest results of these collisions migrates sunward thanks to the Poynting-Robertson effect.

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Certainly! Anything that you see around you that has mass, your dog, your house, your car or yourself, they all have gravitational field and they exert gravitational pull to everything around them. And everything around them exerts that gravitational pull back. This pull however is so weak, that we can't perceive it with our senses. Gravity is a direct result of mass and the bigger the mass that an object has, the bigger its gravitational pull.

You can extrapolate this paradigm to everything that exists in space! From the smallest particles of dusts and comets to the biggest of stars and galaxies. An asteroid that crashes on a planet, is attracted by the planet's gravitational pull but at the same time, the asteroid attracts the planet. Eventually, this is how planets get to grow.

All celestial bodies in our sky would not exist if they hadn't any gravitational field. (1) Small dust particles collide with each other, forming bigger rocks. (2) Bigger rocks collide further to each other (or if they are big enough -several dozen of meters- might attract each other) to form comets and asteroids. (3) Comets and asteroids on their turn, will coalesce with other asteroids and rocks and will form dwarf planets and other terrestrial planets. (4) If those planets gain more mass they will be able to attract gas and they will form gas giants. (5) And if gas giants gain even more mass they will turn into smaller or bigger stars.

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    $\begingroup$ Your analysis is essentially correct, except sometimes agglomeration is controlled by other forces, such as electrostatic, Wan der Waals, or chemical bonding. As the accretion disk gets smaller and flatter, the mass density goes up, but still gravitational accretion only happens on grand scales, or as an organizing force after large collisions. ~ You should also mention that asteroids are currently not coming together to form larger bodies. This happens early in a stellar system's history. ~ Only on the scale of lightyears do gas clouds start to collapse to protostars (see Jeans length). $\endgroup$ – Aabaakawad Dec 19 '15 at 20:05
  • $\begingroup$ Your analysis is fundamentally incorrect with regard to the formation of asteroids, comets, and planets. Small dust particles and rocks collide and form larger dust particles / larger rocks because their masses are so small that gravitation does not come into play. It's only when objects attain a size of several tens of meters across that gravitation starts to be important. $\endgroup$ – David Hammen Dec 19 '15 at 20:47
  • $\begingroup$ Thanks both! Apologies for the error. I edited my response to show that gravity comes into play only when celestial objects are big enough. Cheers! $\endgroup$ – Antonis Dec 19 '15 at 23:36

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