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75 votes
Accepted

Considering our methods of exploration, how likely is it that there are unfound planets (not dwarf planets) in our solar system?

There are no undiscovered planets between the sun and Neptune. Objects closer (to Sun) than Neptune that are large enough to be considered planets (and not dwarf planets) can't remain 'hidden'. If it'...
Ingolifs's user avatar
  • 4,155
69 votes

Why does it take so long to transmit an image from New Horizons to Earth?

New Horizons has just passed the Kuiper Belt Object (KBO) 2014 MU69 also known as Ultima Thule. KBOs form a belt of asteroids (the Kuiper Belt) from Neptune's orbit outwards and of which Pluto is the ...
astrosnapper's user avatar
  • 8,357
60 votes

Is there enough matter orbiting the sun to make a second sun?

The vast majority of the stuff in the solar system other than the Sun itself is contained in one body, Jupiter. The total mass of the solar system is estimated to be about 1.0014 solar masses, or ...
David Hammen's user avatar
  • 34.1k
58 votes

How can comets have tails if there's no air resistance in space?

There are two forces that can cause the formation of a tail: the solar wind and radiation pressure. The first misconception in your question is "the dust [travels] slower than the nucleus". The tail ...
James K's user avatar
  • 123k
51 votes
Accepted

Why won't the Sun set for days at N66.2 which is below the arctic circle?

From the PyEphem Quick Reference Guide: Rising and setting are sensitive to atmospheric refraction at the horizon, and therefore to the observer’s temp and pressure; set the pressure to zero to turn ...
notovny's user avatar
  • 4,788
49 votes
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What makes protoplanetary disks start rotating? (Initial energy needed to rotate)

Two rocks placed in space with no relative motion are going to be attracted by gravity, and hit. 3 rocks, placed in space with no carefully rigged symmetry, will likely miss each other, as the ...
Wayfaring Stranger's user avatar
47 votes

Why does it take so long to transmit an image from New Horizons to Earth?

The other answer mentions it, but this gives a bit more theory as to the why. It's effectively for the same reason that your phone or Wi-Fi don't work as well and slow down when that they are far ...
The_Sympathizer's user avatar
46 votes

Why is the Sun's density less than the inner planets?

The sun isn't the same density all the way through. According to MSFC's solar interior page, the core density at the centre of the sun is a whopping 150,000 kg/m$^3$. Surrounding it the radiative ...
Andy's user avatar
  • 2,467
44 votes
Accepted

Why do the planets orbit in the same direction?

The same reason (almost) all of them rotate in the same direction: because of the conservation of angular momentum. Before a star and its planets exist, there’s just a cloud of disorganized gas and ...
Sir Cumference's user avatar
43 votes
Accepted

Closest star system to Alpha Centauri?

The sun is the nearest star to Alpha Centauri (unless you count Proxima Centauri, which is really part of the same system). There is a very small and dim pair of brown dwarfs, called Luhman 16 that ...
James K's user avatar
  • 123k
42 votes

Why do things float in space, though the gravity of our star is always present?

It is not true that "objects float around" in the solar system. Perhaps you have seen video from the space station, and you can see things floating. This is not because there is no gravity, but ...
James K's user avatar
  • 123k
41 votes
Accepted

Which planet, viewed from which other planet would appear the largest?

Here is a table with planets, their minimum/maximum distances from the Sun, their minimum distance (assuming the planet's aphelion and its neighbour's perihelion coincide, which they don't, so the ...
Glorfindel's user avatar
  • 4,790
39 votes

Is Jupiter a failed star?

No. Besides the 13 Jupiter-masses required to ignite deuterium burning, and make Jupiter into a Brown Dwarf, there is a clear difference between the formation pathways of Brown Dwarves and Gas ...
AtmosphericPrisonEscape's user avatar
39 votes

Can a planet in our system eclipse the sun as seen from another one?

In our solar system, it is possible for one planet to partially eclipse the sun, but it is not possible for any planet to cause a full solar eclipse as seen from another planet. The sun is too big and ...
Connor Garcia's user avatar
  • 16.3k
38 votes

Could we detect the light of cities at night in other solar systems?

No. There are very few exoplanets that can even be imaged as "dots". There are none that can be resolved as discs and it would be well beyond current technology to see lights on the ...
James K's user avatar
  • 123k
37 votes
Accepted

Why do planets and satellites in the Solar system look so wildly different if they came from more or less the same matter?

This questions can be split in two; for planets and satellites. The diversity of planets reflects in part the diversity in terms of chemical composition of the protoplanetary disk. We know that UV ...
Swike's user avatar
  • 3,926
36 votes

How can comets have tails if there's no air resistance in space?

First, there is not just one tail, it is several, but when traveling far from a star, they are "aligned". When it gets closer the different materials behave differently, both depending on the ...
Viktor Mellgren's user avatar
36 votes
Accepted

Which planet came first in our Solar system?

The planets (probably) formed by the accumulation of planetesimals, and they all formed over roughly the same time span. Inner protoplanets had access to more matter, so they probably were able to ...
PM 2Ring's user avatar
  • 14.8k
34 votes
Accepted

How can Io be tidally heated while it is in tidal lock?

How can Io be tidally heated while it is in tidal lock? It is tidally locked in a mean motion sense of "tidally locked". That Io is in an eccentric orbit rather than a circular orbit means ...
David Hammen's user avatar
  • 34.1k
34 votes
Accepted

Where was the solar system born? Alternatively, how far back in time can we retrodict the location of the solar system?

Basically no, and not very far back at all. Star forming regions generally last for at most 10 million years. The "nebula" in which the Sun was born is long gone so cannot be identified. The ...
ProfRob's user avatar
  • 153k
33 votes

Why do things float in space, though the gravity of our star is always present?

To help with James K's excellent answer, a visual representation might help. Let's look at a thought experiment - Newton's Cannonball. Let's say you have a cannon, high enough that it's being held ...
RPBCL's user avatar
  • 421
33 votes

Could we detect the light of cities at night in other solar systems?

I did a few rough calculations. Let's assume a neighbor system (4 light years), and a population of 1 billion people each transmitting 1000W of light into the night sky. Then on Earth, a 100 square ...
Ralf Kleberhoff's user avatar
32 votes
Accepted

Multiply Earth's orbital speed with square root of 2

It is the Solar System "escape speed" for an object in orbit around the Sun, at the distance of the Earth. The energy of an object in orbit is the sum of its kinetic and potential energy. ...
ProfRob's user avatar
  • 153k
31 votes

Why is the Sun's density less than the inner planets?

Fusion inside of a star affects the sun's density (which does not happen with a planet). It produces an outward pressure that balances against the attraction of gravity, thereby reducing the density ...
called2voyage's user avatar
  • 6,322
31 votes

Is it at all possible for the sun to revolve around as many barycenters as we have planets in our solar system?

The short answer is no; there is only one barycenter. Yes, you can count the Sun/Jupiter barycenter or the Sun/Saturn barycenter, or whichever barycenter you want, but the net effect of all Solar ...
Pierre Paquette's user avatar
30 votes
Accepted

How is it possible for a collision to be responsible for Uranus's axial tilt?

For objects on the scale of a planet, the state of matter doesn't really matter much. A colliding planet or planetesimal would not "just pass through" The amount and density of the gas ...
James K's user avatar
  • 123k
29 votes
Accepted

Starting a fire in a cold planet that was full of flammable gas

The outer parts of Neptune are mostly hydrogen and helium. There are small amounts of other gases such as methane, ammonia and water vapour. However, there is no oxygen at all. If you took some of ...
James K's user avatar
  • 123k
29 votes
Accepted

Do the axes of rotation of most stars in the Milky Way align reasonably closely with the axis of galactic rotation?

There is very likely to be a random scatter. Unlike planets orbiting the Sun in the Solar System, most of the stars in the Galaxy did not form at the same time as the Galaxy itself. There is therefore ...
ProfRob's user avatar
  • 153k
29 votes
Accepted

Why did the dust between the planets disappear during the birth of the solar system?

Dust happens in two ways. "Primordial dust" just condenses out of the protostellar material in the disc providing it gets cool enough and dense enough. "Second generation" dust is ...
ProfRob's user avatar
  • 153k
28 votes

Is there enough matter orbiting the sun to make a second sun?

No. The total mass of the planets, asteroids, dust etc in the solar system is only about 0.1% of the mass of the sun. There is not nearly enough to make even a small star.
James K's user avatar
  • 123k

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