Is the size of a planet or star limited by the laws of physics? Could there be a planet as large as our sun, or a star as large as our solar system?

 Yes, the laws of physics limit the radii of planets and stars (we talk about the radius here because mass is heft, while size means dimensions).

No, there could not be a planet as large as the Sun, using the standard definition of planet (that is, not calling a stellar object a planet just because it orbits another star).

No, there could not be a star as large as our solar system. The largest stars yet observed have radii which may approach that of the orbit of Saturn, around 9-10 AU; in contrast, the radius of our solar system as a whole is more like 100,000 AU.

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Such mega-massive stars sit just beneath the Eddington limit, which is the point at which outward radiation pressure exceeds inward gravitational force. Beyond this, they could not cohere as objects — this is what happens when stars go poof.

Stars like this (St2-18, say, or UY Scuti) are extremely low-density; their masses do not scale up with their sizes in the way you might intuitively suppose. If the Sun is a solid baseball, then these guys are gas-filled hot air balloons in comparison.

They’re big puffballs of radiation, shining with extreme luminosity but barely holding together gravitationally.

Models are being refined as scientists gather and analyze more data, but we think this maximum size is somewhere near 2,000 R☉. Two thousand times the radius of the Sun is intimidating enough! But poke it with a toothpick and it bursts (so to speak).

UY Scuti has nearly 2K times the girth of the Sun, but that is still nowhere near the radius of the whole solar system.

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The radius of a planet is constrained by the fact that, the more mass you add, the more gravitational compression you get scrunching the thing down smaller, or at least preventing it from expanding into something larger. (So, more mass = bigger planet only up to a point.)

Eventually, if you add enough mass, this compression will ignite fusion and you get a star instead.

Jupiter is fairly typical in radius for a really big gas giant. We have observed some which are larger — often 1.5–2 RJ, and in rare extreme cases perhaps as large as 6 RJ. But when you get that big, it’s possible you’re looking at a brown dwarf. Gas giants don’t get a whole lot bigger than Jupiter unless they’re quite hot (i.e., in close orbit of a star). Otherwise 2RJ could mean something like twenty times Jupiter’s mass, and that’s inviting deuterium fusion.

You can have rocky planets as large as a few Earth radii. But if you doubled the mass of Earth, you would only increase its radius by around 20–30%. Here again, gravitational compression is a limiting factor.

And they call gravity the weakest force! Well, it is, but it utterly dominates at the scale of everyday objects, including suns and worlds.

Which is the biggest planet and smallest planet of our solar system?

 The largest planet in the solar system is Jupiter, and the smallest is Mercury.

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Hah, you think I am just going to write some super short answer? Please, that’s just is not my style.

So, let’s start off with the smallest, and one of the coolest planets in my opinion:

Mercury!

Ahh, good ol’ Mercury.

Mercury is the smallest, and innermost planet of our solar system. It is quite similar to our very own Moon, both in looks and surface features. In fact, I like to look at Mercury as the inverted color version of the Moon, because the higher areas of Mercury are darker, and the deep craters are brighter (opposite of our Moon.) Some of these craters, in fact, are so deep, sunlight never reaches parts of those craters. And, in some of those craters, there is actually water ice, just hanging out. This is quite amazing, since the day time temperature on Mercury can reach several hundred degrees Fahrenheit.

Another really cool thing about Mercury is that it has an unusually large iron and nickel core for its size. Scientists think that the core makes up 85% of the planets entire radius. This also makes it the second densest planet, only behind the Earth. Scientists have two different theories as to why the core of Mercury is very large.

The first, and older theory is that Mercury had a collision with another planet when it was first forming, and this blew off most of the silicates and lighter stuff off of baby Mercury and the other planetoid, while also merging the two cores of the planets, making one smaller planet, but with a huge core.

The second, and more exciting theory is that Mercury, a long time ago, used to be a gas giant. But, as it got closer and closer to the sun, more and more of the gas was stripped away from the intense solar winds, until all that was left was a small, metallic core that most gas Giants have. This theory is really cool because it would make our solar system seem a bit more normal, since many other stars have gas giants that orbit really close in to its parent star, but ours does not. It is also a nice feeling to think Mercury used to be a giant once upon a time.

Now, for the largest planet in the Solar System:

Jupiter!

Jupiter is a lot different from Mercury, in almost every way imaginable. First of all, Jupiter is HUGE. Jupiter has a radius of 43,441 miles, compared to tiny Mercury, which only has a radius of 1,516 miles. This means that you could place 28 Mercury-sized planets next to each other, stretch them in a straight line above Jupiter, and Jupiter would still have a larger diameter. Jupiter also has a very thick hydrogen atmosphere, with a cloud layer 30 miles thick, before you reach a point inside Jupiter that is very bizarre.

Once you get past that cloud layer, the hydrogen is so dense now that it takes on a liquid form. But, this isn’t a cold liquid, but is extremely hot, like the molten parts of our planet, only made of hydrogen gas. Past this layer, the hydrogen continues to become more dense to the point it acts like a superheated metal. At this point, anything inside would be burnt beyond recovery, but if you were to send something down there, this is as far as you could get. Below the metallic layer of hydrogen, you have a small rock and ice core (similar to Mercury, actually).

Jupiter has the most moons in the solar system as well, counting 67 so far. Most of these moons are small asteroids that were caught by Jupiter’s immense gravity, but 4 of them, the Galilean moons, are quite unique. The closest one in is Io, which is one of the most volcanically active objects in our solar system, and its surface has a weird mix of colors. Europa has a huge water ocean underneath its thick icy crust, and it is a possible contender to support life because of that. Callisto has one of the oldest surfaces in the solar system, and Ganymede is the largest moon in the solar system, and is bigger than Mercury.

So, there you have it, the largest planet in our solar system, and the smallest, along with some cool details and theories about them. And no, Pluto doesn’t count, it’s classified as a dwarf planet, so leave it at that.

What seemingly impossible event has happened on other planets?

 We are possibly witnessing the birth of life on Titan even as we speak.

Ask any origin-of-life researcher why life evolved on Earth. They'll probably give you a list of reasons, with the Earth's location in the Goldilocks Zone around the Sun usually appearing at the top of that list.

The Goldilocks Zone offers the perfect conditions, especially in terms of temperature, for life to originate and thrive.

In the case of the Solar System, this region falls between the orbits of Venus and Mars. Venus, Earth, and Mars are the only planets that fall in this habitable zone.

It's another matter that Venus and Mars don't support life because of other reasons that have little to do with their location.

Any planet farther than Mars and closer than Venus cannot support life.

That's until Titan, Saturn's moon, decided that it's not going to be gaslit by bipedal primates on a far-off rock!

Image Credits: How Does The Surface Of Moon 'Titan' Looks Like? » Space Exploration

The Prebiotic Chemistry of Titan

Titan's atmosphere is mainly composed of nitrogen (~98%) with smaller amounts of methane (~1.4%) and trace gases like hydrogen, argon, and carbon monoxide.

However, solar radiation and cosmic rays are breaking methane into smaller molecules, which are further reacting with each other to form a variety of other complex compounds like ethane, benzene, polycyclic aromatic hydrocarbons, etc.

These complex compounds are further undergoing reactions to form even more complex compounds like amino acids, which you may know as building blocks of proteins!

Mind you, all of this is happening at temperatures in the vicinity of -179°C or -290°F.

Water freezes at 0°C, and the temperature on Titan is frigid -179°C. Yet, it has lakes, rivers, and even seas all over it. But, they're all composed of liquid methane and ethane. Not water!

And it's supporting this complex organic chemistry that's the precursor to life.

To be clear, Titan is far from a crucible for life yet. But scientists believe that its chemistry today is remarkably similar to that of early Earth before life originated here.

Even if (and that's a big IF) life originates on Titan, it will probably take many millions of years, if not billions of years, for it to happen.

However, the fact that we are witnessing this prebiotic chemistry in the works so far away from the Goldilocks Zone is freaking crazy!

That opens up the question: how many other planets and moons are out there with conditions to support life - and perhaps even teeming with life - that we've ignored only because we believe their conditions do not support liquid water!

More importantly, the recent discovery of a likely habitable period in Mars's history and Titan's potential to support the origin of life present an exciting possibility. That's at least 2 opportunities besides Earth for life to originate in the Solar System alone.

The odds alone are astronomically small.

Even without the discovery of life in both cases, it radically increases the prevalence of opportunities (conducive conditions) for the origin of life beyond the Solar System. I mean, if liquid water is not so essential for life, I wonder if anything else is.

What are some of the most unusual planets?

 The universe is full of strange and fascinating worlds. While Earth may feel unique, scientists have discovered planets far more extreme, mysterious, and bizarre. From planets where it rains glass to those orbiting two suns, these unusual exoplanets challenge everything we know about space. Here are my list of some of the most unusual planets -

1.HD 189733b – A Planet Where It Rains Glass
This planet is known for its extreme weather. Winds blow over 5,000 miles per hour, and it rains molten glass sideways! It also has a deep blue color caused by tiny particles in its atmosphere.

2. WASP-12b – A Planet Being Eaten by Its Star
WASP-12b is so close to its star that it is slowly being pulled apart. The star’s gravity is eating away at the planet, creating a long trail of gas and dust behind it.

3. 55 Cancri e – The Diamond Planet
This planet may be made mostly of carbon. With the right pressure and heat, that carbon could turn into diamond. It’s twice the size of Earth and extremely hot.

4. Kepler-16b – A Planet with Two Suns
Kepler-16b orbits two stars. This means it has double sunsets, just like the planet Tatooine from Star Wars. It’s the first planet found in a double-star system

These are some amazing & probably very unheard facts about the universe. There are so many things that we don’t know or ever heard of! Tell me about your opinion below.

Why don't we get eclipses from other planets between the Earth and our Sun?

 We do, actually!

We even have eclipses from other planets between the Earth and the OTHER STARS!

Only they are known by a different name, so you probably don’t know them as eclipses.

Before I get to them, check out this eclipse of Mercury.

Source Image: Transit of Mercury - Wikipedia

See the black dot I’ve marked in the green circle? That’s Mercury between us and the Sun.

The smudge inside the blue circle is the sunspot. Notice how it’s bigger than the planet of Mercury.

In this image, Mercury is fully intent on blocking the Sun’s light from ever reaching us. But the poor guy is too small to have any meaningful impact.

So, we don’t even notice it.

In fact, Mercury’s obstruction of our view of the Sun is so unremarkable that it’s not even called an eclipse. The same goes for Venus, too.

No effect, but A+ for effort!

When is an Eclipse Not an Eclipse?

During a solar eclipse, the Moon blocks the Sun either fully or partially. Either way, the Moon is capable of blocking a substantial portion of the Sun when viewed from the Earth.

Remember, it’s not the actual size of the Moon that matters here, but its apparent size in the sky.

Since its apparent size in the sky, as seen from Earth, is large enough to block the Sun partially or entirely, its obstruction is known as an eclipse.

Now, would you call it an eclipse if an asteroid just happened to whizz past the Earth between us and the Sun?

You wouldn’t, because it’s simply too small to qualify as an eclipse!

That’s what happens when Mercury or Venus ends up between the Earth and the Sun. They’re just too small for us to experience an eclipse.

So, we call those events Transits!

Transits are Among the Most Important Events in Astronomy

Did you know that planets outside the solar system are too dim to be visible to us?

Several factors, including interstellar dust, brightness of the accompanying star, etc., make it almost impossible for us actually to see these exoplanets even with our most sophisticated telescopes.

So, how do we detect them?

We rely on transits.

When a planet passes before its home star, there’s a small drop in that star’s brightness.

Our sophisticated instruments are capable of detecting and measuring that drop in brightness. Based on this, we ascertain the existence of a planet around a faraway star.

In short, these “eclipses” or “transits,” whatever you wish to call them, play a central role in helping us detect planets outside the solar system.

Are there more planets than we know so far?

 Absolutely ! There are roughly 

${10}^{21}$ ( sextillion ) planets in the visible Universe, and at least 106 known exoplanets only within the surrounding sphere with 100 light years radius.

One of my favorite exo-solar system is the Trappist-1 system:

Trappist-1D:

Trappist-1C:

Proxima Centauri-B:

Furthermore !

There can be bizarre Planets such like our newborn Earth was 4.8 billion years ago:

Moreover:

L4L5 Interferometric Exoplanet Spy System (LIESS)

Basic concept:

Launching two spacecraft to the L4 and L5 points of the Earth's orbit path to study exoplanets within 100 light years.

Expected discoveries and resolutions (without claiming to be complete):

Real-time observation of the planets Proxima Centauri B and C at 13.4

meter/pixel resolution and even real-time monitoring of their meteorology.

Imaging of planets in the Trappist star system at a resolution of 134 meter

per pixel and real-time observation of their meteorology.

Costs and implementation:

The implementation, in contrast to the Proxima Centauri approach planned

for 2069, is much simpler than ion-gun acceleration technology and

would provide continuous observation of at least the surrounding

exoplanets within 100 light years, instead of a one-time journey.

… and another 106 exoplanets, if we only take the ones known so far, and possibly more exoplanets and even exomoons may emerge with a terrain map of unprecedented detail. We can boldly say that with this technology we will see the pimple on the ant's dick on the celestial bodies in the Cuiper belt and the Oort cloud, and we will get global maps comparable to the detail of Google Maps of more than 10 times as many planets as we have known so far. For example, it can easily be found that there are not just 106, but actually 268 exoplanets within the surrounding sphere of 100 light-years radius, and they have 778 exomoons. We can boldly say that with this "penny" and relatively simple tool we can find out what the word "weather" means.

Earth-Mars Orbit Configuration

1. Base Distance:
   Base distance: approximately 1.5 astronomical units (AU) = $2.25\ast {10}^{11}$ meters
2. Observation Wavelength:
   Let's assume an optical wavelength of $500nm$ ($500\ast {10}^{-9}$ meters)
3. Angular Resolution Calculation:
   The formula for angular resolution is: $\theta =\frac{\lambda }{B}$
   Where: $\theta$ is the angular resolution in radians
   $\lambda$ is the observation wavelength
   $B$ is the base distance
   
   Substituting the values: $\theta =\frac{500\ast {10}^{-9}m}{2.25\ast {10}^{11}m}\approx 2.22\ast {10}^{-18}rad$
4. Convert to Distance on Proxima Centauri B:
   Distance to Proxima Centauri B: $4.24$ light-years ($4.02\ast {10}^{16}$ meters)
   $d\cdot \theta =4.02\ast {10}^{16}m\cdot 2.22\ast {10}^{-18}rad\approx 8.92$ meters/pixel

Neptune Configuration

1. Base Distance:
   Base distance: approximately $30.1$ astronomical units (AU) = $4.5\ast {10}^{12}$ meters
2. Observation Wavelength:
   Let's assume an optical wavelength of $500nm$ ($500\ast {10}^{-}9$ meters)
3. Angular Resolution Calculation:
   The formula for angular resolution is: $\theta =\frac{\lambda }{B}$
   Where: $\theta$ is the angular resolution in radians
   $\lambda$ is the observation wavelength
   $B$ is the base distance
   
   Substituting the values: $\theta =\frac{500\ast {10}^{-9}m}{4.5\ast {10}^{12}m}\approx 1.11\ast {10}^{-19}rad$
4. Convert to Distance on Proxima Centauri B:
   Distance to Proxima Centauri B: 4.24 light-years ($4.02\ast {10}^{16}$ meters)$d\cdot \theta =4.02\ast {10}^{16}m\cdot 1.11\ast {10}^{-19}rad\approx 4.46$ meters/pixel

These calculations show the angular resolution and pixel distance for both configurations. The Neptune configuration provides a higher resolution due to the larger base distance between the observation points.

Thus, with the above-mentioned method, we will soon be able to easily observe planetary and lunar systems outside our solar system.