# Can you have a planet as bright as Venus orbiting at 1.0 AU with no significant atmosphere?

I'm basically trying to have a hot volcanic planet with lots of visible lava within a short distance of my pov planet, so that it would be large in the sky; orbiting at 0.9-1.1 AU. I want it to be visible with the naked eye. Is there any way you could get such a planet to look like this?

• Are you wanting it to look like that to the naked eye or through a telescope (of unspecified power)? Apr 19 at 6:30
• With the naked eye. The planet would orbit like 10x closer than Venus, or perhaps even closer. Apr 19 at 7:11
• So, a bit like Earth after the impact with Theia, so the hypothesis goes....? Apr 19 at 7:16
• Could you say where you want your POV planet to be, Mars ish perhaps? Apr 19 at 7:47
• I want it to be around 1 AU as it says in the title. Apr 19 at 12:35

TL;DR: you need a binary planet if you want to get that sort of view.

So, the moon is big enough and bright enough to be seen by the naked eye, and large surface details can be identified. Incidentally, less than 15% of the visible light incident upon it gets reflected, whereas Venus reflects more than 75%, but lets just think about size right now, because that's the main restriction on seeing surface detail with the naked eye.

The apparent angular diameter of the moon is ~32 minutes of arc. A Venus-sized world with that apparent size would need to be ~1.3 million kilometers away from the observer. This is within Earth's Hill sphere (which has a radius of ~1.5 million km) which means the two would have to be gravitationally bound as a binary planet (though I'm not sure if they'd be stable over long timescales at that separation... there's a currently open question on that subject). You could certainly tweak the arrangement to provide a suitable companion world that appeared big enough for your needs (it could easily appear much larger than the full moon, for example).

If the observer's world and your hypothetical world weren't orbiting each other, but instead had their own orbits about the sun, then you'd almost immediately get significant gravitational interaction and at least one of those worlds is getting flung into a radically different orbit, or even into interstellar space. You can run an n-body simulation to demonstrate this yourself if you were feeling keen... I've used gravity simulator for this in the past. Its online and free.

An alternate arrangement might to be to have your other world at a Langrange point. Now, the L4 and L5 points are potentially stable, so long as one of the bodies is at least 25 times the mass of the other. Those points are conveniently 1 AU away from Earth, but an object with a moon-like angular diameter at that distance would need to have a radius nearly 10-times larger than Jupiter. At that's problematic, because you can't get planets that big... 10 jupiter radii is a little over the radius of the actual Sun, and adding a second star to a solar system presents issues with habitable zones which will upset all those distance as I'm sure you can imagine. It would at least fulfil the 25x mass requirement, so it isn't all bad news, and it would certainly look pretty firey, through the right sort of filter.

Considering that two planets following the same orbit is a very unstable configuration, I think your only safe bet is to have two planets orbiting around their center of mass around their central star. Sort of Pluto and Charon on a planetary scale.

However if this solves the gravitational aspect of the problem, creates another one: if the planets are twin, and one of them is mostly a ball of molten rock, it's very difficult to explain how the other can still or already be hosting life. Either the planet recently formed or got a very large impact, both of which cannot totally skip the planet's twin.

So, you can have it, but hardly observe it.

## A massive injection of radioactive material

About 50% of the Earth's internal heat comes from radioactive decay. If you were to increase how much radioactive material there was in the Earth, it could heat up and the surface could become molten for millions, maybe even billions of years.

Supernovas create disproportionately large amounts of Radioactive elements which can consolidate into waves of highly radioactive dust flying through space at high speeds passing though many neighboring solar systems. At only ~20ppm of total radioactive isotopes, the Earth gets about 50% of its internal heat from radioactive isotopes like uranium-238, uranium-235, thorium-232, and potassium-40. Because so little radioactive material can cause so much heat, a single wave of supernova dust from a nearby exploding star could passing through the solar system bombarding one of the planets with radioactive dust. If the second planet were shielded by something like a more powerful EM field, or maybe it was in the shadow of the first planet when the dust wave hit, then it could be spared from most of the fallout.

• +1 Or,... or, somehow the magnetic fields of aligned gas giants contrived to focus a huge amount on the one trajectory -> the glowball. Apr 19 at 19:55

Yes, you can have a planet that is large in the sky (like the Moon) which looks like that, but:

• it should have significantly less mass than the other planet - really a large moon, not a planet.

• it won't look like that when illuminated by the sun, it'll look the slightest bit pale orange. But the night side of the moon will look like that.

• it needs a very large, man-made (or alien-made, or magical) heat source to be plausible.

The trick is to do that thermal work without accidentally doing so much mechanical work that you destroy all life on Earth in the process. You can get the moon to look like that naturally by crashing a rogue planetoid into it, or many, many tiny impacts. But that would have extinction-related consequences on Earth. For one big impact: eccentrically orbiting pieces of debris from the impact would crash on Earth, and there might be harmful orbital perturbations of both Earth about the Sun and the Moon about the Earth. For many little impacts: many more would hit Earth!

However, doing this with the actions of a plausible but very industrially well-developed and technologically advanced alien civilization is pretty conceptually easy. The energy is there for the taking, pouring off of the sun into deep space. But to get it you need to start doing engineering projects on a truly huge scale.

The easiest way to do it, I think, would be to use a system of very large (moon-sized), very light orbiting mirrors (automated to use the mirrors as solar sails to maintain their orbits) to shine more sunlight on the Moon. Radiation goes as the 4th power of temperature, so since we want to roughly double the Moon's surface temperature, we need to make it receive about 16 times as much sunlight.

However, unless we want to do this for a very long time and then discard the mirrors, that won't get the specific appearance that you're looking for - the moon will be molten, but it'll just look brilliantly white all the time, and the mirrors themselves may be visible from Earth, depending on their position.

Instead we can have the aliens build a very, very large, distant, solar powered infrared laser and shoot the moon with it. Just make sure not to hit the Earth. This way the moon gets molten hot without getting so bright that we can't see the lava.

As the other answers mention, having them orbiting the star with such close orbits would result in both of them being thrown into eccentric orbits or possibly into the star or out of the system.

The binary-planet concept is possible, with a few adjustments. As Dutch mentioned, they couldn't evolve that way simultaneously. The factors that allow one to support life would apply to both of them. As such, they need to have different origins.

It's estimated that, after the impact that formed the Moon, it took 300-400 thousand years for the Earth's surface to cool. For comparison, the last major extinction event on Earth happened 66 million years ago. This is important because any event that would give an Earth-like planet a binary twin would have to have happened within that time period, and you couldn't do that without significant meteor bombardment of the older planet.

Let's say that, 300,000 years ago, right around the time that evolution was putting finishing touches on Homo Sapiens, something huge ran into an Earth+ sized planet where Mars is in our world. Via the anthropic principal, its primary components all missed the Earth, but a major chunk of it plowed into the Moon, absorbing the Moon and slowing the remaining mass enough to be trapped into an orbit with Earth. The orbital mechanics would be improbable, but not impossible.

You couldn't completely avoid bombardment of Earth, but you could put a Jupiter-sized planet where our asteroid belt is to soak most of the damage. Your PoV planet would be covered with craters, and shooting stars would be very common. You would have a big asteroid belt where the other planet used to be, much of which might be visible to the naked eye.

I can't even guess at the amount of atmosphere that the new planet would have, but that means your readers won't be able to, either. If you presume no magnetosphere, then a lot of the atmosphere would have been blown off by solar wind, and it might have lost more in the collision.

This would be possible, but not very likely. As mentioned above, the chances that a cataclysmic event is severe enough to cause that without affecting the planet which still has life on it are low. But, theoretically, a way this could happen is if some other planet in the system had a large impact, ejecting this planet, which was captured by your host planet. However, this would really be more like a moon than a planet, and would probably have a highly elliptical orbit. Also, about the orbits, I don't think that distance would really work, as the distance needed for the two planets to have any kind of stable orbit around each other would be far outside of a star's habitable zone. However, as a captured moon it would be somewhat possible.