They are still in separate orbits so the straight line between any two varies greatly in length... not to mention intersecting the star regularly.

Regular shuttles for interplanetary travel timed with periodically favorable orbital positions?

At that distance, gravity must be really fun. Tides are not only for water, such tides can also change the height of mountains, the balance of high-rise buildings, infer on volleyball rules, make you fart because of changes in atmospheric pressure, or... be leveraged to generate electricity. Those guys have simili-infinite energy at human scale.

Would they? I mean, we can see geological features on our moon, and our tides are orders of magnitude away from being rock-tearing events. I mean, the moon is small, but... how close do these planets approach, anyway?

The moon is not that small, it is 27% of earth's size. Any other satellite in the solar system is way smaller compared to the planet they orbit. I dare to say that earth and moon are twin planets instead of a planet and a satellite.

Radius 27%, mass 1.2%.

http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html

Unless you are excluding Pluto deliberately for not being "a planet", its moon Charon has half the diameter of Pluto. They are even both orbiting a common center of gravity outside Pluto.

https://en.wikipedia.org/wiki/Charon_(moon)

@ConceptJunkie

> The Moon is about 2% of Earth's mass, not 27%.

2% of mass, yes, but also 27% of the diameter.

You are both right.

My first reaction was WTF?

Then 0.27^3 = 0.0197

And then, only 60% as dense (no core) so 0.0119 :)

One of my scale numbers is, everything between Jupiter and the Sun masses less than 2 Earths -- Venus, Mars, Mercury, the Moon, and all the asteroids mass less than the Earth.

And only just barely! You get to around 99% of the Earth's mass with the rest of the inner solar system.

All of the other rocky bodies in the solar system give you another 10% of Earth's mass out to Pluto and another 10% beyond.

>One of my scale numbers is, everything between Jupiter and the Sun masses less than 2 Earths -- Venus, Mars, Mercury, the Moon, and all the asteroids mass less than the Earth.

Is that correct? Venus alone is about 90% the size of Earth, and Mars roughly 33%. It really seems like all those bodies put together should be greater than 1 Earth mass (but less than 2 Earth masses).

That's size, not mass.

Venus is about 81% Earth's mass, and Mars is only a little over 10%. Mercury gets you another 5.5%, the Moon (as above) another 1.2%, and the asteroids are negligible.

In the outer solar system, it's basically all the Galilean Moons + Titan.

ETA: Hmm, or maybe those are surface gravity numbers you're thinking of?

Yep, I was roughly equating mass with surface gravity, which I see now is incorrect.

I do find it really interesting and a little mind-boggling that the Moon, as huge as it is, with a good 1/6 of Earth's gravity, only has 1.2% of Earth's mass. Same with Mars: a good 1/3 of Earth's gravity, but a mere 10% of its mass. I really thought there'd be more direct correlation between mass and gravity than that. Of course, surface gravity is related to mass with both density and radius, but still, I would have assumed that the densities of these bodies would have been rather similar, as they're all small, rocky worlds.

The densities do vary by almost a factor of 2 (~3 g/cc for the Moon, 5.5 g/cc for the Earth), but the larger problem is that surface gravity is a linear function of the radius, and mass is the cube of the radius.

Consequently, the mass of an object with a given surface gravity is inversely proportional to the square of the density. If the Moon were the same density as the Earth, and had the same surface gravity as it does now, it'd actually mass less than it does now -- and Uranus, despite have 15x the mass of Earth, has a lower surface gravity, because its density is so low.

The rule of thumb for space is "any way your intuition can be wrong, it will be wrong". :-)

You just blew my mind ;)

What do we know about the gas giants' cores?

Gas giant cores are weird. Based on models of solar system formation, we're like, pretty sure they started with rocky (or icy) cores that were dozens of times bigger than Earth. But we're not sure they're still there.

Due to the intense conditions inside a gas giant, we're not sure if they still have a rocky core, or if the cores have liquified or even convected away to mix with the gasses.

So the answer could be "there are dozens of times the Earth's mass worth of rocky material inside gas giants" or "there is no rocky material inside gas giants", or anywhere in-between. We need more detailed study of the gas giants before we can answer that question.

Models of solar system formation also suggest the Kuiper belt started with dozens of times Earth's mass, but we only see about 10% there now, and we only have pretty good guesses about what happened. (I mean, it almost certainly got ejected, but the "why" is harder.)

Thanks. I asked because searching seemed to be telling me that we have only theoretical arguments about gas giants' cores. That surprised me at first, but then I realized how little data we probably have. Basically, mass and its distribution, plus spectroscopic data about surface composition, right?

Juno is hopefully going to tell us more :)

https://en.wikipedia.org/wiki/Juno_(spacecraft)#Scientific_o...

Close enough that the perturbations these planets induced in one another's orbits allowed the team to estimate their masses at between 0.4 and 1.4 Earth-masses. In astronomical terms, that's very close.

So it possible to see Avatar like suspended rocks ? Somewhat orbiting some lagrangian point, which is very close to surface.

The Moon is about 2% of Earth's mass, not 27%.

One of the callers (a biologist, so not a crackpot) on TV suggested that given how close the planets were, he wondered if it were possible for there to be cross contamination between the planets given how close they are to each other. He referred to them being part of the same "ecosystem." I'm not sure how feasible that is as a non-astrobiologist, but it is an interesting idea.

Very likely, since this happens between Mars and Earth:

https://en.wikipedia.org/wiki/Martian_meteorite

Fascinating! Would vira or bacteria beable to survive travelling from Earth to Mars in or on a rock or vice versa?

Tardigrades do survive in space: https://en.wikipedia.org/wiki/Tardigrade

Roughly quoting Neal de Grasse Tyson: Since so many bacteria survive outer space there is a good chance it's because of - yes - survivor bias. If only because early earth was bombarded so often that fragments of the planet with microbes took of in a meteor strike and returned home after a while in space.

Short answer: almost certainly yes, although i presume this is an improbable event.

It's not that improbable. They've found meteorites on Earth that have been identified as having come from Mars based on their mineral contents.

Sure, I was referring to the part where the microbe survives the initial blast, the passage through space, reentry, and landing, and happens to land in a spot where it can survive.

According to some research, microbes survival in meteorites is very likely (if they exist)

https://www.ncbi.nlm.nih.gov/m/pubmed/11543506/

I know enough biology to be skeptical of papers like that (although, I admit I was skeptical when papers from the 50s predicted interplanetary mass transport, and didn't change my mind until we found Martian meteorites). I'd like to see actual evidence of this.

I actually think the rock getting into space with the microbe embedded in it is not that unlikely (many rocks in the earth are laced with bacteria), nor the space transport. I think the excessive heat of rentry, the force of impact, and the difference in target ecosystems are what are going to kill the microbes.

Possible, but cross-section for collision is way way smaller in the Earth-Mars system. Cross-section goes as r^-2 so the closer the bodies, the 'way better' it is.

IIRC, that is a serious hypothesis for how life began on earth, seeded from elsewhere (maybe not Mars).

But certainly someone here knows more than I do about it ...

The theory is called Panspermia

https://en.wikipedia.org/wiki/Panspermia

Considering the time scales involved, it wouldn't surprise me if multiple planets around nearby stars happened to be seeded with earth based bacterial/archaea life.

One of the presenters is an old friend from college. Folks were offering him money to mention "Zod, our Galactic Overlord" on the air. I guess that didn't happen?

No unfortunately, I don't think they did. That would have been hilarious.

The survivability of several organisms has been actually tested https://en.wikipedia.org/wiki/List_of_microorganisms_tested_...

There has been and continues to be cross-contamination between Venus, Earth, and Mars, so it seems likely that planets much closer together, and passing each other more frequently would have more 'communication' of materials.

Continues to be...? Outside of human action, what's the vector?

My understanding is that it is mostly due to solar wind, meteorite impacts, and various other infrequent events.

powerful meteor strikes spreading debris out of orbit.

And the debris isn't heated enough to be sterilized.

Gene Wolfe mined this idea for some of his science fiction works, except it was alien life making the jump ... Fifth Head of Cerberus & Blue's Waters/Green's Jungles.

Even if bacterial material were being transferred all the time, I think it'd be a stretch to call them the same "ecosystem" unless they were actually reliant on each other. But I'm not an biologist either.

I imagine their relative distances change frequently, so probably not.