23

The chance happenstance that Earth happened to be floating along and got captured is minuscule. How did Earth wind up floating through space? There's no established mechanism for terrestrial planets to form on their own. As far as we know, they need a host star to form around. So if our Sun captured Earth, it must've formed around another Star, got ejected ...


16

I think the Earth's orbit is by far the strongest argument you have that the Earth formed around the Sun. The orbit is nearly circular and almost in the Sun's equatorial plane, similar to the other planets. These facts are naturally accounted for if the Earth formed from material that coagulated in the Sun's primordial disc, where circularisation and ...


13

That's basically the Fermi paradox. It seems likely that there are numerous civilizations in the galaxy, and yet we see no trace of them anywhere. The Drake equation is often invoked to calculate the probability of existence of other civilizations, by compounding several other, more simple probabilities: the probability that a star has planets, that the ...


9

What if we are looking for the wrong signs of life on other planets? Are there any fields of astronomy that look beyond the "near-Earth" life comparisons and look for life in other (drastically different) forms? and I'm always bothered by these reports because they are all predicated on the premise that life on other planets will closely ...


8

Why would we assume that the early martian life originated on Mars, rather than Earth? There's still a whole lot we don't know. As Wayfaring stranger points out in the comments, Origin is a whole different question. It's possible that life originated outside our solar system and came to either Mars and/or Earth from outside the solar-system. I don't ...


5

This would seem to be constrained to planets and moons earth has landed on or impacted at all, as we have no way of telling after the fact whether a lander was perfectly sterilized. That list is as follows: Planets: Mercury: USA's MESSENGER in 2015 de-orbited and crashed upon completion of mission. Contamination is unlikely because of high impact and ...


4

It would be really difficult for the Earth to end up in a nearly circular orbit if it came from outside the solar system. Effectively falling from infinity, it would have a hyperbolic orbit and make one swerve around the Sun and depart back into the blackness of interstellar space. What mechanism could get rid of precisely enough energy to keep the Earth ...


3

The earth in its early stages was far too hot for water to be in liquid form and the water vapor that would have been around at the time would have been blasted away via solar wind (http://en.wikipedia.org/wiki/Solar_wind). Anyone claiming to know exactly how water came to be on Earth is likely daft, but comets and asteroids seem to be the most common theory ...


3

We have not found life, bacterial or otherwise, anywhere other than Earth ... yet. I think you mean the difference between liquid water and water ice. This difference relates to the hospitality to life. Life of the type we are used to needs liquid water, even if this is just trapped moisture, unless the life is temporarily in suspended animation. If there ...


3

We only have a sample of 1 - the Earth - for analysis or proposition of potential life-compatible conditions on other planets. Only carbon-based life can exist in this universe. Boron and Silicon are the next 2 elements for potential life, and neither have the ability to provide polymeric bonds required for complex biochemical assembly. Water is essential ...


3

Occam's razor defeats the hypothesis. There are no astronomical anomalies about earth's obit or position that require the complication of earth's insertion from beyond the solar system. Unless some difficulties in known fact are brought forth requiring such a hypothesis as their simplest solution, then the idea may simply be rejected as unnecessary. The ...


2

The experimental data only shows that there is no life and no intelligent life in the parameter space where we could detect it. The experimental data doesn't explain the cause. That we are alone in the Universe, it is still a possibility.


1

There are a number of questions here on the Fermi Paradox with answers ranging from an Infinite Universe, the parameters for the Drake Equation and others. Well worth having a read through them. But my favourite is author Cixin Liu's theory outlined in The Three Body Problem Obviously that theory is not easily proven or disproven in any useful way, so ...


1

If any amino acid chain on Mars is the same or similar to any amino acid chain on Earth, then we know with final certainty that they have the same origin. Simple combinatorics proves it within as many sigmas you wish. Do the math. A median protein on Earth is about 300 amino acids long. That combination has never occurred by chance anywhere in the visible ...


1

My best guess (though I don't know about every mission) is that we have at least seeded our moon with life when we visited it. We have also likely seeded Venus, Mars, and Jupiter with the various probes we have sent to each. As far as I know, we have only done flybys or put probes in orbit for the other planets and moons, so they have likely not been ...


1

We'd analyze the DNA and RNA sequences (assuming they had any) and the coding of amino acids, and compare them with the sequencing and coding on Earth. As far as we are evolutionary from Archaea and Bacteria, we're still a lot closer than an alien life form would be.


1

In addition to already present good answers, I would say that if we found life on Mars on a century there is a noticeably probability of it being made from Earth microbes accidentally taken there by one of our probes or their descendants. Risk of contamination of other planets is addressed by carefully sterilization of probes but no sterilization is complete,...


1

That the Moon and Sun both have about the same angular size is just a coincidence; we (humans) just happen to be around at a time when this is so. Tidal torque is causing the Moon to speed up (which in turn is causing the Earth to slow down and the Moon's distance to increase as it gains orbital velocity), so eventually the Moon's angular size will always be ...


1

As the front image on the wikipedia-page already indicates, a total solar eclipse is not always total. Earth's orbit is slightly elliptic, and so is the Moons orbit around the Earth. Now take the Moon's slight orbital inclination into account and far from all total eclipses are really total. Unlike stated usually. In fact wiki states "On average, the Moon ...


Only top voted, non community-wiki answers of a minimum length are eligible