No other planet in the solar system is a suitable home for human beings; it's this world or nothing. That's a very powerful perception.
If you're sending a message to extraterrestrials, what you want to send is what's special about us and our planet - what is unusual. Now, that's not basic chemistry or mineralogy; it's pretty much the cultural stuff and the consequences of evolution.
Stars, of course, are too hot to support life, so wherever life might exist in the universe, it has to be on planets or moons that are warmed, but not incinerated, by the stars they travel around.
By monitoring auroral activity on exoplanets, we may be able to infer the presence of water on or within an exoplanet.
We think that life develops spontaneously on Earth, so it must be possible for life to develop on suitable planets elsewhere in the universe. But we don't know the probability that a planet develops life.
The future of humanity is going to bifurcate in two directions: Either it's going to become multiplanetary, or it's going to remain confined to one planet and eventually there's going to be an extinction event.
Exploring and colonizing Mars can bring us new scientific understanding of climate change, of how planet-wide processes can make a warm and wet world into a barren landscape. By exploring and understanding Mars, we may gain key insights into the past and future of our own world.
If we do discover more than one type of life on Earth, we can be fairly certain that the universe is teeming with it, for it would be inconceivable that life started twice here but never on all the other earth-like planets.
When we discover New Earth - a planet we could call home - the question of the 'plurality of worlds' will come front and center, reminding us yet again that we are not the center of the universe.
For many centuries, humans have speculated that there might be planetary systems around other stars and that there could be extraterrestrial life there and even intelligent being. However, those were simply speculations, and now we have evidence for the first part of these ideas.
Extraterrestrial intelligence could have sent DNA-seed packets out through space to plant life on hospitable planets such as earth.
A 'serious' scientist in 1992 or 1993 had to admit the possibility that planets were really rare, that most stars might not have planets. We've gone from there to here - where most stars have planets.
So far as we know, Earth is the only planet which supports life, and it is the only planet on which we can survive. Our bodies and our minds are fashioned by it. Our hearts resonate with it. There will be little joy for the human spirit if we destroy the natural fabric of Earth with nothing left to do but go shopping. When we imagine the world a century from now, when we look our great grandchildren in the eye and see them smiling back at us because they know we cared for them, we smile too!
Many different planets are many different distances from their host star; we find ourselves at this distance because if we were closer or farther away, the temperature would be hotter or colder, eliminating liquid water, an essential ingredient for our survival.
Earth is going to lose its oceans in the future, just as Venus did in the past. How long planets retain their oceans is a function of distance from the sun, all other things being equal.
Mars once was wet and fertile. It's now bone dry. Something bad happened on Mars. I want to know what happened on Mars so that we may prevent it from happening here on Earth.
Now we notice that Mars doesn't have any atmosphere either and won't support life. In spite of the fact that it turns green and red and purple with the seasons, it doesn't support life.
Do you ever wonder if--well, if there are people living on the third planet?' 'The third planet is incapable of supporting life,' stated the husband patiently. 'Our scientists have said there's far too much oxygen in their atmosphere.
Mars is the only place in the solar system where it's possible for life to become multi-planetarian.
The story seems to be that almost every star has a planetary system... and, also, the definition of 'habitable zone' has expanded. In our system, it used to be that only Mars and Earth were potentially habitable. Now we've got an ocean on Europa... Titan.
Mars was this water-based planet, and we know there was stable water on the surface for a long time, which is critical for life having a chance to develop.
Venus has a runaway greenhouse effect. I kind of want to know what happened there because we're twirling knobs here on Earth without knowing the consequences of it. Mars once had running water. It's bone dry today. Something bad happened there as well.
Most certainly, some planets are not inhabited, but others are, and among these there must exist life under all conditions and phases of development.
The usual metric for whether a planet is habitable or not is to ascertain whether liquid water could exist on its surface. Most worlds will either be too cold, too hot or of a type (like Jupiter) that may have no solid surface and be swaddled in noxious gases.
Among the plausible niches for extraterrestrial life in our solar system, the clouds of Venus are among the most accessible and the least well explained.
