It's quite possible there's as much lightning on Venus as on Earth.

I do comparative studies of climate evolution, and the interactions between planetary atmosphere and surfaces and their radiation environment, and try to understand the environmental factors that can affect a planet's habitability and how they change over time.

I'd been politically active ever since my parents wheeled me in a stroller in a 'ban the bomb' march in Boston in 1963.

There is a history of thinking about space science from an environmental ethics perspective. And part of what I want to do is turn that back and use that experience to see if it reflects how we think about the Earth.

What if life is not carbon-based? Can life exist as a gas or a plasma? Could planets or stars in some sense be alive? What about an interstellar cloud? Could life exist on such a small or large scale, or move so fast or so slowly that we wouldn't recognize it? Could you have an intelligent virus?

There is a real danger of unintended consequences, of encouraging people to give up. Pessimism, if it becomes a habit, can reinforce a narrative of unstoppable decline. If there is nothing we can do, that releases us from our obligations.

One of the weird things about modern physics is that we do find there are apparently these other dimensions that we don't directly experience that explain some aspects of the overall geometry and reality of our universe.

The planet Mars - crimson and bright, filling our telescopes with vague intimations of almost-familiar landforms - has long formed a celestial tabula rasa on which we have inscribed our planetological theories, utopian fantasies, and fears of alien invasion or ecological ruin.

It will be a long time, if ever, before we get to study Earth-like planets orbiting around other stars, so really, the study of Venus and Mars is the best opportunity that we have, and can imagine having anytime in the future, to understand the evolution of Earth-like planets.

I think that an advanced planetary civilization will modify their own planets to be more stable, to prevent asteroid impacts and dangerous climate fluctuations.

We need to have a vision of the world we want to create so that we can see ourselves as collaborators with future generations in the project of shaping it.

When I first went to college, I went into physics, and my goal was to help perfect nuclear fusion so I could solve the energy crisis and global warming. I probably would have done it, too, if I'd stuck to it.

Fixing global warming is more important than astronomy.

There are other planets besides the Earth and Mars. I'd like to remind you that studying Venus is vital to understanding life elsewhere.

In order to have a decent chance to be a communicating species, you would have to learn to think and plan and act over time scales of a century or a millennium.

We have to learn to become a new kind of entity on this world that has the maturity and the awareness to handle being a global species with the power to change our planet and use that power in a way that is conducive to the kind of global society we want to have.

Now, humans have become a dominant force of planetary change and, thus, we may have entered an eon of post-biological evolution in which cognitive systems have gained a powerful influence on the planet.

Responsible global behaviour is ultimately an act of self-preservation of, by, and for the global beast that modern technological humanity has become.

There was a long history of people believing there was life on Venus. It was about the same size as Earth. It had clouds. It was commonly believed it was tropical - wet, hot and steamy.

We're pretty sure there's plenty of organic material on Pluto. The atmosphere is largely methane, and in sunlight, methane builds organic molecules. We see reddish stuff on the surface that we think is organic material.

We've almost been wiped out as a species many times, going back millions of years, and we've survived by reinventing ourselves and enlarging our circles of awareness, inventing new technologies and social structures.

What I'm interested in is the conversations going on about the Anthropocene and what it means to view ourselves as a part of Earth's geological history.

We have all this very clever technology and all these abilities to manipulate the world in all these ways, yet we are faced with the very real question of whether we can be sustainable on this planet - whether or not, in fact, we can endure.

Ever since the environmental movement was sparked by photos of the whole Earth taken by astronauts onboard Apollo Lunar Modules, I've seen planetary exploration as an extension of a reverence and care for Earth.

Time and time again, our species has escaped existential threats by reinventing ourselves, finding new skills not coded in our genes to survive new challenges not previously encountered.

What I wonder most about the Anthropocene is not when did it start - but when, and how, will it end? Will it end? Or is it possible that our own growing awareness of our role on Earth can itself play a pivotal role in shaping the outcome toward one that we would desire?

As a young planet, Venus was losing hydrogen rapidly to space. The oceans boiled off, and after some period of time, perhaps 600 million years, there was no surface water.

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.

I'm an astrobiologist, and I come from a planetary science background, so in a very broad sense, I study the evolution of planetary environments.

Radiation is one of the important factors in evolution. It causes mutation, and some level of mutation is actually good for evolution.

Mars does not have an atmosphere and does not have a magnetic field today, so the planet doesn't have the protection from radiation that our atmosphere and magnetic fields provide us on Earth.

You cannot study other planets without referring to Earth and without applying the techniques and the insights of Earth science. And you cannot really do a good job understanding the Earth without the insights from planetary exploration.

I will defend the NASA Earth Science Division with everything I've got.

I think Pluto has to be considered among the places in the solar system that are possible homes for life.

What we should really be thinking about is what it would look like for a truly intelligent technological species to be interacting with their planet's atmosphere.

We definitely don't want to go through another Ice Age or another natural cycle of global warming. Both happen over a long period of time. It would be disastrous for our civilization, and not just for us but many other species.

Titan has rivers and lakes of liquid methane and ethane, methane weather systems of clouds and storms that mirror Earth's hydrologic cycle, and seasonal cycles that rival Earth's in complexity.

If you were on the surface of Venus, assuming you could see the Sun, which, you know, would be hard because it's so cloudy there, but the Sun would actually rise in the west and set in the east. And, it would do so very, very slowly, because the planet rotates incredibly slowly.

It turns out one of my dad's best friends was Carl Sagan when I was little. They were both Harvard professors.

I do a lot of work with NASA and am involved in research projects studying planetary evolution, Earth-like planets, and potential conditions for life elsewhere.

We're going to get off fossil fuels, no question. We may not do it quickly enough to avoid some pain, and I'm quite worried about that. But by the 22nd century, there's no way we'll be on fossil fuels.

As Earth's climate changes, we can expect more destructive hurricanes. As sea level and surface temperatures rise, more solar energy is trapped in the atmosphere, revving up the hydrological cycle of evaporation and precipitation and sometimes manifesting in terrifying storms.

Through space-based climate studies, my colleagues and I have learned that a stable and comfortable climate is not something to take for granted.

Thinking about the new epoch - often called the Anthropocene, or the age of humanity - challenges us to look at ourselves in the mirror of deep time, measured not in centuries or even in millennia, but over millions and billions of years.

Humanity has at least a dim, and growing, cognisance of the effects of its presence on this planet. The possibility that we might integrate that awareness into how we interface with the Earth system is one that should give us hope.

The mature Anthropocene begins when we acquire the ability to live sustainably and become a lasting presence on this world.

Once we become a multiplanet species, our chances to live long and prosper will take a huge leap skyward.

The future peopling of Mars is much more than a scientific endeavor. It is a step of historic and spiritual importance for the human race.

Mars does not belong to 'America,' nor to Earth, nor to human beings.

My high-school friends and I felt part of a community of smart, forward-looking space and technology freaks.