There has only ever been one Earth, there is only one Earth, and there will only ever be, one Earth.
Now, that's not to say there are no other planets in the galaxy that can support life, because there almost certainly are. No, what I'm trying to say comes from two different perspectives. The first, is that although other planets could support life, it is almost completely unlikely that any of them would sport flora and fauna like Earth's. Some of them may even look similar on the surface, but they'll have uniquely complex chemistry that almost certainly couldn't be reproduced elsewhere on the universe, Earth included. The second perspective is that even if such a planet exists, we'll never get there.
Earth is an incomprehensibly complex combination of interacting physical, chemical, and biological systems. These interactions range in scale from the atomic, to the planetary.
Three billion years ago, a lightning strike provided the heat energy needed for the synthesis of the first amino acids and ribonucleic acids, the building blocks of life. Primordial elemental soups created from volcanic eruptions and other geological and meteorological processes allowed for the eventual spontaneous formation of lipid bilayers, and with these ingredients in place, the first cells were born. These cells almost certainly were anaerobic, and depended on sources other than oxygen for their survival. They were also microscopic, invisible to the naked eye.
Eventually, a few of these cells randomly developed a mutation that allowed them to collect energy from sunlight, through specialized pigment molecules. A few of THESE cells randomly mutated the ability to store this energy in the form of chemical bonds. Evolution favoured these mutations because it led to increased survival. So many of these cells developed that their energy harvesting began to change the composition of the atmosphere. Then, a few OTHER cells mutated physically, enabling them to engulf, or "hunt", these energy-producing bacteria, and digest them for nutrients and energy. And this is what life was for several billion years, until one day, something amazing happened.
A large cell tried to engulf and digest a smaller energy-producing cell, but instead of being digested, it kind of just, stayed there, and the two entered a symbiotic relationship. Now, the larger predatory cell had a miniature factory living inside it to produce energy, what we now call the "mitochondria". Pretty useful if you ask me. But despite this amazing, chance event, all life on earth was STILL microscopic, and completely invisible to the naked eye.
Until around 500 million years ago. Then, for reasons scientists are still unsure of to this day, in an event called the "Cambrian explosion", life suddenly went from simple, single cells, to vastly complex organisms composed of thousands, millions, billions, of SPECIALIZED cells. They swam in the oceans, and then, some of them, by chance, mutated little appendages that ended up being pretty useful for traveling on land. And once they got on land, I'm sure they were as amazed as I would have been, to find that the entire planet, the soil, the air, and everything else in between, had been completely and utterly transformed by plants. And some of them liked the taste of plants, so they stayed up there. And some of them liked the taste of the others that ate the plants, so they stayed up there too. And millions and millions and millions of more chance mutations and random happenings, until the Earth is in the state it is in today.
Why am I telling you the history of the Earth? Well, because it's amazing of course. And I want to try and convince you why studying the environment is the most noble field a human can undertake. It's because, with all of these amazing coincidences in tow, studying the environment is the only practical field that transcends the needs of the human being. It transcends the needs of human society, because it takes into consideration the needs of the entire biosphere of planet Earth, and lasting effects on the planet that will linger long after we disappear. It is the only field that even attempts to pay homage to the billions of years of amazing physical, chemical and biological change that allowed for the miracle of human existence.
Yes, the space program is cool, and a lot of people will say, "oh, we don't need to worry about Earth, because we'll find a new planet to live on in the future anyways!" But think about that. Do you want a future in which grandparents have to tell their grandkids that humans are the only species in the known universe that have been able to destroy an entire planet? Because I sure don't. I want a future in which grandparents want to tell their grandkids about how the noble scientists of the world realized what they were doing before it was too late, did their best to fix the problems they had caused, and then left to let the Earth heal from its wounds.