When philosophers talk about the world, they don't refer to Earth. They usually mean the whole of existence, everything that can be experienced, material or immaterial.
Philosophy, the love of wisdom, has a long history of studying fundamental questions such as "What can we know about the world?" or "Can the world be known as it is?" Over the millennia, many branches have appeared; metaphysics (study of reality), ethics (study of morality), natural philosophy (astronomy, medicine, physics), etc.
During the Age of Enlightenment, natural philosophy became science and took on its own life. The scientific method has since become the de facto standard of objective truth. Science has replaced philosophy and is the only serious pursuit when it comes to knowing about the world, or so many scientists have disdainfully declared—and they have science's tremendous instrumental success to back their claim.
But is that really so? What if the world is bigger than what science can fathom? What if it is so vast that it doesn't exist?
Why the World Does Not Exist, Markus Gabriel
Markus Gabriel is a philosopher and author and holds the Chair for Epistemology (theory of knowledge) at the University of Bonn. He is one of the new "rockstars" of philosophy if that's even a thing.
Calling himself a new realist, he has been vocal about his belief that science doesn't have a monopoly on truth. "Physics" he says, "is blind to everything that it does not investigate." The universe studied by "hard" sciences is just one of many aspects of reality. Sherlock Holmes may be a fictive character, but my thoughts about him are real. They are part of reality. But just from a different domain of reality.
So why does the world not exist? Well, it is a bit of a philosophical stunt.
Fields of sense are domains in which something appears in a certain way. For example, there is a field of sense of empirically observable physical objects. Physics has access to this field. But there is also a field of sense of conscious experiences. Physics doesn't seem to have access to that one.
Gabriel defines existence as the circumstance that something appears in a field of sense. For a pen to physically exist, it has to appear in the field of sense of empirically observable physical objects. Existence, however, is not a property of the object appearing. It is a property of the field of sense itself, the property that the existing object appears in it.
Now, if we state that the world is the entirety of reality, we may define it as the domain of all domains. As such, my thoughts about Sherlock Holmes are part of the world because there exists a domain—the domain of conscious experiences—in which my thoughts appear.
But for the world itself to exist, there would need to be a field of sense in which it appears. If there were such a field, it would already be included in the set of all domains and therefore contained within the world (the domain of all domains). We end up with a self-embedding infinite regress.
world = {field, world} => world = {field, {field, world}} => ... => world = {field, {field, {field, ....}}, world}
Because of the infinite regress, there cannot be a field of sense that includes the domain of all domains, and therefore, the world does not exist. Q.E.D.
As I said, a bit of a stunt. But that's not really what this book is about.
It is about our relationship with the truth. It warns us against the tyranny of objectivity and the arrogance of science in its belief that it has a monopoly on epistemic and ontological truths. The world is much bigger than the universe. And that's a good thing. Our existences cannot and shouldn't be reduced to lucky circumstances of particles randomly shattered as a result of entropy.
Philosophy is far from being obsolete, and no, science hasn't replaced it. Not even close. Pondering the difficult questions about our human nature and the meaning of our lives is as relevant as it has always been. And there is more to our relationship with our own nature than reducible principles. Our world is not reducible because such a world does not exist.