But here on terra firma, life teems in all forms, from microbes up to magnificent beasts like the tiger. Many species are in trouble, however, as man destroys habitat, pollutes soil and air, and kills animals for pleasure or profit.
How much lonelier the Earth will be if we lose our fabulous diversity of creatures. The tiger, for instance, appears to be dwindling in number to only a few thousand in the wild, in five subspecies.
The most numerous is the Royal Bengal tiger, which is thought to number 2,000 to 3,500 animals in India. That compares with an estimated 100,000 tigers there a century or more ago.
In view of this, the UN agency responsible for controlling the trade in endangered species has criticized the Indian government for inadequate protection of its tigers and slowness to act.
The agency, known as CITES, has written to Indian Prime Minister Manmohan Singh requesting an urgent meeting to talk about the tigers' plight.
CITES enforcement officer John Sellar says India needs to be more affective in protecting the striped cats from poachers. He recommended as an example what he called the successful defense of the rare Amur tiger in Far Eastern Russia.
"One of the things we often hold up as an example is what has taken place in the Far East of Russia; and that is the establishment of what is now known as 'Inspection Tiger,' which is a group of law enforcement officers who are devoted to fighting environmental crime," Sellar says.
This dedicated unit is typically staffed by professional hunters and former military men and police officers. Sellar says the unit has had a "great deal of success."
"They have the equipment they need, namely four-by-four vehicles, they have firearms if they need them, they have proper radio communications, they have in-depth training, as regards to intelligence and working with informants, and preparing prosecutions," Sellar says.
He also says there are now indications that the Far East tiger and leopard populations are relatively stable. More should be known about that by June.
Susan Lieberman, the director of the Global Species Program at the international environmental group WWF, says a major survey of the Amur tiger population has just been completed.
It involved sending a thousand people into the vast steppes and woodlands of Far Eastern Russia to look for clues to the population, which is thought to number as few as 400 animals.
"I'm optimistic," Lieberman says. "I don't yet have the numbers on the Amur tiger, but I am hopeful that I am going to be able to report that all the hard work has paid off, and that [its numbers] are stable or increasing."
But with so much attention going to what high-profile animals like tigers, whales and elephants, there is a risk that other, less impressive, species will be forgotten.
"There is a danger here that we are focusing too much on the tiger," Sellar says. "We are also seeing where shipments [of illegal animal parts] are being intercepted before they can leave India, or when they arrive in Tibet [on their way to China], and in several of these shipments we have had leopard skins involved, and I remember one shipment in particular where 120 leopard skins were seized; so the great danger in focusing on the tiger is that we ignore what might be happening with other species."