When I write or give public talks about exoplanets - alien worlds orbiting other stars - the question I am most often asked is, "When will we find another Earth? "
It's a good question. As we've discovered, the cosmos is filled with a huge number of very different worlds, and it's natural to wonder if Earth 2.0 exists, or if they're all really what's called "alien".
There are hundreds of billions of stars in our Milky Way galaxy. Given that local planets are as common as stars according to the results of research, there could be trillions of planets in our galaxy alone. Of course, in reality this does not mean that every star has a planet, rather some have none at all, while others have their own huge solar systems.
Exoplanets come in a wide variety of sizes, some of them incredibly bizarre: planets the size of Jupiter but passing so close to the surface of their star that the boiling heat of their atmospheres strips them of air, turning them into megacometas. Worlds larger than Earth but smaller than Neptune, which are the most common type of exoplanets. Strange planets abound.
And yes, there are many Earth-sized worlds on the list. Of the roughly 5,500 exoplanets discovered to date, about 100 are close in size to our home planet. But it's not just the size of the planet that matters.
If you're looking for an exact replica of, say, Earth's size, mass and composition, with habitable air and potable water, the chances of discovering such a planet are slim.
Planet formation is a stochastic process; many random variables affect how a planet forms and evolves over time.
Even small changes can lead to dramatically different planetary evolution, and many of these variables influence each other.
For example, a planet slightly warmer than Earth - orbiting a hotter star or closer to a cooler star - could create a greenhouse effect that would cause its oceans to boil and its desiccated surface to heat to the melting point of lead.
At the mercy of Venus
As we can see, even a relatively small change in atmospheric carbon dioxide can have a significant impact on the environment. It is unlikely to make the Earth uninhabitable, but the changes are happening fast enough to be alarming.
In addition,
Earth was not always like the Earth we are used to. For two billion years, our planet had no habitable atmosphere, and it was only through global environmental changes that oxygen became available.
It is also possible that our planet has experienced at least one period of complete glaciation, a hypothetical era called "Snowball Earth." While this idea is controversial, it is clear that for long periods of time the Earth was not the beautiful home we know now.
Moreover, there is a growing discourse in the scientific community that Mars was once more habitable than its current thin atmosphere and arid surface would suggest. Perhaps Mars was more Earth-like a few billion years ago. Even
Venus may have once been habitable.
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The very concept of "habitability" is more vague than one might think. There are icy satellites outside the solar system, with oceans lurking beneath their frozen surfaces, as well as other potentially life-friendly factors. Eternal darkness at temperatures just above zero doesn't sound like Eden, but it could be a paradise for the life that evolved there.
It is premature to say that we have found planets similar to our Earth. First, we don't know enough about the atmospheres and chemical composition of these worlds to say with certainty whether they are Earth-like. Of the 100 planets mentioned, only three have roughly Earth-like masses and receive about the same amount of light and heat from their star.
Third, it's small magnitude. Our current detection methods do a better job of finding large, hot planets. Small, faintly glowing planets like ours are much harder to detect.
But methods are constantly improving, and we may not have to wait long for astronomers to announce that they have found an Earth analog. And when that happens, what then?
It's not like we can go there. We don't have the Enterprise to board and transport to the nearest Earth 2.0, and without FTL travel, the trip won't be a long one. Even the fastest spaceship will take millennia to reach the nearest star system, Proxima Centauri.
So many science fiction movies tell us about the need to leave the Earth that it has become commonplace. However, this is more science fiction than science: humanity is increasing its population by more than 70 million people a year. To cope with this increase, 2,000 SpaceX ships need to be launched every day, even if we don't take into account the length of flights. Reducing the demographic crisis through interstellar emigration is not the answer.
Establishing a settlement is no easy task either. We don't even know how to do it in low Earth orbit,
on the Moon or on Mars. We're still a long, long way from setting up store on the "other Earth," even if we could easily get there.
When people ask me about Earth 2.0, the question inevitably arises as to whether we can get there and live there. In a nutshell, no. So why look for it if we can't go there?
We're looking because we want to know
Scientifically, we look for other planets because we want to understand how they form, how conditions change their physical properties, how they differ from or mirror those of our
Solar System.
But from a human perspective, we long to see another pale blue dot somewhere in the depths of space. To know that somewhere, sometime, conditions were the same as on Earth. Or similar.
Certainly the mere realization that there is a similar planet somewhere will profoundly change the way we think
about the universe and our place in it. Such a discovery would emphasize the contrasting nature of our Earth and help us understand it better.
It can also help answer the most important question mankind has ever had: How did we get here? For millennia, this question has been the source of speculation, myths, beliefs and philosophies. With a distant blue-white world hovering in the eyepiece of a telescope, it becomes scientific. Cognizable.
If we find another habitable world, we can dare to open the door to the next Big Question: Are we alone?