Are there planets that are better for life than Earth?

Philosophers have long debated whether Earth is the best of all worlds. More powerful telescopes are finally giving us a better chance of answering this question, writes astronomer Chris Impey.

The following is an extract from our Lost in Space-Time newsletter. Each month, we hand over the keyboard to a physicist or two to tell you about fascinating ideas from their corner of the universe.

A super-Earth orbiting its red-dwarf


Over 300 years ago, the German polymath Gottfried Wilhelm Leibniz argued that ours was the best of all possible worlds. The word “world” comes from Old English, originally meaning Earth, and later extending to the physical world in the broadest sense, or the universe. Leibniz was trying to address the thorny theological issue of the existence of evil. He agreed with many other philosophers that even an all-powerful God must be constrained by the laws of geometry and logic. He believed it would be impossible for God to create a world containing logical contradictions and therefore could not avoid it containing evil. While our world is not flawless, Leibniz argued that it must be the best possible world God could have created.

Not everyone agreed. The French writer and philosopher Voltaire lampooned this idea in Candide, a short novel published in 1759. The Lisbon earthquake had recently killed 30,000 people, and so many were asking how God could have allowed it to happen. Voltaire parodied Leibniz as someone clinging to optimism even as catastrophes rained down all around him.

Neither Leibniz’s nor Voltaire’s point of view was vindicated in their lifetimes – they had very little evidence about what another world might even be like. And right up until the mid-1990s we hadn’t identified a single planet outside of our solar system. Yet, today, discovering such exoplanets has become routine. Since the first detection in 1992, more than 5200 have been found, along with 9000 further candidates awaiting confirmation. So how does our world, Earth, stack up?

There is huge variety among the exoplanets that we know of but a good place to start in attempting to settle whether our planet is the best of all worlds is to identify which of them are like Earth. Our planet is still the only one that we know of to be home to life, and while we’ve yet to find a true Earth 2.0 that shares most of Earth’s properties, we’ve found hundreds of exoplanets close to Earth in both size and mass, and that orbit a star similar to our sun. Most importantly, they are in the habitable zones of their stars, the region with the right temperature for liquid water to exist on the surface.

However, if we relax the requirement that the planet should orbit a star like our sun, there are several planets that are very near twins of Earth – within 10 per cent of Earth’s mass and size and have orbits close to a year. The main difference is that they orbit red dwarf stars.

Most stars are far less massive than the sun. The lowest-mass stars are red dwarfs. They live for hundreds of billions of years, and there are a hundred red dwarfs for every star like the sun. Twenty of the 30 stars nearest to the sun are red dwarfs, including the closest, Proxima Centauri. Red dwarfs have slender habitable zones that are much closer to the star than the habitable zones of sun-like stars. Nevertheless, the vast number of red dwarfs means that their habitable “real estate” is greater than the habitable real estate around stars like the sun. Other factors weigh against them as places where life might develop, such as the X-rays they emit that would irradiate any planet nearby. The habitability of red dwarf planets is hotly debated, but their surface conditions may allow life. The number of them plus their long lifetimes mean that finding life on red dwarf planets may be far more likely than finding life on the planets of solar-type stars.

But if Earth is really the best of all worlds, then it should be the best of them all, not just the Earth 2.0s. As the exoplanet discoveries have piled up, one surprise has been the abundance of planets larger and more massive than Earth. So-called super-Earths lie between the size of our planet and ice giants like Uranus and Neptune. There are no super-Earths in our solar system, but elsewhere in the Milky Way they are common. A third of known exoplanets fall into this category and the nearest is just 6 light years away.

There may be as many as tens of billions of habitable super-Earths in the Milky Way alone, most of which will orbit red dwarfs. To top it off, super-Earths may be super-habitable – in other words, even better suited for life than Earth and so able to support more biodiverse ecosystems. There is a working set of criteria that may make a planet super-habitable, such as having an average temperature of 25°C (77°F) and an atmosphere thicker than Earth’s but with more oxygen. These super-Earths would be super-sheltered environments supercharged for biology. And in 2020 two dozen such exoplanets were identified.

However, we are yet to detect life on any exoplanets, super-Earth or not. The way we search for life is to look for biosignatures, spectral imprints of biology in a planet’s atmosphere. NASA’s James Webb Space Telescope was designed before exoplanets were discovered so it’s not optimised for exoplanet observations, but it will target several super-Earths with massive oceans in its first few years of operation. Exoplanet atmospheres will also be studied by a new generation of giant, ground-based telescopes: the Extremely Large Telescope, the Thirty Meter Telescope and the Giant Magellan Telescope. We know the ingredients for life are out there, but habitable does not mean inhabited. It is still possible that life on Earth is a unique accident.

Even though we haven’t found an Earth clone yet, it almost certainly exists. The exoplanets detected so far are nearly all within 4000 light years. That covers just 1 per cent of the Milky Way, which is 100,000 light years across. With tens of billions of terrestrial planets in the galaxy, probability and common sense say that many will be like Earth. Multiply by a hundred billion galaxies in the observable universe, and the projection becomes ten billion billion (1019) terrestrial planets around sun-like stars and fifty times more around red dwarfs. The nearest exact twin might be on the far side of the galaxy or in another galaxy millions of light years away. Finally, if we embrace the speculative multiverse theory, where our entire observable universe is one region in a potentially infinite ensemble of universes, not only are there many exact copies of Earth, but there are many exact copies of you and me!

Leibniz claimed that we live in the best of all possible worlds. We can look for Earth cousins and Earth clones, but is our planet really the gold standard when it comes to defining habitability? Maybe you like your parents – you might even think they are the best. But to objectively judge them, you would have to find out about all the other parents. It’s statistically unlikely that your parents are the best of all possible parents. The same goes for planets and habitability.

Rather than being the best of all possible worlds for life, Earth might be inferior. Life is found almost everywhere there is water, but it has trouble thriving in arid deserts, frigid polar regions and nutrient-poor oceans. Far from being inevitable, the habitability of Earth has always been “touch and go”. Earth has suffered extreme conditions and catastrophic changes in the distant past. For 4 billion years, the climate has veered from ocean-boiling heat to planet-wide deep freeze. Simulations suggest the long-term habitability of our planet was not inevitable but was contingent. It could have gone either way. We’re literally lucky to be alive.

Reference: New Scientist. Read original article here.

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