OASIS EARTH
As the search for Earth-like planets intensifies, we’re realising just how lucky our own planet may have been in overcoming the overwhelming number of barriers to habitability.
Our hospitable world should not exist. If Earth had not collided with another planet, it would have been barren today – and that is only one of many ifs. As scientists zero in on alien worlds, they are realising that Earth may be the luckiest planet in the entire universe.
EARTH FORMS
1. Supernova provided heat
2. Saturn stopped the potential devastation caused by Jupiter
3. Earth ended up in a narrow ‘Goldilocks’ zone
LIFE BECOMES POSSIBLE
4. Collision provided Earth with the perfect size
5. The Moon stabilises our seasons
6. Active geology is Earth’s thermostat
LIFE IS PROTECTED
7. Free oxygen paved the way for sophisticated life
8. Jupiter prevents meteor strikes
9. The magnetic field was saved at the last minute
High above the planet’s dark side, a probe silently passes with its sensors aimed at the dark landscape 1000km below. It is collecting data about the atmosphere’s composition, looking for any light sources on the surface, and listening for radio noise, trying to find out whether there is life on the planet.
The answer is clearly yes, as the planet that the probe is flying above (back on 8 December 1990) is our own world, the Earth. The probe looking for life on our planet is called Galileo, and this part of its mission is not as crazy as it might sound. Based on Galileo’s observations, astronomer Carl Sagan and his colleagues are able to conclude that an inhabitable planet will emit certain evidence that technological equipment can detect from far away.
Five years later, in 1995, astronomers discover the first exoplanet orbiting a star that is reminiscent of the Sun. Suddenly, Galileo’s peculiar mission becomes highly relevant. The planet, 51 Pegasi b, is a red-hot Jupiter-like giant which orbits its star very closely, so it is almost certainly both lifeless and uninhabitable. But the discovery triggers a new space race – the search for inhabitable worlds in other solar systems and hence the answer to one of the biggest questions in science: did life find other oases in the universe, or is life on Earth just a highly unlikely stroke of luck?
Searching for another Earth
By 2019, the number of confirmed planets outside our Solar System had reached 4071 (and rising), distributed across 3043 star systems, of which 659 have more than one planet. But although scientists now have thousands of planets to choose from, the vast majority of them can be immediately disqualified as Earth-like worlds.
Scientists quickly discount gas giants, but most rocky planets do not qualify either, because their orbits are either too close to or too far from their star, or the planets differ too much in size from our world, or have the wrong type of star. As new exoplanets are discovered and astronomers learn more about some of the ones they already know, the list of the most probable candidates is updated. The one currently considered to come closest to matching Earth is the exoplanet of K04878.01. It has an ESI (Earth Similarity Index) value of 0.98, where Earth is defined as 1. The planet K04878.01 is almost the same size as Earth and is located in an orbit which means that it receives only 3% more radiation than Earth from its star, which is also very similar to the Sun.
However, other studies involving factors that are not included in the ESI value indicate that K04878.01 has an atmosphere with 10 times higher pressure than Earth, making it unlikely that life as we know it can thrive. Another candidate, the TRAPPIST-1e planet, has almost no atmosphere. Though these two planets are reminiscent of Earth in many ways, they have apparently developed differently from our own.
For life to have time to originate and evolve, it’s crucial that both the planet’s orbit and the star’s radiation have remained fairly constant over billions of years. Furthermore a planet’s habitability is not only measured by external conditions, but also in the planet’s interior. An active underground is vital for life on Earth, and that is probably also the case on other worlds. Plate tectonics stabilise conditions on the planet’s surface by adjusting the atmosphere’s content of greenhouse gases. The carbon
cycle adds CO2 to the atmosphere in case of volcanic eruptions, while old crust plate carries carbon into the planet’s interior again.
In 2015 Japanese scientists headed by geologist Takehiro Miyagoshi simulated the interiors of rocky planets that are bigger than Earth. It turned out that the crust becomes too thick and the pressure in the planet’s interior too high for plate tectonics to work. Other analyses have shown that plate tectonics are difficult to trigger even on rocky planets that combine the right size with the right temperatures and composition. Scientists do not even know for certain how the process was initiated on Earth.
Super telescope to find oxygen
Today, astronomers can observe only exoplanets’ diameters and the distance to their stars, but new telescopes will provide them with much more detailed information. One of those will be the Extremely Large Telescope (ELT), which will be the world’s biggest when it is aimed at the universe for the first time in 2025. The telescope will be a part of Europe’s southern observatory in the Atacama Desert of Chile, and will have a diameter of 39 metres. The diameter of the biggest Earth-based telescopes currently are around 10 metres, and the ELT will produce images that are 16 times sharper than those coming from the Hubble telescope.
One of the telescope’s central tasks will be to find out how many Earth-like systems at different development stages exist in the galactic neighbourhood of our Solar
System. That will involve analysing protoplanetary discs around newborn stars, with astronomers particularly interested in the distribution of elements, molecules and mass. This information can fill in holes in the history of Earth’s birth as well, and reveal whether our Solar System’s childhood was normal or special. The ELT should also provide data about the mass of exoplanets. Knowing the planets’ diameters from the space telescopes, astronomers will also be able to calculate their density and consequently provide us with a qualified guess of their chemical make-up. With this knowledge, scientists might be able to find out if there is a chance that the planet has a protective magnetic field and active geology, including plate tectonics like Earth.
Astronomers also hope that the ELT will prove to be so powerful that the telescope can analyse the atmospheres of many of the known Earth-like planets – not least the top candidates of K04878.01 and TRAPPIST-1e. The scientists would particularly like to find free oxygen. The gas reacts very easily with other compounds, so it would only remain in the atmosphere of a rocky planet if oxygen is continuously produced. And that probably only happens where there are complex biochemical processes like Earth’s photosynthesis to provide a basis for higher forms of life. Over the next 10 years, the ELT and other new instruments will bring us closer to an answer for whether life has developed in other places, or if Earth is just the luckiest planet in the universe.