When Did Life Develop? Kepler-444 and the Future of Terrestrial Planets

 In 2015, astronomers discovered the oldest planets so far: all of them are smaller than Earth and just two billion years younger than the universe itself. But could life even exist on such old planets?

11 billion years ago, the universe wasn’t as we know it today: stars bigger than any in our present-day universe lit up young galaxies still in the making. When our telescopes look back at this period of time, we find a chaotic universe — in crowded regions, three or more galaxies merge into larger ones, and the centers of young galaxies are defined by highly energetic quasars, active supermassive black holes. 

It was also a time when the universe was strewn with much less heavy elements (anything heavier than helium), which are essential for life to develop. This means that there were much fewer terrestrial planets like our Earth — but they did exist. 

Artist's impression of Methuselah orbiting its pulsar and white dwarf; Source: Wikipedia

The oldest planet we know of so far received the name Methuselah, sometimes also referred to as the Genesis planet. It’s 12.7 billion years old, orbiting its dead star in the constellation Scorpius. Being a Jupiter-like gas giant, it’s composed mostly of hydrogen and helium. 

While Methuselah is quite famous for being such an ancient planet, we are much more interested in rocky planets where life might be able to form. And this is where Kepler-444 comes into play. 

Kepler-444 is a very complex system. It consists of three stars, an orange dwarf and two red dwarfs. Red dwarfs are some of the most common stars in the Milky Way because of their long lifetimes; they can have stable lives for trillions of years. Orange dwarfs are intermediate between red dwarfs and Sun-like stars (yellow dwarfs). 

Orange dwarfs are very interesting in the search for extraterrestrial life; their properties are still somewhat similar to the Sun, though they are smaller and dimmer, and therefore they live much longer — for up to 70 billion years! In comparison, the Sun has a stable lifetime of about 10 billion years. 

The greater was the surprise when astronomers announced in 2015 that they found planets orbiting the orange dwarf; and all of them are as ancient as their star. 

A total of five planets are known to orbit Kepler-444. All of them are very small, most about Mars-sized. The largest planet in the system has three-fourth of the Earth’s size. They are also all extremely close to their star; all the five planets would fit comfortably within Mercury’s orbit and there would still be a big empty space. 

But that also has consequences; it means that none of the planets are hospitable to life. However, the system is still very fascinating, not only for its old age. 

Size comparison of the Kepler-444 system with terrestrial bodies of the Solar System; Source: EurekAlert

Finding small planets is tricky; but thanks to asteroseismology, astronomers could estimate the properties of the five planets orbiting Kepler-444, thanks to the Kepler space telescope. In asteroseismology, the oscillations in stars are studied, and brightness variations help astronomers measure properties of any planets that may lie in a star’s orbit.

The fact that such low-mass rocky planets already existed at this early time in the universe opens up the possibility that life could have been able to develop much earlier than anticipated. 

When astronomers measure a planet’s habitability, not only its properties matter. Does the planet orbit a calm star that doesn’t flare much? Does the star live for a long time? Are there any neighboring stars that could go supernova? Is the planet far enough away from the galactic center where radiation levels become toxic? Is the galaxy’s center relatively calm, meaning that there isn’t an active supermassive black hole?

A lot has to be considered.

And in the young universe, chaos prevailed. It was much more crowded with many small galaxies, stars were bigger and lived shorter lives… it may have been a very toxic place for life to develop. 

In 2015, a study suggested that if we’re looking for habitable worlds, the future is the door we have to open. At the time the Earth was born 4.6 billion years ago, the study says, only eight percent of all habitable planets were born so far! 

The remaining planets that will be habitable aren’t even born yet — but why?

A lot has to do with the fact that the universe of the future will, to but it plainly, be boring. There will be fewer but bigger galaxies, less material for supermassive black holes to swallow, the stellar family will be defined by small and long-lived stars, and of course, there will be more heavier elements that facilitate the formation of life on terrestrial planets. 

So, though we know with certainty that Kepler-444 is no habitable haven, life itself may have had a much difficult time of sustaining itself — the existence of terrestrial planets at this time opens the possibility that life existed; but it may as well be long dead. 

Instead, life and civilizations are a thing of the future. And the existence of us humans places us at the very beginning of this epic cosmic journey. 

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