By Harry Keller
Editor, Science Education
With all of the hoopla over exoplanet discoveries in recent years, it’s a big surprise that this one did not receive more attention. Kepler-444 is a small star, about 25% smaller than ours, and is 11.2 billion years old. According to measurements made by the 600-million dollar Kepler space telescope, it has five rocky planets ranging in size from Mercury to Venus.
The above information is sufficient to generate great excitement. When you realize that the universe is only about 13.6 billion years old, you know that this star and its planets formed in the early days of a very young, only about 2.5 billions years old, universe. Our own star is less than half as old at 4.6 billion years and has an expected lifespan of around 10 billion years.
The second piece of information you require to understand this announcement fully is that the universe began with only three Periodic Table elements: hydrogen, helium, and lithium. Almost all of this matter was hydrogen. A modest amount, maybe 5%, was helium, and a smidgen was lithium. You just cannot make rocky planets from those elements. Before you can build a planet, you must have heavier elements. Silicon is among the most important of those because it’s so prevalent in our crust. Silicon is element 14 and so is in the third row of the Periodic Table, just below carbon. Our planet’s core is mostly iron, element 26, which is even further down in the table.
These heavier elements require some serious stellar processes to make them and major stellar explosions, novas and supernovas, to spread them out into space where they can be attracted to newly forming solar systems and make planets. Simulating this sort of thing is quite difficult. How many stars in the early universe were large enough to nova or supernova? What were their lifespans? How much heavy element material was dispersed? What fraction of each explosion might be expected to accrete in a newly forming solar system?
Until now, the age of a galaxy in which rocky planets of substantial size could form was pure guesswork. Now, we know that rocky planets close to the size of our Earth formed in the first one-fifth of the universe’s life. This information tells us that planets are even more common than previously estimated. More importantly, it suggests that our galaxy, and the universe, have plenty of very old rocky planets, much older than ours. The Milky Way has billions of stars, of which maybe half have planets. Perhaps, one-tenth of those have rocky planets anywhere near the correct distance for liquid water. Even if only one percent are really old, that’s still tens of millions of potential old Earths.
The Kepler-444 planets have very brief orbits, less than ten days. They are too close to their star to support liquid water, way too hot to develop life. This, however, is just the first such discovery. If space were filled with heavy elements, enough to make entire planets, then it should also have been full of water, cyanogen, and other simple chemicals that are important to initiating life on planets. A rocky planet in the right location could have had life emerge at least five billion years before the Earth formed. Given that it took nearly five billion years for life to evolve into today’s variety and complexity, some planet out there could have completed this process before the Earth was born.
This astounding conclusion should give us all pause and should have generated huge headlines.