What came first: galaxies or planets? The answer has always been galaxies, but new research is changing that idea, reports say universetoday.com.
Could habitable planets really have formed before galaxies existed?
In the immediate aftermath of the Big Bang, there were no heavy elements. There was only hydrogen, which made up about 75% of the mass, and helium, which made up the remaining 25%. (There were probably also traces of lithium, even beryllium.) There was nothing heavier, meaning there was nothing from which rocky planets could form. After a few hundred million years, the first stars and galaxies emerged.
As successive generations of stars lived and died, they forged heavier elements and spread them across the universe. Only then could rocky planets, and by extension, habitable planets form. That has been an axiom in astronomy.
However, new research yet to be published suggests that habitable worlds may have formed in the early stages of the Cosmic Dawn, before the formation of galaxies. The title is “Habitable worlds formed during the Cosmic Dawn”, and is available on the prepress site arxiv.org. The lead author is Daniel Whalen of the Institute of Cosmology and Gravitation at the University of Portsmouth in Great Britain.
The research revolves around primordial supernovae, the first stars in the universe to explode. These massive stars lived fast and died young during cataclysmic explosions. They peaked around redshift 20 when Population III stars, which were extremely massive, exploded as pair-instability supernovae. Simulations show that these stars formed in dark matter halos, where temperatures allowed large amounts of molecular hydrogen to accumulate.
According to Whalen and his co-researchers, when these stars exploded, low-mass stars were formed in the aftermath. Planetesimals formed around these stars, leading to the formation of potentially habitable, rocky worlds. This all happened before the first galaxies formed. These results are based on simulations that the research team conducted together with Enzo.
It starts with a star formation with a mass of about 200 solar masses. It only lives for about 2.6 million years before exploding as a PI supernova. The explosion enriches the supernovae to a high metallicity. In the aftermath, hydrostatic instabilities cause a dense core to form about 3 million years later, with a mass of 35 solar masses.
“All known conditions for planet formation in this core are met: dust growth, dust enhancement in a dead zone, the onset of flow instability and the conversion of dust into planetesimals,” the authors explain.
In this respect, this study differs from previous studies. Because the PI supernova explodes and creates gas with a high metal content, the gas cools faster. This allows the next star to form faster, and thus planetesimals and planets.
Ultimately, a protostar with a mass of 0.3 solar masses formed. Then planetesimals formed between 0.46 and 1.66 AU from their star. Life needs water, and the researchers’ simulations also showed that the young solar system contained an amount of water comparable to that of our own solar system.
Planetesimals formed in the circumstellar disk around the low-mass star, and over time they combined to form planets. Because the original primordial supernova created elements like carbon, oxygen and iron, all the necessary ingredients were likely present to form rocky planets and even life.
The remarkable thing is that this could have happened before the first galaxies formed. If this is true, it would change our understanding of the universe and of life. However, this is only one simulation. How can observations support this?
“These planets could be detected in future exoplanet searches as extinct worlds around old, metal-poor stars in the Milky Way,” Whelan and his fellow researchers write in their paper.
According to the authors, if conditions were just right, rocky planets could have formed even earlier than their simulations indicate. If that is true, it changes the entire course of events in the evolution of the universe.
However, this is just one study. And it depends on primordial supernovae. Did they even exist? At least there is some evidence that they did.
Clearly, attempts to observe primordial supernovae are extremely difficult. They took place so long ago that they are extremely distant and vague. With current technology that is probably impossible.
There is also a lot of uncertainty about the Population III stars that were the precursors of primordial supernovae. Their exact masses, lifetimes and explosion mechanisms are uncertain. Astronomers do not have a clear understanding of the extreme conditions in the early universe. It’s still evolving, just like our understanding of supernovae. All in all, that’s a lot of uncertainty.
Yet all these challenges do not mean that primordial supernovae did not exist. So astronomers cannot rule them out, nor can they rule out very early habitable planets.
As things stand, there is no way to prove or disprove this research. However, it does open up a new mindset and new possibilities.
