Ancient Galaxy Holds Planet Chemistry Surprise
When did planets first form in the Universe? Though we’ve been finding hundred of exoplanets and thousands of planet candidates in our Milky Way Galaxy, we need to look at processes in much more distant galaxies to find the earliest hints of our chemical ancestry.
Life as we know it evolves on a planet. Planets form from the debris left over when a star is born. Planetary formation requires elements heavier than hydrogen and helium, but the very first stars were made of just those two elements formed in the Big Bang. So, it had to take some time and several cycles of stellar life and death to build up the heavier elements through nuclear fusion and supernovae. But the question remains… how early in the Universe’s history were these elements around to form planets?
A group of astronomers led by Jens-Kristian Krogager, a Ph.D candidate at the Niels Bohr Institute, took a detailed inventory of a very distant galaxy, around at a time when the Universe was about 2.8 billion years old — around 11 billion years ago. (For reference, our sun is about 5 billion years old, so this was well before it was formed in its own nascent cloud.)
The galaxy blocks some of the light from an even more distant quasar, so its spectrum can be studied for absorption lines. Individual elements in a gas can remove or block certain wavelengths of light from a background source, and for these kinds of systems, that tells you the redshift, giving you the distance to the galaxy. This galaxy also have spectral emissions lines from gas that has been excited by radiation given off by the star formation regions.
Using the Very Large Telescope in Chile and the Hubble Space Telescope, the astronomers looked at various emission and absorption lines of oxygen, nitrogen, zinc, iron, silicon and magnesium to accurately determine how much of these heavier and potentially planet-building elements existed in the gas forming new stars. They determined it to be about one-third of the heavy elements found in the sun. These elements had to have been formed by earlier generations of stars that lived and died, making way now for the potential of planet formation 6 billion years before our sun was even born.
The imaging results were added to get a more complete picture of the galaxy, which appears to be forming stars at a rate of about 13 solar masses per year (compared to our Galaxy’s paltry one solar mass per year). The galaxy is a small, elongated disk shape, probably see nearly edge-on, with a mass of 2 billion solar masses, much smaller than the grand spirals and ellipticals we see in the Universe today.
The gas that was studied in absorption and emission lies well outside the disk, indicating that a “galactic fountain” is at work. This occurs when so much star formation creates a large number of supernovae that expel interstellar gas outside of the galaxy, thus shutting down the star formation. That gas can later “rain” back down on the disk, starting a new wave of formation.
Young, small, star-forming and heavy-element rich galaxies were the building blocks of galaxies like our Milky Way. They brought with them their stars, planets, and gas through a series of mergers to form the bustling stellar metropolis we live in today. Did such systems have rocky, even habitable planets on them? Could life have begun even further into the distant past than our own planet? And if so… where did everyone go? These are the big questions that we can only speculate on, but at least we learn some amazing astrophysics along the way.
Image: The galaxy in question is just a teeny tiny smudge as seen to the upper left (circled) of a much brighter but more distant quasar. The quasar is subtracted from the images so that the galaxy itself can be studied. From Krogager et al, 2013.