Illustration by Soner Ön
Life on Earth May Owe Its Existence to Black Holes
The behavior of black holes in the universe could very well be connected to the origins of life. It sounds outrageous, but not when you look carefully at the chain of phenomena that we think go into making stars, planets and living things.
A spider scuttles across the wall. A flower blossom unfurls its petals. A dog idly barks at something real or imagined, and deep in the ocean a school of fish darts and swoops around a cloud of krill. Something slimy grows on the underside of a muddy rock while, together with the 100 trillion bacteria in our guts, we sit quietly, electrical pulses zipping around our brains. This is life.
It is a collection of phenomena involving molecular machines that organize and reorganize matter in self-sustaining processes. Quadrillions of tiny life-forms can alter a planet’s atmosphere and modify its surface chemistry. Eventually, they may even produce a busy multicellular beast that leaves behind its own trail of disorder in the search for food and energy.
We have only one example of life to study: that existing on a small rocky planet orbiting a modest star in the 14 billionth year of this universe. Nothing about the nature of life on Earth, however, suggests it is anything but a fair sample of the mechanisms that could work anywhere. Consider that terrestrial life consists of carbon, hydrogen, oxygen and nitrogen, plus some other elements. The chemical bonding among these compounds is such that an extraordinary array of energy-efficient molecular structures can form -- from amino acids to DNA. There is no obvious example of an alternative chemical set that can do this.
We don’t really know the when, how or why of life’s origins, but it’s clear that some prerequisites are fundamental. The first is the elemental mix needed to produce biologically important molecules. The second is a location, or sequence of locations, in which that chemistry can be incubated. A third requirement emerges as the wheels of life are set in motion, and that is a supply of energy, in our case energy from the sun and geothermal energy from the Earth. In short, the recipe for life calls for ingredients, pots and pans, and a continually hot oven.
And this list points to the connections between life and its broader cosmic environment. We have learned how stars build the heavier elements of the universe. While hydrogen and helium will always be the most abundant cosmically, the next in line are oxygen and carbon. These elements are generally made deep in the cores of stars more than eight times the mass of the sun, and also, briefly, during their violently explosive deaths as supernovae.
Over time, the heavier elements pollute the interstellar and intergalactic gases, providing material for new stars. Precisely how this matter condenses into stars and planets is a question at the forefront of modern scientific inquiry, not least because we are in the midst of searching for other worlds that might harbor life. From at least ancient times, we’ve questioned whether ours is one of many such worlds in the cosmos. Now that we have the technological means to detect planets around other stars, we’re finally discovering other solar systems.
Stars form at the center of thick disks of gas and dust that can be a thousand times wider in radius than the distance from the Earth to the sun. Planets condense from these disks in a variety of ways. Over a few million years, what was once a pristine wheel of matter becomes pocked and lumpy with coagulating worlds. And, gradually, the disk of material boils off.
One thing that seems clear from observations is that the more heavy elements we detect in a star, the more planets are likely to exist around it, and the more massive they typically are. This makes a lot of sense. Greater quantities of substances like carbon and silicon mean more raw material for efficiently forming planets.
Frozen water also provides much of solid matter. A large fraction of the solid interiors of planets such as Uranus and Neptune are composed of water ices.
Earth happens to be located in an orbit about its parent star that allows for a temperate surface environment, so that liquid water can flow freely. And a planet’s potential for liquid water seems to be a reasonable signpost for life. Water is both an essential biochemical solvent and a planetwide contributor to geophysics and climate.
Our astronomical observations of young stellar systems also reveal lots of chemical mayhem, but there are clear hints that systems producing smaller stars may contain different chemistry from those with massive stars. So the chemical makeup of planets may well be related to the size of their stellar parent.
All this means that getting a world that has the right chemical and energetic richness to produce and sustain living organisms hinges on many things. Our next step is to find the connection between these more local phenomena and those on a truly cosmic scale.
The first place to look is up, straight up, to the galaxies. Each one has evolved over billions and billions of years, as dark matter, gas, dust and stars have coalesced, orbited, bumped, exploded, wafted and circulated. But galaxies are not all alike, and their global properties can affect the smaller details significantly. Less-elemental richness, for instance, can mean less-efficient cooling of nebular gas, which means fewer stars will form. The ingredients list can also influence the comparative numbers of big stars and small stars. And a dearth of heavy elements directly impacts the raw chemistry that takes place around forming planets.
If that space chemistry makes a lot of carbon-based, organic molecules, it may provide a “prebiotic” mixture for life. Instead of life having to wait millions of years for a young world to build complex molecules in some puddle, a rich starter mix could come from space. This is admittedly speculative, but not unreasonable.
No doubt, other factors are also at play. A planet subjected to blasts of intense cosmic radiation may be poorly suited for the growth of complex molecules, for example. I’d bet that no Earth-like planet exists inside the jets feeding supermassive black holes. That would surely be a horrible place for delicate biochemistry.
Nevertheless, black holes are capable of molding the universe around them. And so it makes sense to wonder how their behavior might affect the formation of stars and planets that have the potential to make and sustain life. To address that question, we have to look at the origins of supermassive black holes.
The very first black holes -- large and small -- left an imprint on all subsequent generations of stars and galaxies, and these early effects influence the production of new elements, the opportunities for planetary systems and the long-term behavior of galaxies and the black holes they contain. Places that offer relatively infertile terrain for life may be poorer in condensing elements, and are probably poorer in fresh stars with pristine new worlds. To explore whether this is true, we can ask what is special about Earth’s particular cosmic history.
The Milky Way, which contains our solar system, is a big spiral galaxy. The 200 billion stars in this disk stretch across a diameter of 100,000 light-years. Our parent star is positioned toward the outer edge of it. Every 210 million years, our solar system circumnavigates the galaxy. Since the sun formed more than 4.5 billion years ago, it has made this round trip more than 20 times.
Our biggest neighbor is the Andromeda galaxy, separated from the Milky Way by a gaping void 2.5 million light-years across. Our eyes see Andromeda as only a hazy patch. In reality, its light spreads across the sky in a great band some six times the size of the full moon. Andromeda is a giant spiral, too, but appears older than the Milky Way. Its baby stars form at one- third to one-fifth the rate of those in the Milky Way.
In 4 billion to 5 billion years, the curved space-time containing Andromeda and the Milky Way will cause them to merge. In fact, they have already started falling toward each other. Although this encounter will happen at a velocity of more than a hundred miles a second, it will not exactly be a collision. There is so much space between the tiny points of condensed matter in stars that the galaxies will simply drift and flow into each other over hundreds of millions of years. Eventually, the combined content of these two great systems may settle into something resembling an elliptical galaxy, and Andromeda and the Milky Way will be no more.
Regardless of the outcome, by the time this slow collision begins, our sun will have used up the hydrogen fuel in its core, which will contract inward. The shrinking interior will get hotter, flooding the solar atmosphere with radiation, and the sun will grow to a bloated red-giant star, engulfing what remains of Earth and the other inner planets. This tiny scrap of rock and water that, in just a few billion years, nurtured life from microscopic single-celled organisms to beings like us will be erased. Until then, we have a chance to understand what makes the Milky Way tick, how it compares with all other galaxies and what exactly made one of its planets come to life.
(Caleb Scharf, an astrophysicist, is the director of Columbia University’s Astrobiology Center and author of the blog Life, Unbounded. This is the fourth of five excerpts from his new book, “Gravity’s Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars and Life in the Cosmos,” which will be published on Aug. 14 by Farrar, Straus and Giroux. The opinions expressed are his own. Read Part 1, Part 2, Part 3, Part 5.)
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