In our hectic world, we seldom have time enough for life: time for family, time for friends, or time for the pleasures of life. In studying the universe, Astrobiologists face a different problem: which stars might provide time enough for life? The answer depends on the life of the star. It may seem odd to speak of the life of a star, but stars go through processes akin to birth, maturity, and even death. It is the pacing of those stages in a stars life that determine whether biological life has a chance on an orbiting planet.
The life of a star is an ongoing struggle between the force of gravity trying to collapse the star and the energy released by nuclear fusion which heats the gas and makes it expand. The "metabolism" of a star, how fast it consumes hydrogen, is determined by this simple balance of forces. The more massive the star, the greater the force of gravity pulls it together, and the more it must "burn" hydrogen to fight the collapse. In the case of a star, there is no question of "nature vs. nurture." Its fate is determined the day it is born.
All stars begin the same way, generating energy by converting hydrogen into helium. During this phase, a stars energy output is fairly constant and life might have a chance to evolve on a nearby planet. Eventually, the amount of hydrogen in the core gets low enough that the rate of nuclear fusion decreases. As this happens, gravity gets the upper hand and causes the core of the star to shrink. As the core shrinks, it becomes hotter and denser. What happens next depends on the mass of the star.
For stars like the Sun, the core will become hot and dense enough for a new nuclear fusion reaction: helium forms carbon. This new source of energy turns the tide against gravity and the star will expand a hundredfold or more. The swollen star is thousands of times more luminous, but its surface temperature is lower giving it a reddish hue. The star is a "red giant" and is nearing the end of its life. It is also the end for life on any planets orbiting the star. When the Sun enters this phase about four billion years from now, its surface will touch the Earth.
Stars much more massive than the Sun have a flashier but shorter life, and death. Because the force of gravity is much stronger for massive stars, they use up the hydrogen much faster. The steady phase only lasts millions rather than billions of years. The red giant phase is equally short and spectacular, ending in a supernova explosion. In its death throes, a massive star will briefly outshine its galaxy and sear any orbiting planets.
But which stars have time enough for life? To begin with the obvious, stars like the Sun. Our Solar System formed about 4.5 billion years ago. Life on Earth started shortly after the young planets crust cooled enough for liquid water to form. Scientists have found fossils of micro-organisms that date back more than 3.5 billion years. Simpler life forms must have started even earlier. In spite of such an early start, life stayed at the single cell stage for about three billion years. For the Earth, it took more than another half billion years for our earliest hominid ancestors to appear. During most of that time, except for its early years, the Sun has provided a fairly steady source of energy for life on Earth. So, life needs time. In our case, it needed several billion years.
It is nearly as obvious that massive stars are unlikely to host life-bearing planets. With stable lifetimes of only millions of years and planet-sterilizing, explosive deaths, it is very unlikely that life would have a chance to evolve.
An intriguing question considered by Astrobiologists today is whether stars much less massive than the Sun could support planets with life. For many years the conventional wisdom said no. The argument went as follows. Such stars are small and relatively cool. In order for a planet to support liquid water, necessary for life as we know it, it would have to orbit close to the star. So close, that it would become "tidally locked" to the star, as the Moon is locked to the Earth. Just as the Moon only shows one side to the Earth, so too the tidally locked planet would always have one side facing its star. The other side of the planet would have perpetual night. The nightside temperature would be cold enough for the gasses in the planets atmosphere to freeze out. Such a planet would have two climate zones, a bright, barren desert and a dark, frozen wasteland.
Adding to the bleak picture, some small stars also exhibit enormous flares, gigantic versions of the solar storms that create auroras and disrupt communications on Earth. The radiation from these flares would rain down on the dayside desert.
All of the conventional wisdom assumed that our Solar System was typical, that a low mass star would have small, planets rocky planets in close. The discovery of other planetary systems has changed that picture. We now know that gas giant planets, larger than Jupiter, can form close to stars. A large planet with a thick atmosphere, even if tidally locked, could distribute heat to the nightside, keeping the atmosphere from freezing out. The thick atmosphere would also offer protection from stellar flares. It could be that small stars with gas giant planets may be excellent sites for life, because these stars have more than enough time for life, tens of billions of years.
A team of SETI Institute scientists, as part of the NASA Astrobiology Institute, is bringing together experts to reconsider the possibility of life on planets orbiting small stars. Such stars are the most common. Over three-quarters of all stars are less than half the mass of the Sun. If such stars are suitable hosts for habitable planets, life may be far more widespread than we ever imagined.
To learn more about the lifetime of stars, tune in to SETI Radio this Sunday featuring Dr. Jeffrey Kargel of the US Geological Survey. For more information on this weeks schedule (including a list of stations that carry SETI Radio) please click here.