If we ever have contact with extraterrestrial life from outside our solar system, it will be intelligent, which may be the only thing we have in common with them. Life on our planet is carbon-based, but argues Oxford evolutionary-biologist Richard Dawkins, it’s not totally out of the question to have it based on silicon – robotic, machines in other words. “So if and when we meet, it’ll also transform our understanding of ourselves and the Universe we live in.”
A possible abode of such advanced technological species formed only a billion years after the Big Bang, was discovered in July 2003, when the Hubble Space Telescope helped make the astounding discovery of a planet called PSR B1620-26 b, 2.5 times the mass of Jupiter, located in globular cluster M4 orbiting a peculiar pair of burned-out stars — white dwarf and a pulsar– in the crowded core of a spherical cluster of more than 100,000 stars. It contains several tens of thousands stars and is noteworthy in being home to many white dwarfs—the cores of ancient, dying stars whose outer layers have drifted away into space.
The PSR B1620-26 system lies around 5,600 light years away in M4, directly west of red supergiant star Antares in Constellation Scorpius. Its age is estimated to be around 13 billion years—almost three times as old as the Solar System. It is also unusual in that it orbits a binary system of a white dwarf and a pulsar.
The existence of a 13-billion year old planet, if in fact it still exists, highlights the fact that our Solar System exits in a universe that is estimated to be between 13.5 and 14 billion years old. Some astronomers, including Sir Martin Rees of Cambridge University believe that there could be advanced civilizations out there that have existed for 1.8 gigayears (one gigayear = one billion years) and longer, which means that the median age of technological civilizations should be greater than the age of human civilization by the same amount.
A few intrepid astronomers have concluded that the most productive to look for planets that can support life is around dim, dying stars white dwarfs so prevalent in M4.
“In the quest for extraterrestrial biological signatures, the first stars we study should be white dwarfs,” said Avi Loeb, at the Harvard-Smithsonian Center for Astrophysics (CfA) and director of the Institute for Theory and Computation. Even dying stars could host planets with life – and if such life exists, we might be able to detect it within the next decade. This encouraging result comes from a theoretical study of Earth-like planets orbiting white dwarf stars. Researchers found that we could detect oxygen in the atmosphere of a white dwarf’s planet much more easily than for an Earth-like planet orbiting a Sun-like star.
When a star like the Sun dies, it puffs off its outer layers, leaving behind a hot core called a white dwarf. A typical white dwarf is about the size of Earth. It slowly cools and fades over time, but it can retain heat long enough to warm a nearby world for billions of years. Since a white dwarf is much smaller and fainter than the Sun, a planet would have to be much closer in to be habitable with liquid water on its surface.
A habitable planet would circle the white dwarf once every 10 hours at a distance of about a million miles. Before a star becomes a white dwarf it swells into a red giant, engulfing and destroying any nearby planets. Therefore, a planet would have to arrive in the habitable zone after the star evolved into a white dwarf. A planet could form from leftover dust and gas (making it a second-generation world), or migrate inward from a larger distance.
If planets exist in the habitable zones of white dwarfs, we would need to find them before we could study them. The abundance of heavy elements on the surface of white dwarfs suggests that a significant fraction of them have rocky planets. Loeb and his colleague Dan Maoz (Tel Aviv University) estimate that a survey of the 500 closest white dwarfs could spot one or more habitable Earths.
The best method for finding such planets is a transit search – looking for a star that dims as an orbiting planet crosses in front of it. Since a white dwarf is about the same size as Earth, an Earth-sized planet would block a large fraction of its light and create an obvious signal.
More importantly, we can only study the atmospheres of transiting planets. When the white dwarf’s light shines through the ring of air that surrounds the planet’s silhouetted disk, the atmosphere absorbs some starlight. This leaves chemical fingerprints showing whether that air contains water vapor, or even signatures of life, such as oxygen.
Astronomers are particularly interested in finding oxygen because the oxygen in the Earth’s atmosphere is continuously replenished, through photosynthesis, by plant life. Were all life to cease on Earth, our atmosphere would quickly become devoid of oxygen, which would dissolve in the oceans and oxidize the surface. Thus, the presence of large quantities of oxygen in the atmosphere of a distant planet would signal the likely presence of life there.
Image credit: White dwarf planet with thanks to InvaderXan