The biggest challenge facing astronomers hunting for a bona fide "Earth-like" exoplanet is, where do you look? In a galaxy stuffed with hundreds of billions of stars, it's difficult to narrow the search.
Naturally, a key motivational factor behind planet-hunting missions (like NASA's Kepler space telescope) is to hunt for small rocky worlds — not too dissimilar to Earth — that may play host to life as we know it.
So, Kepler astronomers have focused their search on sun-like stars in the hope of detecting Earth-sized exoplanets orbiting at a similar distance our planet orbits the sun — a "sweet spot" (or "habitable zone") where the temperature may be just right for liquid water to exist on the surface. In our experience, where there's liquid water, there's life.
But still, even though we look toward "sun-like" stars with the hope that they may host a system of worlds not too dissimilar to our solar system, astronomers are still taking a proverbial "shot in the dark."
In an effort to find sun-like stars that are more sun-like than just their outward appearance, researchers from the University of Turku in Finland are carrying out a search for our sun's siblings.
When the sun was a baby, some 4.5 billion years ago, it was nourished in the same stellar nursery as thousands of other baby stars. After a billion years, the cluster of young stars went their own ways, dispersing throughout interstellar space.
But, like any family, these stars have a lot in common. And like nursery mates here on Earth, they may have shared some germs and viruses during their formative years when they were in close proximity. But in this case, "cosmic chicken pox" may have formed the building blocks of life that eventually flourished on Earth. If there's life on Earth, might there be life on the planets that formed around our sun's siblings?
This might sound a little outlandish, but there is a hypothesis that may have acted as the stellar nursery "cosmic chicken pox transmission mechanism" for the building blocks of life.
"The idea is if a planet has life, like Earth, and if you hit it with an asteroid, it will create debris, some of which will escape into space," astronomer Mauri Valtonen, of the University of Turku, told SPACE.com. "And if the debris is big enough, like 1 meter across, it can shield life inside from radiation, and that life can survive inside for millions of years until that debris lands somewhere. If it happens to land on a planet with suitable conditions, life can start there."
Valtonen is describing "panspermia" in its most basic form — life, as bacteria or even just some strands of DNA, hopping from planet to planet after meteorite impacts.
Usually astronomers will point out that panspermia could be used to explain how life may be common on Earth and Mars (if it is discovered). Perhaps meteorites made from Earth or Mars rock carried life throughout the solar system and beyond?
But what if this mechanism spread the earliest form of biology billions of years ago through our sun's cosmic creche? Suddenly we have targets for missions hunting for true sun-like stars, bona fide Earth-like planets and (possibly) Earth Brand™ Life.
However, in a galaxy filled with stars, how can we find the stars our sun was born with?
Using a star catalog compiled by the European High Precision Parallax Collecting Satellite, or HIPPARCOS, mission between 1989 and 1993, Valtonen is identifying the stars with similar velocities (through interstellar space) as our sun. Of the 100,000 stars in the catalog, there are two candidates — HIP 87382 and HIP 47399 — that not only have similar velocities, they also have a similar metal content and are of a similar age to our sun. They are currently 100 light-years away. There is a high probability that these two stars originate from the same place of birth as the sun.
Now that the first candidates have been identified, Valtonen wants to search for the presence of exoplanets around these stars. Should planets be present, it would be interesting to see if any small Earth-sized worlds also exist. This would form a very sound basis for the hunt for extraterrestrial life. Should there be life in these systems, it may be less extraterrestrial and more terrestrial in nature.
Sadly, even if true "Earth-like" worlds are present, we'd need an ingenious way of detecting life in these distant systems.
Image: A stellar nursery inside the Orion Nebula. Credit: NASA/JPL/Caltech.