Best cosmic ‘cradles of life’ may be elliptical in shape

There’s plenty of real estate in the universe, but some neighborhoods are more desirable than others.

Giant elliptical galaxies that are more than twice as massive as the Milky Way are far more likely to harbor habitable worlds, researchers suggest. These galactic giants can produce up to 10,000 times as many Earthlike planets as our galaxy, astrophysicist Pratika Dayal and colleagues report online July 16 at arXiv.org.

“This is a starting point to think about astrobiology in a more cosmological sense,” says Dayal, of Durham University in England.

Dayal and colleagues combed through data on about 140,000 nearby galaxies, looking for oases that strike the right balance for sustaining life. The potential prolificacy of ellipticals is partly due to sheer size. But the bare trickle of star formation — as low as one-tenth that of the Milky Way — helps. Bustling stellar nurseries produce young, massive stars that explode within a few dozen million years. By keeping star birth to a minimum, giant galaxies protect fledgling worlds from being blasted by high doses of radiation and sterilized by supernovas. These galactic heavyweights also have an easier time holding on to gas that’s enriched with planet-building ingredients.

“This is a great step forward in researching habitability,” says Mike Gowanlock, a computer scientist at MIT. Future simulations, he says, can explore these findings in more detail. Not all stars in elliptical galaxies, for example, are suitable for supporting life. Many travel on orbits that take them through the crowded cores of these galaxies where the gravity of other stars might trigger mass extinctions by knocking loose a few comets or even jostling the planets themselves.

Dayal next plans to study how a galaxy’s habitability changes over time. The Milky Way formed from smaller galaxies smashing together. Each collision triggered a wave of star birth. “Supernovas going off everywhere would have destroyed what habitable zones had already formed,” she says. Fortunately for us, Earth came along a couple billion years after about 90 percent of the galaxy was assembled.