How common Are Earth-Like Planets? At Least They're Made of the Same Stuff.

On the first Tuesday of each month, I write an astronomy-related column piece for the Oklahoman newspaper. On the following day, I post that same column to my blog page.

This is reprinted by permission form the Oklahoman and www.newsok.com.

With nearly 4,000 known planets orbiting other stars (exoplanets), a few questions inevitably come up from both scientists and lay people alike: How similar are they to Earth? Do they have a composition similar to our planet? Can they support life? The problem with answering these questions is that exoplanets are tiny and extremely faint compared to the stars they orbit. Any signal from them that might help answer these questions is drowned out by the parent star.

Now, scientists have figured out a way to answer one of those questions, that of the composition of other planetary systems. We can’t directly measure the composition of the planets, but as parent stars age and evolve, they present a way to determine planetary composition.

When a sun-like star evolves to its final state, a white dwarf, it contains almost nothing but hydrogen and helium. As Dr. Siyi Xu of the Gemini Observatory in Hawaii and one of the authors of the new study explained, “White dwarfs’ atmospheres are composed of either hydrogen or helium, which give out a pretty clear and clean spectroscopic signal. However, as the star cools, it begins to pull in material from the planets, asteroids, comets and so on which had been orbiting it, with some forming a dust disk, a little like the rings of Saturn. As this material approaches the star, it changes how we see the star.”

Gemini South Observatory. Credit NSF

The star’s light shines through the dusty rings allowing astronomers here on Earth to determine the composition of the dust. It turns out, as Dr. Xu explains, “Most of the building blocks we have looked at in other planetary systems have a composition broadly similar to that of the Earth.” Such studies don’t yet tell us if the planets have water, believed to be a prime ingredient necessary for life. But they reveal that Earth’s overall composition is rather common. And since water is one of the most abundant compounds in the universe, it seems likely that if other factors are similar to our own solar system then water exits in those planetary systems as well.

Dr. Robert Jedicke of the University of Hawaii studies our moons. That’s right, plural. We are all quite familiar with our big, bright Moon in the sky. But our solar system occasionally picks up hitchhikers in the form of small asteroids that pass near us. These mini-moons, as Dr. Jedicke calls them, allows us to study wandering asteroids to get a better look at them than we can from their distance in the Asteroid Belt. Not only will we learn more about them, Dr. Jedicke tells us they offer an even more exciting possibility. "Mini-moons are perfect targets for bringing back significant chunks of asteroid material, shielded by a spacecraft, which could then be studied in detail back on Earth." Such access to asteroids opens up both scientific and, possibly, financial opportunities, as asteroids contain significant amounts of precious metals and, perhaps more importantly, rare-earth metals, essential for our computer technology.