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Stars Can Be Late Parents

By David Reneke

Astronomers weigh a proto-planetary disk while miners set their sights on passing asteroids for gold and other valuable minerals.

This new decade has already heralded in a significant number of exciting cosmological discoveries, and the surprises continue in the relatively new field of extra-solar astronomy. Using the unique capabilities of the European Space Agency’s Herschel Space Observatory, astronomers have accurately “weighed” a star’s disc and found it still has enough mass to spawn 50 Jupiter-sized planets several million years after most other stars have already given birth.

Proto-planetary discs contain all the raw ingredients for building planets. They are composed mainly of cold molecular hydrogen gas, which is highly transparent and essentially invisible.

Usually it is much easier to measure the emissions from “contaminants” like the small fraction of dust mixed in the gas to estimate the total disc mass. In the past this technique has caused significant uncertainties, but thanks to the sensitivity of Herschel, astronomers have used a new, more accurate method involving a close relative of molecular hydrogen called hydrogen deuteride, or “heavy” molecular hydrogen.

Since the ratio of “normal” and “heavy” molecular hydrogen gas is well-known from measurements in our local solar neighbourhood, this approach provides a means to “weigh” a star’s total disc mass with ten times greater accuracy than ever before.

Using this technique, a substantial mass of gas was detected in a disc encircling TW Hydrae, a young star just 176 light years away in the constellation of Hydra. “We didn’t expect to find so much gas around this 10 million-year-old star,” says Prof Edwin Bergin of the University of Michigan.

The star selected has significantly more mass than required to make our own solar system, and could make a much more exotic system with giant gas planets like Jupiter and Saturn. Observing such a massive disc around TW Hydrae is unusual for stars of this age because, within a few million years, most material is typically incorporated into the central star and planets, or has been swept away by its strong stellar wind.

The new method of “weighing” a disc means that the volume of materials available, including water, could have been underestimated in this system and in others. “There may be different outcomes regarding planet formation for systems of varying ages,” says Prof Thomas Henning of the Max Planck Institute for Astronomy in Germany.

Just as the ages at which people have children span a range, TW Hydrae seems to lie at the edge of that range for stars, showing that this particular system may have needed longer to form planets, and that it might be a late parent. A re-evaluation of the masses of discs around other stars of varying ages will provide more insight into the planet-building process.

The Asteroid Gold Rush

The asteroid 2012 DA14, which made an extremely close pass of Earth recently, could be worth up to US$195 billion in metals and propellant according to Rick Tumlinson, Chairman of Deep Space Industries (DSI), a privately held American company.

Pie in the sky, you say? Not really, as most astronomers agree that the asteroid belt between Mars and Jupiter could be full of precious metals and gases worth trillions, even at today’s prices.

But achieving low Earth obit is costly. Sending fuel, water and building materials into orbit costs at least US$10 million per tonne, even using new lower-cost launch vehicles.

“Getting these supplies to serve communications satellites and coming crewed missions to Mars from low orbit space sources like asteroids is key if we are going to explore and settle space,” said Tumlinson. “While this recent visitor wasn’t going the right way for us to harvest it, there will be others that are, and we want to be ready when they arrive.”

How valuable might such an asteroid be were it harvestable? According to DSI experts, space programs could save around US$65 billion if 2012 DA14 contains 5% recoverable water that can be split into hydrogen and oxygen to create rocket fuel. And it could be worth an additional US$130 billion if just 10% of its mass is easily mined iron, nickel and other metals.

The obvious value of any given asteroid, if we begin to use them in space, will be as a space oasis for refuelling and resupply, but at present little is known about most of them. That’s why DSI is beginning with a prospecting campaign hitching cheap rides to space as secondary payloads accompanying the launch of large communications satellites.

DSI believes there are thousands of near-Earth asteroids that will be easier to chase down than 2012 DA14, and is laying the groundwork for potential space mining operations in about 2020. Several other private companies are known to be working towards the same aim, with some even planning to extract the ore from future manned lunar refining stations.

David Reneke is an astronomy lecturer and teacher, a feature writer for major Australian newspapers and magazines, and a science correspondent for ABC and commercial radio. Subscribe to David’s free Astro-Space newsletter at www.davidreneke.com