Cool stars have different life forming ingredients

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 09 April, 2009

New data from NASA’s Spitzer Space Telescope suggests that planets around cooler stars than our own Sun might have a different set of life-forming ingredients to our Solar System’s primordial soup.

Astronomers used Spitzer to search for a prebiotic chemical called hydrogen cyanide in the planet-forming discs swathing different types of stars. Hydrogen cyanide is a component of adenine, a basic element of DNA, which of course can be found in every living organism on the Earth. The team examined the proto-planetary discs of 17 cool and 44 Sunlike stars, covering an age range of one to three million years old.

Artist impression of a young planet around a cool star, with a soupy mix of possible life-forming chemicals seen pooling on the surface. Observations from Spitzer hint that planets around cool M-dwarfs and brown dwarfs might possess a different mix of life-forming chemicals than our young Earth. Image: NASA/JPL-Caltech.

“Prebiotic chemistry may unfold differently on planets around cool stars,” says Ilaria Pascucci of Johns Hopkins University, and lead author of the new study featuring in the 10 April issue of the Astrophysical Journal. All stars begin life inside a cocoon of dust and gas, which flattens out into a disc that can eventually spawn planets. The building blocks for life on Earth may have begun in this way, with prebiotic molecules, such as adenine, raining down to the surface via meteoroid impacts.

Pascucci and colleagues used Spitzer’s infrared spectrograph to split the light around different stars into its constituent parts in order to search for the fingerprint of hydrogen cyanide, and then compare it to a baseline molecule, acetylene, a simple hydrocarbon.

Athough acetylene was detected around the cool stars, the results revealed that both the cool M-dwarf stars and brown dwarfs had no hydrogen cyanide at all, while 30 percent of the Sunlike stars did. “Perhaps ultraviolet light, which is much stronger around the Sunlike stars, may drive a higher production of the hydrogen cyanide,” says Pascucci.

Graph showing the Spitzer observations. Data from stars like our Sun are yellow, and data from cool stars are orange. The signature of a baseline molecule, called acetylene (C2H2), was seen for both types of stars, but hydrogen cyanide was seen only around stars like our Sun. Image: NASA/JPL-Caltech/I. Pascucci (Johns Hopkins University).

The findings have implications for the large Earthlike planets that have recently been discovered around M-dwarf stars, although none of them are believed to orbit in the habitable zone, which would allow water to be liquid. But even if such a planet was discovered, it may not be able to sustain life, since M-dwarfs have extreme magnetic outbursts that could prevent life taking a firm hold. But as the new Spitzer data shows, these planets may be deficient in life’s building blocks anyway.

“Although scientists have long been aware that the tumultuous nature of many cool stars might present a significant challenge for the development of life, this result begs an even more fundamental question: Do cool star systems even contain the necessary ingredients for the formation of life? If the answer is no then questions about life around cool stars become moot,”
says Douglas Hudgins, the Spitzer program scientist at NASA
Headquarters.

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