Peering Through the Dust–“Milky Way’s Central Region Best Bet for Ancient Habitable Planets” (Wednesday’s Most Popular)



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Clouds of dust permeate the Milky Way, blocking our view of the galaxy's stars. Today, researchers have a suitable map of our galaxy's spiral structure, but, like early explorers charting new territory, they continue to patiently and meticulously fill in the blanks.

"The sun’s location within the dust-obscured galactic disk is a complicating factor to observe the galactic structure," said Denilso Camargo, from the Federal University of Rio Grande do Sul in Brazil. The job of mapping our own Milky Way galaxy from planet Earth, situated about two-thirds of the way out from the galaxy's center, is similar to trying to create a map of your house while confined to only the living room. You might peek through the doors into other rooms or look for light spilling in through the windows. But, in the end, the walls and lack of visibility would largely prevent you from seeing the big picture.


Researchers have turned to a new mapping method that takes advantage of data from NASA's Wide-field Infrared Survey Explorer, or WISE. Using WISE, the research team has discovered more than 400 dust-shrouded nurseries of stars, which trace the shape of our galaxy's spiral arms. Seven of these "embedded star clusters" were described in a study published in 2015 in the Monthly Notices of the Royal Astronomical Society.

The results support the four-arm model of our galaxy's spiral structure. For the last few years, various methods of charting the Milky Way have largely led to a picture of four spiral arms. The arms are where most stars in the galaxy are born. They are stuffed with gas and dust, the ingredients of stars. Two of the arms, called Perseus and Scutum-Centaurus, seem to be more prominent and jam-packed with stars, while the Sagittarius and Outer arms have as much gas as the other two arms but not as many stars. 



The WISE study finds embedded star clusters in the Perseus, Sagittarius, and Outer arms. Data from the Two Micron All Sky Survey (2MASS), a ground-based predecessor of WISE from NASA, the National Science Foundation and the University of Massachusetts, Amherst, helped narrow down the distances to the clusters and pinpoint their location.

Embedded star clusters are a powerful tool for visualizing the whereabouts of spiral arms because the clusters are young, and their stars haven't yet drifted away and out of the arms. Stars begin their lives in the dense, gas-rich neighborhoods of spiral arms, but they migrate away over time. These embedded star clusters complement other techniques for mapping our galaxy, such as those used by radio telescopes, which detect the dense gas clouds in spiral arms.

"Spiral arms are like traffic jams in that the gas and stars crowd together and move more slowly in the arms. As material passes through the dense spiral arms, it is compressed and this triggers more star formation," said Camargo.

WISE is ideal for finding the embedded star clusters because its infrared vision can cut through the dust that fills the galaxy and shrouds the clusters. What's more, WISE scanned the whole sky, so it was able to perform a thorough survey of the shape of our Milky Way. NASA's Spitzer Space Telescope also uses infrared images to map the Milky Way's territory. Spitzer looks along specific lines of sight and counts stars. The spiral arms will have the densest star populations.

In 2011, a large new spiral, outer arm of the Milky Way peppered with dense concentrations of molecular gas was discovered by two Harvard astronmers. What are the odds that this new arm might host an Earth-like planet capable of evolving advanced form of life?

Radio wavelengths can peer through the dust, however, and molecules like carbon monoxide that emit in the radio and concentrate in the galaxy's spiral arms are particularly good tracers of their structure. Using a small 1.2-meter radio telescope on the roof of their science building in Cambridge, CfA astronomers Tom Dame and Pat Thaddeus used carbon monoxide emission to search for evidence of spiral arms in the most distant parts of the galaxy, and discovered a large new spiral arm peppered with dense concentrations of molecular gas.

The new spiral is the far end of the Scutum-Centaurus Arm, one of the two main spiral arms thought to originate from opposite ends of our galaxy's central bar (see figure). If their proposal is confirmed, it will demonstrate that the Milky Way has a striking symmetry, with the new arm being the symmetric counterpart of the nearby Perseus Arm.




The central core, above,  which appears orange, is about 25,000 light years away and is thought to harbor a supermassive black hole. The reddening of the stars here and along the Galactic Plane is due to scattering by the dust; it is the same process by which the sun appears to redden as it sets.  What appear to be large, white stars are actually globular clusters.

Virginia Trimble, of the University of California, Irvine and a leading astronomer specializing in the structure and evolution of stars and galaxies, believes that it is highly probable that most of the stars that are both rich enough in metals to harbor habitable terrestrial planets and are more than five billion years old exist considerably closer to the center of the Milky Way than we are.

Stars in spiral galaxies such as the Milky Way have been divided by the world’s experts in galactic structure into four general categories. The huge, outer halo is thinly populated with some 500 million stars all more than twice the age of our Sun with less than 10% solar metal content. It’s here, in the outer halo that terrestrial planets may never have formed but if they did, could be ten billion years in advance of homo sapiens of Planet Earth.

Somewhat more metal rich than the outer halo, the inner halo and thick disk populations of stars present a more crowded, younger star-scape, making up about 10% of the Milky Way’s total star population. Like the outer halo, though, there is about a 10 billion-year jumpstart on Earth but, again, perhaps with few or no terrestrial planets as hosts for Spore-like evolutionary events in the heavy-element deficient halo and thick-disk stars.

The rest of the galaxy’s stars belong to the thin disk –home to the Sun- and Milky Way’s central bulge, a region that up until recently with the launch of the Spitzer infrared and Chandra x-ray space observatories has been difficult to study because of the dense interstellar dust surrounding the massive Sagittarius A black hole. The Milk Way's center is small too -a 600 light years across while the galaxy itself is 100,000 light years across.

The thin disk and central bulge stars are the best bet to find habitable terrestrial planets –an Earth’s twin with the added bonus of a possibly vastly advanced technological civilization based solely on the potential amount of time they’ve had to think about things like warp drive systems and time travel.

The average metallicity of thin disk solar objects is about two-thirds of solar. It is likely that most bulge stars are significantly older than the sun and the average K giant and has twice the solar iron abundance. Main sequence stars like the Sun are too faint to be studied directly in the central bugle but are not expected to be chemically different from the giants.

The Daily Galaxy via Harvard Smithsonian Center for Astrophysics (CfA) and Virgina Trimble, Galactic Chemical evolution: Implications for the Existence of Habitable Planets, pp. 184-191, Extraterrestrials, Cambridge University Press.

Image: Credits: NASA/JPL-Caltech/R. Hurt (SSC/Caltech) and




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