A New, Distant Arm of the Milky Way Discovered: What are the Odds of It Hosting a Life-Zone?


A large new spiral arm of the Milky Way peppered with dense concentrations of molecular gas has been 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?

Our Milky Way galaxy, like other spiral galaxies, has a disk with sweeping arms of stars, gas, and dust that curve around the galaxy like the arms of a huge pinwheel. The Sun, Earth, and solar system are located in a spur of material that lies between two of the spiral arms, collectively orbiting around the galaxy about 25,000 light-years from its center. Because the Milky Way contains copious amounts of dust that blocks our optical views, it is extremely difficult to study the galaxy from our vantage point within the disk.

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 CfA scientists suggest that 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.

Now, to our earlier question: What are the odds of the "new arm" of the Milky Way hosting an Earth-like planet capable of evolving advanced forms of life?

Virginia Trimble, of the University of California, Irvine and one of the world's leading astronomers 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 (all the elements except for hydrogen and helium are called "metals") to harbor habitable terrestrial planets (such as Earth) and are more than five billion years old exist considerably closer to the center of the Milky Way than we are.

It’s as if the Milky Way had formed from the inside out, with the older disk stars forming in the dense galactic center about 12 billion years ago. The upshot is that a 6 billion year old terrestrial planet has a potential 1.5 billion-year technology headstart to produce some pretty awesome next-generation iPods.

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.

Spore-306-tidal-080829Somewhat 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 (image below) 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.

Milan Cirkovic of the Astronomical Observatory in Belgrade, points out that the median age of terrestrial planets in the Milky Way is about 1.8 gigayears (one billion years) greater than the age of the Earth and the Solar System, which means that the median age of technological civilizations could be greater than the age of human civilization by the same amount.



Casey Kazan via CfA and  Virgina Trimble, Galactic Chemical evolution: Implications for the Existence of Habitable Planets, pp. 184-191, Extraterrestrials, Cambridge University Press.

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Image top of page: The Milky Way's basic structure is believed to involve two main spiral arms emanating from opposite ends of an elongated central bar. But only parts of the arms can be seen – gray segments indicate portions not yet detected. Other known spiral arm segments–including the Sun's own spur–are omitted for clarity. Credit: T. Dame


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