PRESS RELEASE, January 2008
HABITABLE REAL ESTATE AROUND NEARBY STARS
Justin R. Cantrell
Todd J. Henry
Astronomers aren't terribly worried about the ups and downs of the real estate market in the Sun's neighborhood. In fact, they're willing to look at all of the options, regardless of what type of star provides the light and heat. A group from Georgia State University is using the data they have collected on the nearest stars to estimate what they call the ``habitable real estate" around each of the Sun's neighbors. This habitable real estate is defined as the region around a star where a planet could sustain liquid water.
The nearby real estate market is being investigated by the RECONS group, which has been using relatively small telescopes to study nearby stars at the National Science Foundation's Cerro Tololo Inter-American Observatory in the Chilean Andes since 1999.
``Some might call us nosy neighbors, but we feel that there's a lot more to learn about the habitable zones around the nearest star systems," explains Justin Cantrell, who is leading the real estate work. ``We want to know how much habitable area there is associated with each stellar system, and for each different class of star found among the Sun's neighbors."
The team is using measurements of the stars' observed brightnesses at optical and infrared wavelengths and the stars' distances found via the trigonometric parallax method. Together, these data allow them to derive the stars' intrinsic luminosities, colors, and distances, which ultimately yield accurate estimates of the stars' temperatures and sizes. The team is the first to make a concerted effort to make the measurements for all of the nearest stars, and the first to look at each different type of star in the sample --- whether the stars are bigger and hotter than the Sun like white Sirius, similar to the Sun like yellow Alpha Centauri A, or smaller and cooler than the Sun, like the orange and red stars that make up most of the nearest stars, including Alpha Centauri B and Proxima Centauri.
``Once we have good values for the temperatures and sizes of the nearby stars, we can estimate how hot planets will be at different distances from the stars," Cantrell explains. ``We consider those stars that would have surface temperatures suitable for liquid water to be in the traditional habitable zone."
The group was keenly interested in the habitable real estate around red dwarf stars, which are 50-90% smaller than the Sun and much cooler. Although they comprise more than 70% of the Galactic population, they are often overlooked as hosts of planets suitable for life because they shine so feebly. To the group's surprise, although there are only three Sun-like stars of spectral type G and 44 red dwarfs of spectral type M within 5 parsecs (16.4 light years), all the red dwarf habitable real estate added together did not equal the habitable zone of even one Sun like star. It's much like finding that a single large island has more good places to live than several dozen small islands.
Cantrell's graduate advisor, Todd Henry, nonetheless thinks red dwarfs might still be intriguing places to look for planets, and life on them. ``When you have forty four options for planetary systems around red dwarfs versus only three for stars like the Sun, it seems we shouldn't give up on the Sun's smaller cousins just yet. You never know where the liquid water may turn out to be." We now know there is liquid water on Saturn's moon Enceladus and suspect water on Jupiter's moon Europa, both of which are far removed from the prime real estate in our Solar System.
Another area of interest for the team was to understand the effects on habitable zones when more than one star is in a system. Roughly half of stars like the Sun have companion stars (called binary star systems), while about a third of red dwarfs have stellar companions. In many such systems, the stars are separated by enough distance to leave good real estate around each star where life could settle in comfortably.
For example, the nearest star system contains three stars --- the Sun-like star Alpha Centauri A and the somewhat cooler star B in close orbit around one another, and the red dwarf Proxima Centauri that is much farther away. Stars A and B orbit in such a way that the distance separating them changes from similar to the distance between the Sun and Saturn to a bit larger than the distance between the Sun and Neptune. ``It's like having a second Sun shifting from where Saturn is to where Neptune is," Cantrell explains. The surprising result was that when the light and heat of the two stars was combined, the team found that the other star in the system did not significantly change the size of the habitable zone, regardless of where it was in its orbit. ``We expected that Alpha Centauri A and B might interfere with each others' habitable zones, but the areas of the available good real estate around each star is affected by less than 1%," says Cantrell. Distant Proxima is completely unaffected by the other two stars. Apparently, the real estate market in the nearest star system has not one, but three promising locations for life.
This research has enjoyed the consistent support of the National Science Foundation (grant AST-0507711), the Cerro-Tololo Inter-american Observatory (part of the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy Inc. (AURA), under a cooperative agreement with the National Science Foundation), The SMARTS Consortium, NASA's SIM PlanetQuest, and Georgia State University.
The relative sizes of traditional habitable zones are shown around four of the nearest stars. Sirius A is the brightest star in the night sky, while Alpha Centauri A, B, and Proxima Centauri are three stars forming the nearest star system, which is a triple. At this scale, the habitable zone around the red dwarf Proxima is so small that it is only about the size of a period at the end of this sentence.