Habitability: Betting on 37 Gem


"We are witnessing the birth of a new observational science: the discovery and characterization of extrasolar planetary systems."
- G. Marcy, UC Berkeley

If life does exist elsewhere, it’s likely to be on a middle-aged star in the constellation of Gemini, according to astrobiologist Maggie Turnbull, of the University of Arizona in Tucson, as reported in a New Scientist article.

Profile: Margaret Turnbull

Margaret Turnbull’s career has included construction, setup, testing and repair of detectors and readout system for an Antarctic Muon and Neutrino detector array.

At the University of Wisconsin in space physics, payload construction, she was involved in detector preparation for the X-ray Quantum Calorimeter sub-orbital launch experiment.

At Lowell Observatory in Arizona, she participated in the examination of main-belt and Earth-crossing asteroid orbital parameters and trends of ephemeris uncertainty with orbital parameters.

She has done Monte Carlo modeling of high-resolution Hubble Space Telescope images of protostars in the Taurus-Auriga dark cloud at the Harvard-Smithsonian Astrophysical Observatory. And in the radio domain, she has analyzed large-scale HI 21 cm emission from Seyfert galaxies imaged by the Very Large Array, and conducted a search for obvious distortions caused by unseen companion galaxies.

Turnbull has compiled a shortlist of 30 possibly habitable planets and stars and one called 37 Gem is her top choice.

"This stable, middle-aged star is just a bit hotter and brighter than our sun. And if alien life is anywhere, it’s likely to be there," New Scientist magazine said. The star is in the constellation, Gemini, which gives it the astronomical name, 37 Gem.

Turnbull made the list for NASA’s Terrestrial Planet Finder (TPF), a space telescope project that will search for habitable planets after it is launched in about 10 years time. In an earlier article published this year, "Target Selection for SETI: I. A Catalog of Nearby Habitable Stellar Systems," in the Astrophysical Journal, their survey identified 17,129 potentially habitable hosts for complex life.

For such a study, Maggie Turnbull and adviser Jill Tarter have been in the process of compiling this much larger list called HabCat, or Catalog of Nearby Habitable Systems. HabCat was created from what is known about habitable stars, or ‘habstars’, near our sun. Seventy-five percent are within 140 parsecs, or around 450 light years. These Sun-like, habitable stars have just the right distance, constancy, and light to qualify in a forthcoming enlarged radio search.

The amount of heavy metal present when the star is formed and its age were important criteria for Turnbull. Young stars typically have high rotations, they emit soft X-rays above and beyond their estimated temperatures, or they haven’t burned through enough light elements to produce metals (heavy elements like iron).

But Gem 37, the 37th brightest star in the constellation of Gemini, came out on top because it looks most like our sun.

Luminosities are "perhaps the most important information", Turnbull told Astrobiology Magazine, "we use in determining the habitability of nearby stars" for complex life, because luminosity indicates which phase of life the star is in, and that in turn dictates how long the star will remain stable.

To be included in the short list of habitable stars, each candidate stellar systems is weighted for X-ray luminosity, rotation, spectral types or color, kinematics, metallicity, and Strömgren photometry. Hotter, more massive stars have always been considered less likely to harbor life, though not because they would be too hot. Planets could still enjoy temperate climates, just at orbits farther away from the star.

The problem is one of time, not temperature. Hotter stars tend to burn out faster — perhaps too fast for life to develop there.

Our sun, considered a typical G2V dwarf, is approximately 4.5 billion years old. G stars are characterized by the presence of metallic lines and weak hydrogen, Given the billions of years required for evolution of life on earth, scientists could question whether life would stand a chance in a shorter-lived solar system. The star identified in the Turnbull survey, 37 Gem, is a G0V dwarf.

"Priority will be assigned," said Turnbull, "based on location in the color-magnitude diagram [CMD] and on distance. A plot of the CMD score would look like a bull’s-eye centered on the sun’s absolute visual magnitude and B-V color, with the highest score at the center. The distance scoring will go as the inverse square of distance, or the detectability of a given signal".

"We are still considering how to weight these two factors, but most likely the distance score will dominate within a certain distance", Turnbull told Astrobiology Magazine. "One idea is that, given the first terrestrial transmissions of a century ago, HabCat stars within the possible ETI [extraterrestrial intelligence] response distance of 50 light years should have scores dominated by distance".

"This work falls within the broader themes of, first, the overall habitability of the Milky Way, and, second, the remote sensing of biosignatures", said Turnbull. "My dissertation work includes topics such as characterizing the Earthshine signal, including spectral signatures of life; constraining the fraction of habitable stars in the solar neighborhood; identifying suitable targets for the Terrestrial Planet Finder mission; and examining the concept of a habitable belt in the Galaxy".

The task is challenging, not just because of the large number of stars to consider, but also the difficulty of defining conditions for habitability. As Turnbull and Tarter concluded in their Astrophysical Journal article: "Despite the broad array of data used to assemble this catalog, this exercise has forced us to admit that we are defining habitability from a position of considerable ignorance…For SETI this humbling situation is amplified when we consider that we have no indisputable definition for life itself, to say nothing of the precise conditions that are necessary and sufficient for life to evolve into a technological civilization detectable by a SETI search program."

What’s Next

For HabCat surveys, the current best set of stellar distances comes from what astronomers call Hipparcos parallax measurements . These measurements give values accurate to ~ 1 milli-arcsecond. Several new astrometric satellites are now being planned to measure star distances, which will improve the selection criteria for habitability.

The most ambitious planned star cataloguing projects are NASA’s Space Interferometry Mission [SIM] and ESA’s Galactic Census Project, or GAIA mission, which may yield large numbers of parallaxes with precisions better than ~ 10 micro-arcseconds. SIM is scheduled to operate from 2006 to 2011 while GAIA, if accepted by ESA, could launch in 2009 with a 5 year lifetime.

SIM would provide astrometric measurements of 10,000 stars and GAIA would measure around 10 billion positions. The next scheduled astrometry mission, however, was the now cancelled FAME (Full-sky Astrometric Mapping Explorer), which was scheduled to be launched in 2004.

When the Allen Telescope Array turns on in 2005, it will be capable of searching to the farthest of 17,000 habstars, just beyond 300 parsecs.

Scientists hope to launch six new space-borne missions over the next few years to search for terrestrial planets.

They include France’s small-scale COROT, NASA’s more-ambitious Kepler mission, the European Space Agency’s (ESA) Eddington and NASA’s Space Interferometry Mission (SIM).

The French COROT mission, approved and due for launch in late 2004, will study asteroseismology, or oscillations within stars, and likely will be the first orbiting telescope to search for extrasolar planets.It will look at 50,000 to 60,000 stars and should find a few dozen terrestrial planets and several hundred close-in gas-giant planets during a two- to three-year mission, says Pierre Barge, an astronomer at the Laboratory of Astrophysics in Marseille and leader of COROT’s exoplanets group. COROT – for Convection, Rotation and Planetary Transits – is a mission of CNES, the French National Center for Space Studies, in partnership with ESA, Italy, Belgium and Germany. When searching for extrasolar planets, COROT’s 27-centimeter (10.6-inch) telescope will use a method called photometry, in which sensitive light detectors look for a slight drop in a star’s brightness as a small planet "transits" the star (crosses the face of the star as viewed from COROT).

SIM reference design
SIM, scheduled for launch in 2009, will determine the positions and distances of stars several hundred times more accurately than any previous program.
Credit: NASA / JPL

The Kepler mission is scheduled for launch into solar orbit in October 2006. Kepler will simultaneously observe 100,000 stars in our galactic "neighborhood," looking for Earth-sized or larger planets within the "habitable zone" around each star – the not-too-hot, not-too-cold zone where liquid water might exist on a planet. To highlight the difficulty of detecting an Earth-sized planet orbiting a distant star, Borucki, Kepler’s principal investigator, points out it would take 10,000 Earths to cover the Sun’s disk. One NASA estimate says Kepler should discover 50 terrestrial planets if most of those found are about Earth’s size, 185 planets if most are 30 percent larger than Earth and 640 if most are 2.2 times Earth’s size. In addition, Kepler is expected to find almost 900 giant planets close to their stars and about 30 giants orbiting at Jupiter-like distances from their parent stars. Because most of the gas giant planets found so far orbit much closer to their stars than Jupiter does to the Sun, Borucki believes that during the four- to six-year mission, Kepler will find a large proportion of planets quite close to stars. If that proves true, he says, "We expect to find thousands of planets."

In 2008 or later, the European Space Agency hopes to launch the Eddington mission (named for the late British astronomer Arthur Eddington). Eddington primarily would study stars’ interior structures and the processes that govern how stars evolve, but it would spend three years scanning 500,000 stars for planets, including terrestrial planets in habitable zones. Like Kepler and COROT, Eddington would use a camera with a large optical telescope, in this case a wide-field 1.2-meter (47-inch) one, to detect planetary transits using photometry.

Also due for launch in 2009 is the almost $1 billion NASA-ESA Next Generation Space Telescope, or NGST [James Webb Space Telescope], a near-infrared telescope that will succeed the Hubble Space Telescope. Planet hunting will be a minor part of its job. Like Hubble, NGST will be a general-purpose telescope with an emphasis on cosmology. But it will investigate stars with dusty disks – the early stage of planet formation – and may also be able to study Jupiter-size planets.