51 Pegasus. The first planet discovered outside our own Solar System was found orbiting around this star.
Credit: NASA
The search for signals from extraterrestrial civilisations will benefit from a new network of radio antennas, called the Allen Telescope Array, now under development. Forty two of the planned 350 telescopes in the array should be operational this year.
Turnbull's top candidate star for such radio scans is beta CVn, a sun-like star about 26 light-years away in the constellation Canes Venatici (the Hound Dogs). Astronomers have been using currently available technology to search the star for accompanying planets but none has been found so far, Turnbull said. Her other top candidates for a SETI search:
- HD 10307, another solar analogue about 42 light-years away. It has almost the same mass, temperature and metallicity of the Sun. It also has a benign companion star.
- HD 211415, about half the metal content of Sun and a bit cooler, this star is in just a little farther away than HD 10307.
- 18 Sco, a popular target for proposed planet searches. The star, in the constellation Scorpio, is almost an identical twin to the Sun.
- 51 Pegasus. Already famous. In 1995, Swiss astronomers reported they had detected the first planet beyond our solar system in orbit around 51 Pegasus. An American team soon verified the finding of the Jupiter-like object and the rush to find more extra-solar planets was on. Turnbull thinks 51 Pegasus could harbor terrestrial planets as well.
In choosing candidate stars for a terriestrial planet finding mission, Turnbull went for stars with enough intrinsic luminosity to suggest good prospects for a habitable zone but not so bright as to overwhelm efforts to images their planets. In her Goldilocks solution, the best candidates were K-class stars, objects that are intrinsically dimmer than the Sun.
Turnbull's top choice is epsilon Indi A, a star only about one-tenth as bright as the Sun. It is nearby, about 11.8 light-years away in the constellation Indus. The star is among the top 100 targets for a planet finding mission.
Her other good candidates:
- epsilon Eridani. A star somewhat smaller and cooler than our Sun, located about 10.5 light-years away in the constellation Eridanus (the River).
- omicron2 Eridani. A yellow-orange star about 16 light-years away, roughly the same age as the Sun.
- alpha Centauri B. Part of the closest stellar system to the Sun, just 4.35 light-years away. Long considered one of the places in the Milky Way that might offer terrestrial conditions. This star is part of a triple star system.
- tau Ceti Unlike the candidates in this group, Tau Ceti is a G-class star, the same brightness category as our Sun. Metal-poor compared to the Sun but long-lived enough for complex life forms to evolve.
Efforts to take direct images of Earth-like planets - the goal of the planned Terrestrial Planet Finder (TPF) mission - are extremely challenging. Astronomers want to find Earth-like planets orbiting close enough to the star - but not too close - for there to be an environment capable of having liquid water, a key ingredient of life. But such planets in orbits relatively close to the star simply get lost in the glare of the host star.
Turnbull acknowledges that it is a toss up when it comes to naming just a few candidate habstars. "There are inevitable uncertainties in how we understand these stars," Turnbull said. "If I took the top 100, it would be very difficult for me to tell which one is the best." But the exercise is worthwhile, she said, and her selection criteria really did drive her toward a couple top choices and a handful of other candidates.
