Water Worlds
Some extrasolar planets have water in their atmospheres, an Italian astronomer announced at the Second European Workshop on Exo/Astrobiology in Austria.
At least, that’s one interpretation of the data obtained by Cristiano Cosmovici and his team from the Institute for Cosmic and Planetary Sciences in Rome. Using the 32-meter Medicina radio telescope to receive water MASER emissions, Cosmovici and his team looked at 17 stars that are thought to have planetary systems.
MASER stands for Microwave Amplification by Stimulation Emission of Radiation. Like a LASER, which is an amplified beam of ultraviolet, visible, or infrared light, a MASER is a beam of amplified microwave radiation. Natural MASERs develop in regions of space where the normal balance of atoms absorbing and releasing energy is altered.
For instance, MASERs can form from the disk of gas and dust that surrounds a young star. As the star emits electromagnetic radiation, the densely packed atoms and molecules in the disk absorb some of these photons. The frequency of the photon absorbed depends on the type of atom or molecule. This absorption pushes the atoms and molecules into a higher, “excited” state. Atoms don’t remain in this state for long, and they soon emit photons in order to relax back to the lower energy, “ground” state.
The MASER beam arises when groups of atoms or molecules are stimulated to simultaneously make a transition to the ground state, all releasing their energy at the same wavelength. Since the photons are the same wavelength, the MASER beams are extremely focused. Scientists can use radio telescopes to receive these beams, and then determine which type of atom or molecule is responsible based on the frequency of the emission.
The water MASER line is approximately 22 gigahertz (GHz). For radio astronomers, this is one of the brightest spectral lines in the radio universe. Water production is a common result of star formation, as newly ignited stars send huge shock waves into the surrounding cloud of material. This energy blast causes some hydrogen and oxygen molecules to bind together, creating excited water molecules that relax by emitting microwaves at the 22 GHz wavelength.
Over the years, many water MASERs have been detected from the regions around newborn stars as well as from the circumstellar disks of young stars. But for older stars, the shock waves have long since subsided and the circumstellar disks have dissipated. Another explanation for the presence of water near such stars must be found.
The Italian astronomers found three stars with the signature water MASER emissions: Upsilon Andromedae, Epsilon Eridani, and Lalande 21185. The Italian team suggests these water MASER beams might arise as water molecules in a planet’s atmosphere become excited by the infrared light of its star. According to Cosmovici, such water MASER signals are “a powerful diagnostic tool for planetary searches outside the solar system.”
“This result is astonishing if it’s true,” says Geoff Marcy, a planet hunter from the University of California at Berkeley. “It definitely needs to be checked by other observers.”
The Stars
Upsilon Andromedae is about 44 light-years away. The star is about 3 billion years old, and a little more massive and just slightly hotter than the Sun. Doppler measurements of Upsilon Andromedae indicate three gas giant planets orbit the star. The most massive planet is 4.6 Jupiter masses, although astrometry measurements suggest it may be as much as 10 Jupiter masses. This planet orbits at 2.5 AU and completes an orbit in 1,267 days. Another planet is about twice as massive as Jupiter, orbits at .83 AU and completes an orbit in 241 days. The third planet is .71 Jupiter masses and orbits at a mere .059 AU, completing an orbit in only 4 days.
The star Epsilon Eridani is located 10.5 light-years away. Somewhat smaller and cooler than our own Sun, Epsilon Eridani is also less luminous. This may be because the star is so young: Epsilon Eridani is believed to be only 500 million to 1 billion years old. The existence of a planet near Epsilon Eridani is still under debate because, like other young stars, Epsilon Eridani is magnetically very active. The California planet hunting team led by Geoff Marcy says, “The Doppler behavior of magnetically active stars remains poorly explored, because so few stars are as active as Epsilon Eridani. Thus, [the] interpretation of a planet remains controversial.”
If the planet orbiting Epsilon Eridani does exist, it is very much like Jupiter, with a similar mass and an orbit of 3.3 AU from its star (Jupiter orbits our Sun at 5 AU). Other scientists have commented that the Epsilon Eridani system probably resembles what our own solar system may have looked like 4 billion years ago.
The planets around Lalande 21185 are also controversial. Rather than the Doppler measurements normally used to detect planets, these were found by optical astrometry. This observational method relies on the fact that a planet and its star orbit a common center of mass, making the star appear to move back and forth in the sky. University of Pittsburgh astronomer George Gatewood analyzed photographic plates taken between 1930 and 1984 that tracked the movement of Lalande 21185. He also followed the star’s motion for eight years using a photoelectric detector. His measurements indicated the presence of at least one, and possibly three, Jupiter-mass planets.
Only 8.1 light-years away, Lalande 21185 is one of our closest neighbors. The star is a red dwarf – a relatively cool, low-mass star – and it is believed to be between 5 to 10 billion years old. (Interestingly, water MASER emissions have been detected from the outer atmospheres of red giants and supergiants – older stars approaching the end of their lifetimes.
On Earth, liquid water is essential to life. By searching for liquid water elsewhere, we may find “life as we know it,” increasing the odds that we will recognize alien life if we ever come across it. Because Lalande 21185 is such a dim star, any worlds orbiting it will be cold and dark, and water could not exist there in a liquid state. Nor can water remain liquid on Jupiter-mass bodies due to temperature and pressure conditions. Thus, all of the supposed planets in these three star systems would only be able to retain water as a vapor in their atmospheres.
But water MASER emissions from these distant star systems also could be due to other water sources, such as cometary clouds. Still, knowing that other stars have water in their solar systems gives astronomers hope that Earth-like worlds could be out there.
