On The Hunt For The Very Closest Alien Earths

Posted: Jan 18 2014, 4:36am CST | by , in News


On The Hunt For The Very Closest Alien Earths
Photo Credit: Forbes

Awash in a sea of long-lived stellar Red dwarfs, our own sun seems to be somewhat of a local anomaly.

Until recently, detecting these faint M-spectral type stars — much less their potential earthlike planetary companions — remained just out of technological reach.

But thanks to vast improvements in spectrographic instrumentation, such surveys are now feasible.

To that end, late this year, a German-Spanish collaboration will begin a ground-based survey of 300 stars at an average distance of only 42 light years.

The hope is that the survey will be sensitive enough to detect a habitable “super-earth” (an earthlike planet a few times the mass of our own planet) in the astrobiological “sweet spot.” That is, where liquid surface water is abundant and temperatures are “pleasant” enough to spawn complex life.

The key to it all are two highly-precise visible and near-infrared stellar $10.8 million spectrographs that will soon be mounted on the 3.5 meter telescope at the Calar Alto Observatory in the far south of Spain.

The project, dubbed CARMENES for (Calar Alto high-Resolution search for M dwarfs with Exo-earths with Near-infrared and optical Echelle Spectrographs) should complete its observations by the end of 2018.

“CARMENES will enormously improve our understanding of M dwarfs,” said Andreas Quirrenbach, an astronomer at the University of Heidelberg, and the project’s Principal Investigator. “We will have a much better handle on the question whether M dwarfs are ‘good’ potential hosts for habitable planets, or whether life is restricted to planets around much [larger] more solar-like stars.”

Because these M dwarfs are less massive and less luminous, Quirrenbach says their radial velocity signals should allow for the detection of exo-earths of some 5 earth-masses.

“We hope to be the most sensitive survey for habitable super earths around stars with masses primarily between 0.2 to 0.5 solar masses,” said Quirrenbach.

When mounted on telescopes, spectrographs facilitate the measurement of a celestial body’s “radial velocity,” or the speed of its movement toward or away from us along our line of sight.

Radial velocity surveys have been used to detect everything from the expansion of the universe to the first Jupiter-like extrasolar planets circling their parent stars.

In exoplanet detection, radial velocity surveys look for a periodic wobble in a target star’s spectral lines, caused by the telltale gravitational influence of a planetary body orbiting its parent star.

“CARMENES is not designed for getting atmospheric spectra of planets, but in principle, we could try to do that,” said Quirrenbach. “We may also find targets that are suitable for follow-up by the James Webb Space Telescope (JWST).”

The project’s target stars are distributed more or less all over the Northern sky with a few targets dipping into the Southern hemisphere, says Quirrenbach and are much closer than any previous such survey. Thus, he says, any extrasolar planets found by CARMENES will be among the nearest yet discovered.

“But the question of whether a planet is suitable for life is much more complex than just finding out whether it can have liquid water at its surface,” said Quirrenbach.

While understanding a star’s given astrophysics is crucial when characterizing any putative habitable planetary system, it’s particularly important when understanding low-mass red dwarfs which may have earthlike planets on very short orbits. That’s because their stellar irradiance and sunspot variability over time have to the potential to dramatically impact a close-in planet’s atmosphere.

Because small M dwarfs are very red, the project is building two spectrographs, one for the visible, one for the near-infrared. Incoming light from the target star will be split into the visible and near-infrared and sent via separate optical fibers to the two different spectrographs. However, Quirrenbach says that the project will be unique in its ability to simultaneously take spectra in both the visible and near-infrared wavelengths.

Observing in both the visible and near-infrared wavelength ranges gives Quirrenbach and colleagues “a lot of leverage” to distinguish between spectral data that could either be interpreted as an extrasolar planet, or data that may simply be the signature of repetitive phenomena in the stellar atmosphere itself, such as star spots or pulsations.

Quirrenbach says that a planet in the habitable zone of a 0.4 solar mass star is expected to have an orbital period of only 25 days, while a planet in the habitable zone of a 0.2 solar mass star would orbit its star in just 12 days.

“For planets in the habitable zone,” said Quirrenbach, “observing lower-mass stars has a double [radial velocity] advantage. I get a [stronger] signal because of the smaller stellar mass and I also get a [stronger] signal because of the planet’s smaller orbital distance.”

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Source: Forbes

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