Some 75 percent of all stars in the solar neighborhood are faint red dwarfs, which are both less massive and much cooler than our own sun. But given their numbers, astrobiologists have been salivating at the idea of looking for habitable planets circling such M-type stars.
Trouble is, we know very little about these stars' chemical makeup. Taking spectra of their light in a way that could reveal their chemistry has to be done both at high resolution and in the near infrared. That's a combination that heretofore has never been possible.
Enter Natalie Hinkel, a Louisiana State University astrophysicist and her team of twenty so researchers from LSU, the Southwest Research Institute, Indiana University, and the University of Texas in Austin. This year will mark the fourth time her team is applying for a $20 million NASA grant to launch a high-altitude balloon from McMurdo station in Antarctica.
Equipped with a high-resolution spectrograph and near-infrared instruments, from its vantage point 36km above the Antarctic ice, this balloon-borne 60cm telescope would not be subject to the constraints of a day-night cycle as are optical telescopes. Nor would it be subject to the contaminating influence of Earth's atmosphere.
Named for the Egyptian goddess, SAKHMET (Spectroscopic Abundances to Know the Heritage of M-dwarf Environs through Time) would enable the first-ever high resolution, near infrared spectroscopic star survey.
We know almost nothing about M-dwarf compositions, SAKHMET team leader Hinkel, told me via email. Given that M-dwarfs are fantastic places to observe small, earth-sized planets; it's important for us to understand their stellar and planetary chemistry, especially if we expect to learn about what it means for a planet to be habitable, she said.
SAKHMET would be sensitive to measuring 12 different elements in 300 stars that roughly range in size from an eighth to half that of the sun.
Light or darkness only affects observations in the visible wavelengths, said Hinkel. Since we are looking at the infrared, we are not impacted by whether the sky looks dark or bright, she said. That means that we can observe 24 hours a day, said Hinkel.
SAKHMET would be a fully automated system controlled from the ground.
We would send the balloon a set of targets and their coordinates on roughly a daily basis using a datalink, said Hinkel. The on-board computer would then take that list and cycle through it until we send it an updated list, she said.
And if a given star needs to be put back into the telescope's observing queue, the team can do so.
Right now, we have 398 total measurements of elements inside of M-dwarf stars, said Hinkel. The 398 measurements were not determined from one group or one survey but are a result compiled from 20 different smaller surveys, she said. These 20 surveys used different telescopes with varying sensitivities and resolutions, said Hinkel.
In contrast, SAKHMET would measure the abundances of 12 elements in 300 stars all from one survey, with the same telescope and same abundance calculation methodology, said Hinkel.
Because red dwarfs are smaller and less massive than solar type stars, a given planet's transit depth (the amount of light blocked when a planet passes in front of its star) is also larger, thus making it easier to detect. The same principle applies when astronomers use doppler spectroscopy to detect a given star's barycentric reflex motion caused by the gravitational effects of a planet as it orbits around its parent star.
And if the physical process of planet formation is as robust as everyone suspects, then our galaxy must be loaded with rocky planets circling these small stars.
Flying a high-resolution instrument is the only way to go if we want to understand anything about M-dwarfs, said Hinkel. The better we can measure the elements in a star, the better we understand the composition of their planets, and that's important for determining potential habitability, she said.
That's because a host star's abundance of rock building elements can act as a one-to-one proxy for the exoplanet's composition.
With a proposal deadline of early April, the team won't learn if they will receive funding until at least October.
SAKHMET would be launched from McMurdo Station during the Antarctic summer of 2029 and be expected to make two full revolutions around the pole, the proposal notes.
The 30-day mission could be also extended as long as the weather, the balloon and the winds allowed.
Due to Antarctica's circumpolar winds, we have the best chance of recovering our telescope, instrument, onboard computer and then analyzing all the data that we aren't able to downlink, said Hinkel. That means we could potentially fly the instrument again, she said. Flying from any other location on earth means that we would lose the instrument and anything on board, said Hinkel.