Red dwarfs or brown? The question relates to finding targets as the James Webb Space Telescope gets closer to launch. We’re going to want to have a well defined target list so that the JWST can be put to work right away, and part of that effort means finding candidate planets the telescope can probe. Yesterday’s white paper on a proposed search for brown dwarfs using the Spitzer Space Telescope lined up a number of reasons why these objects are good choices:
* for a given planetary equilibrium temperature, the orbit gets shorter with decreasing primary mass, increasing the probability of transit and providing 50+ occultations per year (and 50+ transits);
* the planet to brown dwarf size ratio means transiting rocky planets produce deep transits and permit the detection of planets down to Mars’ size in a single transit event when using Spitzer;
* the reliability of the detection is helped by the absence of known false astrophysical positives: brown dwarfs have very peculiar colors, small sizes, and being nearby, have a high proper motion allowing to check what is within their glare
All this is in addition to the fact that the fainter the star, the greater the contrast between the primary and the planet. But interest in red dwarfs remains high as well. Here again we are dealing with small stars where the habitable zone can be closer than the distance between Mercury and the Sun, making for easier transit detections than with G or K-class stars. Daniel Angerhausen (Rensselaer Polytechnic Institute) and team are thus proposing a project of their own called HABEBEE, for “Exploring the Habitability of Eyeball Exo-Earths.”
Eyeball? This online feature in Astrobiology Magazine lays out the background. Angerhausen knows that tidal lock will set in on a closely orbiting planet, with the night side likely covered in ice while the day side could offer, at the right orbital distance, clement conditions for life. The article cites the disputed candidate planet Gliese 581g as a possible ‘eyeball’ world, but there seems to be little need to single out such a controversial object. Planets meeting this description should be relatively common given that M-dwarfs make up 70-80 percent of all stars in the galaxy, so that it’s possible they are the most abundant locations of life.
“A little bit closer to the star — that is, hotter — they would completely thaw and become waterworlds,” Angerhausen tells Astrobiology Magazine’s Charles Choi; “[A] little bit further out in the habitable zone — that is, colder — they would become total iceballs just like Europa, but with a potential for life under the ice crust. These planets — water, eyeball or snowball — will most probably be the first habitable planets we will find and be able to characterize remotely. Thats why it is so important to study them now.”
Image: This artist’s impression shows a sunset seen from the super-Earth Gliese 667 Cc. The brightest star in the sky is the red dwarf Gliese 667 C, which is part of a triple star system. The other two more distant stars, Gliese 667 A and B appear in the sky also to the right. Finding planets in the habitable zones of red dwarfs and characterizing their atmospheres will be a major component in our search for life in the universe. Credit: ESO/L. Calçada.
There’s plenty to work with, especially given the flare situation on younger M-dwarfs, which can cause ultraviolet radiation spikes of up to 10,000 times normal levels. We have copious information about M-dwarf flares that has been gathered by observers over the years, while new observations of likely JWST candidate stars should help us characterize those more likely to host habitable planets. Radiation experiments involving the Brazilian National Synchrotron Light Source at Campinas will help the team understand the effects of radiation on ice.
The plan is to put together various models for red dwarf planets in the habitable zone that will help astronomers predict how well existing and future telescope surveys can find them. The team also hopes to travel to Antarctica to gather microbes in places that are transition zones between ice and water. They’ll use a planetary simulation chamber that was originally designed at the Brazilian Astrobiology Laboratory to mimic conditions on Mars. There the Antarctic microbes can be tested under various conditions of radiation and atmosphere to simulate M-dwarf possibilities.
So many unknowns, no matter what kind of star we home in on. I suspect that Amaury H.M.J. Triaud (Kavli Institute for Astrophysics & Space Research), who heads up the brown dwarf team, and Angerhausen himself would agree that we can’t be doctrinaire about where we look for life. Their proposals focus on brown or red dwarfs respectively not because they think these are the only possibilities for life, but because a case can be made that finding rocky worlds in the habitable zones of such stars will be quicker and the planets more easily characterized than the alternatives. The more we learn now, the better we’ll be able to use our future instruments.