Observing Red Dwarf Stars May Reveal Habitable “Super-Earths” Sooner

A planet orbiting a red dwarf (NASA)

OK, so if you’re an exoplanet hunter, which stars would you focus your attention on? Would you look at bright blue young stars? Or would you look at dim, long-lived red stars? If you think about it, trying to see a small exoplanet eclipse (or transit) a very bright star would be very hard, the luminosity would overwhelm any attempt at seeing a tiny planet pass in front of the star. On the other hand, observing a planet transiting a dimmer stellar object, like a red dwarf star, any transit of even the smallest planet will create a substantial decrease in luminosity. What’s more, ground-based observatories can do the work rather than depending on expensive space-based telescopes…

Red dwarfs are very common stars. They are small and burn hydrogen in their core at a much slower rate than larger stars like our Sun. As a consequence, a red dwarf’s lifespan can be in the realms of trillions of years. Generally speaking, the upper mass limit is 40% the mass of the Sun, and the lower limit is around 8% the mass of the Sun (these small stellar objects are generally believed to be the upper mass limit of proto-stars, or brown dwarfs). At most, the luminosity of red dwarfs can reach 10% the luminosity of the Sun with a maximum surface temperature of 3,500 K. Calling these stars “dim” is a fairly accurate description.

H-R diagram

They may not be very bright, but they are very useful for the search for exoplanets (i.e. planets orbiting other stars), specifically planets orbiting within the habitable zone of red dwarfs. For example, a planet twice the size of Earth, orbiting 8AU from its parent star, would cause a 5% dimming effect on the luminosity of the star as it transits when viewed from Earth. A 5% reduction in luminosity can be detected by ground-based observatories, so Jonathan Irwin and colleagues from the Harvard-Smithsonian Center for Astrophysics are carrying out a survey of 2000 M-type red dwarfs in the hope of finding habitable super-Earths.

Their survey uses 8 independent 0.4m robotic telescopes (at the Fred Lawrence Whipple Observatory, Mount Hopkins, Arizona), seeking out M5 dwarfs with a luminosity 0.5% that of our Sun. In this case, the red dwarf will have a radius of approximately 0.25 Solar radii and a mass of around 0.25 Solar masses and signs of an orbiting planet of 7 Earth masses and 2 Earth radii will be sought out. For the planet to exist within the “habitable zone” of a red dwarf, it would have to be orbiting the dim star at a distance of only 0.074 AU (and would therefore have an orbital period of 14.8 days).

But there’s a catch. It is expected that there is only a 1% probability of finding such a red dwarf with a closely orbiting super-Earth; the planet must pass in front of the star for it to be detected by instrumentation. This geometric constraint means that at least 2000 stars need to be included in the survey. Even so, the US astronomers are hopeful that the first habitable super-Earth will be discovered by ground-based telescopes before space-based missions…

Read the full arXiv paper
Original source: arXivblog.com

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