In the Milky Way, our beloved, cozy, golden sun is rather uncommon. The
vast majority of stars are far smaller, colder, and only have up to half the
mass of the sun. In our galaxy, many planets orbit these common dwarf
stars.
These planets would have to be extremely near to their little stars in
order to accumulate enough heat to be habitable, making them vulnerable to
strong tidal forces.
University of Florida astronomers have shown that two-thirds of the planets
near these commonplace tiny stars might be sterilized by these tidal
extremes by roasting them in a new study based on the most recent telescope
data. But that still leaves one-third of the planets—hundreds of millions
around the galaxy—that may be in an orbit that's just right for holding onto
liquid water and perhaps supporting life.
The week of May 29, research by UF astronomy professor Sarah Ballard and
doctoral candidate Sheila Sagear was published in the Proceedings of the
National Academy of Sciences. Exoplanets, or planets that orbit stars other
than the sun, have long been the subject of research by Ballard and
Sagear.
Because attention is turning to this group of stars, Sagear said, "I think
this result is really important for the next decade of exoplanet research."
These stars are great places to seek for tiny planets in orbits where it's
possible that there may be liquid water, which would make the planet
potentially livable.
A sample of more than 150 planets orbiting these Jupiter-sized M dwarf
stars were analyzed for eccentricity by Sagear and Ballard. An orbit's
eccentricity is directly correlated with how oval it is. An eccentric orbit
can cause a planet to experience tidal heating if it circles its star
sufficiently closely, at a distance similar to that of Mercury's orbit
around the sun. Friction warms the planet as it is stretched and twisted by
shifting gravitational forces on its erratic orbit. At its most extreme,
this might roast the Earth and completely eliminate any prospect of liquid
water.
Ballard claimed that only for tiny stars is the zone of habitability close
enough for these tidal pressures to be significant.
The information was gathered by NASA's Kepler telescope, which records data
on exoplanets as they pass in front of their home stars. Ballard and Sagear
concentrated particularly on how long it took the planets to cross the faces
of the stars in order to calculate the planets' orbits. New information from
the Gaia telescope, which calculated the distance to billions of stars in
the galaxy, was also used in their analysis.
"The distance is really the key piece of information we were missing before
that allows us to do this analysis now," said Sagear.
According to research by Sagear and Ballard, stars with many planets are
more likely to have the type of circular orbits necessary to support liquid
water. The likelihood of tidal extremes that might sterilize the surface was
highest for stars with only one planet.
Since one-third of the planets in this sample had orbits that might
potentially support liquid water, the Milky Way most certainly contains
hundreds of millions of intriguing prospects for the search for
extraterrestrial life.
Provided by
University of Florida
