Proxima Centauri
Credit: NASA, ESA, K. Sahu and J. Anderson (STScI), H. Bond (STScI and Pennsylvania State University), M. Dominik (University of St. Andrews), and Digitized Sky Survey (STScI/AURA/UKSTU/AAO). Release Images
The nearest star to our Sun, the red dwarf Proxima Centauri, is on a
course for a rare conjunction with two distant background stars. This
alignment will offer astronomers a unique opportunity to look for
planets orbiting close to Proxima Centauri. In addition, astronomers
will be able to precisely measure the mass of this isolated red dwarf.
Observations from NASA's Hubble Space Telescope were used to plot the
star's trajectory across the sky with enough precision that
astronomers can predict two "close encounters" of the stellar kind with
Proxima Centauri. The red dwarf passes nearly in front of a
20th-magnitude background star in October 2014 and a 19.5-magnitude star
in February 2016.
The warping of space by Proxima Centauri's gravitational field will
cause the image of each star to be very slightly offset from their real
positions on the sky. The amount of offset can be used to measure
Proxima Centauri's mass — the greater the offset, the greater the mass
of Proxima. If Proxima Centauri has a planet, it may produce a small
second position shift due to the companion planet's gravitational
field.
The stars will shift very slightly in their apparent position, an
estimated 0.5 milliarcsecond and 1.5 milliarcseconds, respectively. (A
milliarcsecond is the angular width of a nickel in Honolulu, Hawaii, as
viewed from the distance of New York City.) Hubble can make measurements
as small as 0.2 milliarcsecond. The European Space Agency's Gaia space
telescope and the European Southern Observatory's Very Large Telescope
on Mt. Cerro Paranal in Chile may be able to make comparable
measurements. These so-called microlensing events will last from a few
hours to a few days.
Because Proxima Centauri is so close to Earth, the area of sky warped
by its gravitation field is larger than for more-distant stars, and
this makes the observations easier to look for shifts in apparent
stellar position caused by this effect.
So far all attempts to detect planets around Proxima Centauri have
been unsuccessful. Radial velocity and astrometric observations, which
can measure the gravitational wobble of a star due to the pull of an
unseen companion, have not detected the presence of planets.
Astrometric observations rule out giant planets with anything larger
than 80 percent of Jupiter's mass with a period of less than 1,000
days. Likewise, radial velocity observations exclude the presence of any
planet the mass of Neptune or larger within an orbital radius equal to
Earth's orbit (assuming the orbit is not face-on to Earth or similarly
inclined at a steep angle). Nor have planets been found transiting
across the face of the star, which would happen if the planet's orbit
were edge-on to Earth. But now, by looking for microlensing effects
during these rare stellar alignments, smaller terrestrial planets
around Proxima Centauri could be detected.
A team lead by Kailash Sahu of the Space Science Telescope Institute,
Baltimore, Md., first searched 5,000 stars in the Luyten Half-Second
catalogue of stars that have a high rate of angular motion across the
sky for possible alignment events. "Proxima Centauri's trajectory
turned out to offer one of the most interesting opportunities because
of its extremely close passage to the two stars," says Sahu. Because it
is so close to Earth, Proxima Centauri also moves across the sky at a
comparably high angular rate. It crosses a piece of sky with the
apparent width of the full Moon every 500 years.
Red dwarfs are the most common class of stars in our Milky Way galaxy
— any such star that has ever been born is still shining today,
because their lifetimes are longer than the age of the universe.
Therefore, getting a precise determination of mass is critical to
understanding a star's temperature, diameter, intrinsic brightness, and
longevity.
For every star like our Sun, there are approximately 10 red dwarfs in
space. Because lower-mass stars tend to have smaller planets, red
dwarfs are ideal places to go hunting for Earth-sized planets.
At a distance of 4.2 light-years from Earth, Proxima Centauri is just
0.2 light-years from the more-distant binary star Alpha and Beta
Centauri. These three stars are considered part of the triple-star
system, though Proxima Centauri evolved in isolation from the two
Sun-like companion stars.
This paper has been submitted to the Astrophysical Journal and Dr.
Sahu is presenting these results at the meeting of the American
Astronomical Society in Indianapolis, Indiana.
CONTACT
Ray VillardSpace Telescope Science Institute, Baltimore, Md.
410-338-4514
villard@stsci.edu
Kailash Sahu
Space Telescope Science Institute, Baltimore, Md.
410-338-4930
ksahu@stsci.edu
