Close encounter could reveal planets

NASA’s Hubble Space Telescope will have two opportunities in the next few years to hunt for Earth-sized planets around the red dwarf star Proxima Centauri. The opportunities will occur in October 2014 and February 2016 when Proxima Centauri, the star nearest to our Solar System, passes in front of two other stars. Astronomers plotted Proxima Centauri’s precise path and predicted the two close encounters using data from Hubble.

Red dwarfs are the most common class of stars in our Milky Way galaxy; there are about 10 for every star like our Sun. Red dwarfs are less massive than other stars, and because lower-mass stars tend to have smaller planets, they are ideal places to go hunting for Earth-sized planets.

Previous attempts to detect planets circling Proxima Centauri have not been successful. But astronomers believe they may be able to detect smaller Earth-sized planets, if they exist, by looking for ‘microlensing’ effects during the two rare stellar alignments.

The projected motion of the red dwarf star Proxima Centauri

The projected motion of the red dwarf star Proxima Centauri (green line) over the next decade, as plotted from Hubble Space Telescope observations (the path appears looped due to Earth’s motion around the Sun. In 2014 and 2016 Proxima Centauri will pass almost in front of two background stars, affording astronomers a rare opportunity to study the warping of space by Proxima’s gravity. The amount of warping will be used to calculate a precise mass for Proxima Centauri and look for the gravitational footprint and any planets orbiting the star. 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)

Microlensing occurs when a foreground star (the ‘lens’) passes close to our line of sight to a more distant background star (the ‘source’). The appearance of the background star may be distorted, brightened and multiplied depending on the alignment between the foreground lens and the background source.

These microlensing events, which range in duration from a few hours to a few days, will enable astronomers to precisely measure the mass of Proxima Centauri. Getting a precise determination of mass is critical to understanding a star’s temperature, diameter, intrinsic brightness and longevity.

Astronomers will measure the mass by examining images of each of the background stars to see how far the stars appear to be shifted from their real positions in the sky. The shifts will be the result of Proxima Centauri’s gravitational field warping space. The degree of shift can be used to measure Proxima Centauri’s mass; the greater the shift, the greater the mass. If the red dwarf has any planets, their gravitational fields will produce a second small position shift.

Diagram explaining microlensing as Proxima Centauri appears to pass close to a background star

The upcoming conjunction between the nearest star to our Sun, Proxima Centauri, and a distant background star. Proxima’s gravitational field distorts space like a funhouse mirror and bends the path of light from the background star. The result is that the apparent position of the star will shift slightly during Proxima Centauri’s passage, as seen in the upper right diagram. If an unseen planet is orbiting Proxima Centauri, the star’s apparent position will be further offset, as seen at lower right. Credit: A. Feild (STScI)

At a distance of 4.2 light-years from Earth, Proxima Centauri is just 0.2 light-year 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.

Because Proxima Centauri is so close to Earth, the area of sky warped by its gravitation field is larger than for more distant stars. This makes it easier to look for shifts in apparent stellar position caused by this effect. However, the position shifts will be too small to be perceived by any but the most sensitive telescopes in space and on the ground. The European Space Agency’s Gaia space telescope (due for launch later this year) and the European Southern Observatory’s Very Large Telescope in Chile might be able to make measurements comparable to Hubble’s.

Adapted from information issued by NASA and the Space Telescope Science Institute.

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