I normally think about gravitational lensing as a way of finding planets that are a long way from home. That’s just the nature of the beast: Lensing as an exoplanet detection tool depends upon a star with planets moving in front of a background object, its mass ‘bending’ space enough to cause slight changes to the image of the farther star. Monitor those changes closely enough and you may see the signature of a second disruption, flagging the presence of a planet around the closer star. Occultations like these are rare enough and more likely to be found in a crowded starfield, such as looking toward galactic center.
It’s a remarkable fact that instruments like the Hubble Space Telescope can make measurements down to 0.2 milliarcseconds, a milliarcsecond being (as this Space Telescope Science Institute news release notes) the angular width of a nickel in Honolulu when viewed from New York City. Comparable measurements, within range of the European Southern Observatory’s Very Large Telescope (Chile) and ESA’s Gaia space telescope, may be able to offer confirmation of what we find when we study two upcoming occultation opportunities involving not a distant star but the closest one.
Image: Proxima Centauri (Alpha Centauri C). Credit: NASA, ESA, K. Sahu and J. Anderson (STScI), H. Bond (STScI and Pennsylvania State University), M. Dominik (University of St. Andrews).
For STScI’s Kailash Sahu has gone to work on stars that have a high angular motion across the sky, reasoning that we might be able to find a nearby star on which to make microlensing observations. In Proxima Centauri, Sahu hit the jackpot, a star that crosses a stretch of sky with the apparent width of the full Moon every 500 years. Not one but two occultation events loom in Proxima’s near-future, the first being its passage in front of a 20th-magnitude background star in October of 2014, the second an occultation of a 19.5-magnitude star in February of 2016.
Reading about this induced one of those ‘slap yourself on the forehead’ moments when I realized how prescient it was of Sahu and team to use Hubble to plot Proxima’s trajectory, looking for precisely this kind of event. And if the instruments above can measure down to 0.2 milliarcseconds, this method could pay off, for the displacement of the background stars induced by Proxima’s mass is estimated at 0.5 milliarcseconds and 1.5 milliarcseconds respectively. If there are planets around Proxima Centauri, this method may find them.
Image: This plot shows the projected motion of the red dwarf star Proxima Centauri (green line) over the next decade, as plotted from Hubble Space Telescope observations. Because of parallax due to Earth’s motion around the Sun, the path appears scalloped. Because Proxima Centauri is the closest star to our Sun (distance, 4.2 light-years), its angular motion across the sky is relatively fast compared to much more distant background stars. This means that in 2014 and 2016 Proxima Centauri will pass in front of two background stars that are along its path. This affords astronomers a rare opportunity to study warping of space by Proxima’s gravity, as will be evident in the apparent displacement of the two stars in sky photographs. This effect is called gravitational lensing. 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. The background image shows a wider view of the region of sky in the southern constellation Centaurus that Proxima is traversing. 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).
Earlier efforts to find planets around Proxima have been intensive. Work with the UVES spectrograph at the European Southern Observatory, coupled with radial velocity studies of the star from the 1990s, helps us understand what we won’t find around Proxima. No planet of Neptune mass or larger is detectable there out to a distance of 1 AU. And we can go further: No super-Earths larger than 8.5 Earth masses have been found. Within Proxima’s habitable zone (assuming roughly 0.022 to 0.054 AU, which corresponds to an orbital period ranging from 3.6 to 13.8 days), no planets larger than 2-3 Earth masses have been detected in circular orbits.
You can see how much we leave out with this description, including planets of Earth mass or smaller in the habitable zone. And we always have to contend with the limitations of radial velocity work, which is sensitive to the orientation of the planetary system to us — in other words, we don’t know the angle at which we are observing the system, so we wind up with the still faint possibility of larger planets if we are seeing the Proxima system face-on. We do know that no transits have been found, indicative of an edge-on orientation, and the upcoming microlensing study coupled with ever more precise radial velocity measurements may help unlock the puzzle.
For more on recent Proxima Centauri work, see Endl and Kürster, “Toward Detection of Terrestrial Planets in the Habitable Zone of Our Closest Neighbor: Proxima Centauri,” Astronomy and Astrophysics, Volume 488, Issue 3, 2008, pp.1149-1153 (abstract). Kailash Sahu will be presenting his findings at the meeting of the American Astronomical Society in Indianapolis, with a paper already submitted to the Astrophysical Journal.