Astrobiology Magazine contacted Kailash Sahu of the Space Telescope Science Institute, Baltimore, Maryland, with some additional questions about the research:
Astrobiology Magazine (AM): The press release mentions Hubble, Gaia and VLT. Are there any other instruments that will be used to make observations of this event?
Sahu: Some clarification first. We mentioned these telescope which we thought can be used for these observations. The idea of the paper is to alert the astronomical community about this possibility so that they can plan, request for telescope time, etc. We do have approved time with Hubble for a limited set of observations (to measure the mass of Proxima). But searching for planets needs lots of time and effort which HST alone will not be able to do, so efforts from other telescopes (such as VLT and GAIA) will be very useful. Some other facilities such as GEMINI may also be helpful.
There is one part where even smaller, 1-meter class telescopes may be able to contribute. If Proxima has a planet that comes very close to the source star, the source star may brighten due to microlensing by the planet. Since the planet's Einstein ring is small (about 8 milliarcsec for a Jupiter-mass planet), the probability of such photometric brightening is small. But the payoff would be high since the timescale of brightening provides a direct measure of the mass of the planet. Also, if Proxima has a planetary system, the probability of brightening by a planet increases. Such brightening would last anywhere between a few hours to a few of days, and can happen anytime within a period of about 100 days. So such an event may be best observed by a network of smaller telescopes.
AM: In an ideal world, where would be the best place on Earth or in orbit for astronomers to view the event?
Sahu: Proxima is a southern (declination of –62 degrees) object, so it is best observed from the southern hemisphere. Fortunately, it is a circumpolar object in places like Chile, Australia and South Africa, so it can be observed all through the year. Space based telescopes have some extra advantages of good image quality and no atmospheric scintillation which help the astrometric observations, but ground-based telescopes with adaptive optics can also be used for astrometry.
AM: Previous observations have ruled out Neptune and Jupiter sized planets around Proxima Centauri. Is there any indication yet that smaller planets may be in orbit, or is this the first chance to make the type observation needed to determine this?
Sahu: A small correction: The radial velocity observations rule out planets with (m sin i) larger than Neptune mass up to 1 AU (where i is the inclination angle). So, if there is a Neptune mass planet with an orbit that is face-on or farther away, that won't be detected by radial velocity measurements. This current method is sensitive, in principle, to detecting Earth-mass planets, but the probability of detection decreases with mass of the planet.
AM: Have there been similar conjunctions in the past where observations of other stars have been made?
Sahu: No such observations have been reported so far, although such conjunctions for some other stars have been predicted in the past.