Anyone who follows this site is well aware of David Kipping’s work as Principal Investigator of The Hunt for Exomoons with Kepler, which sifts through the voluminous Kepler data in search of exoplanet satellites. Now based at the Harvard-Smithsonian Center for Astrophysics (CfA), David lists a number of research interests including the study and characterization of transiting exoplanets, the development of novel detection and characterization techniques, exoplanet atmospheres, Bayesian inference, population statistics and starspot modeling. Yesterday he wrote with news that will get the attention of anyone interested in stars near the Sun. A transit search of Proxima Centauri, never before attempted, is about to begin.
By David Kipping
I wanted to let Centauri Dreams readers know that I’m leading an upcoming observing campaign with MOST this month and the mission’s PI, Jaymie Matthews, recently shared with us an important decision by the Canadian Space Agency (CSA) on May 1st which discontinues funding for the MOST (Microvariability & Oscillations of STars) space telescope as of August 2014 (click here for more). As you probably know, MOST is a suitcase sized space telescope in operation since 2003 (>10 years!). Jaymie Matthews is the Principal Investigator and he is naturally disappointed and seeking alternative ways of funding MOST to keep it going for longer. To be clear, there are no hardware failures onboard which would prevent the mission from continuing for much longer.
MOST is sometimes called the “Humble Space Telescope” and there was a running joke that it was the first astronomy telescope which weighed less than its PI! In many ways, MOST is a model for small cubesats and for example the upcoming CHEOPS mission. Unlike Kepler or COROT, this telescope is functioning very well still, so many of us are very disappointed by this decision. The cost of running MOST each year is also relatively low, at just $450K per year. I believe Jaymie is looking for ways to slim that down in efforts to fund MOST privately or via a crowd-funding platform. [We’ll track this effort as it develops - expect more soon - PG].
MOST has discovered a great deal of exciting science both in terms of stellar astrophysics and exoplanet research. Perhaps its most famous discovery was the detection of transits of 55 Cancri e, the first naked eye star with a transiting planet (and I think still the only one!). This kind of high-risk high-gain science is perfect for MOST and nothing else really fills the gap right now. [You can read about MOST and 55 Cancri e in A Super-Earth in Transit (and a SETI Digression)].
Let me also tell you about a very exciting observing campaign for MOST from May 13th-May 28th which fits right into that category. In a 15-day continuous staring run, I am leading a campaign to observe Proxima Centauri in order to search for transits. This is the first transit survey of Proxima to date, as far as we are aware, which is quite extraordinary given it is the closest star.
I was inspired by the discovery of the KOI-961 system (Muirhead et al. 2012) to propose for this target. KOI-961 is a late M-dwarf in Kepler’s field with three planets discovered by Muirhead and colleagues. By Kepler’s standards it is a very rare star since M-dwarfs usually appear too faint for Kepler. As you know though, these stars are intrinsically very common in the cosmos with M-dwarfs comprising ~75% of all stars. The three planets in question found were all tiny, sub-Earth sized (0.73, 0.78 and 0.57 Earth radii) and therefore likely very low mass, roughly 1/3 Earth mass or less based on mass-radius scaling relations for terrestrial planets.
So the key point is that current radial velocity surveys would never have seen such low-mass planets; they just don’t have the sensitivity. This all makes sense from the emerging trend that smaller stars tend to host smaller planets. If RVs can’t find a KOI-961 system of planets around Proxima (which are broadly similar stars, remember), then could transits possibly succeed?
The answer is a resounding yes – because Proxima is so small yet relatively bright at V=11, transits of a system of planets like KOI-961 would cause transits depths of 2.6 to 1.4 mmag, within the grasp of MOST. By the way, an Earth-sized planet would cause a whopping 4 mmag transit! But things get even better: KOI-961 also hosts a very compact set of planets with orbital periods of 0.45, 1.21 & 1.86 days. Being so close to their star, the transit probability of such worlds is enhanced to 11.4, 5.9 and 4.5 percent respectively. That’s not bad at all! Finally, we know that the occurrence rate of planets around M-dwarfs (although not quite as small as this star) is very high, with Dressing & Charbonneau (2013) for example estimating ~1 planet per M-dwarf. On this basis, we argue our chances of success are around 10%.
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).
So yes, our chance of success is just 10%, a modest but respectable figure. Yet this probability should be weighed against the potential reward if we succeed. Just think about the possibilities of not only our nearest star having a planet but the unprecedented opportunities for following-up a bright, tiny M-dwarf hosting a transiting rocky planet(s). Any planet found would become everyone’s favorite overnight and JWST would be able to smell the atmosphere quite easily.
But the most compelling reason of all to look for a planet around Proxima is that such a world may provide the impetus needed to build the first interstellar space craft – we could fly there within our lifetimes and send back a photo.
If you need any more icing on the cake, a planet receiving the same insolation as the Earth around Proxima would have an orbital period of about 8.7 days and so our 15-day campaign should see a transit like that too.
Anyway – I wanted to share with you the news about MOST and the exciting observations coming up, which I think highlight the unique opportunities MOST is able to pursue.