Imagine that you’re going house-hunting, looking for an appealing place to invest in. But this is an unusual kind of property search. Very often the only part of the house you get to look at is the front door. The fact that it has one is good, and gives you confidence that there might be other house-like qualities; such as some walls, a roof, and rooms. But it’s up to you to guess at those features, to divine what the real architecture is.
Such an excruciatingly restrictive approach might be very odd for earthly real estate (and a truly bizarre bit of reality TV, “Guess the House!”) but it’s pretty much the norm for exoplanetary science.
We typically have scant information about any given stellar system and the planets it harbors. More often than not we have a single detected planet, and because of the sensitivity of our detection techniques that is usually a world on a small orbit and likely the largest of any inner objects in a system. Of course, sometimes we get lucky, and there have been systems where as many as 8 planets (Kepler-90) have revealed themselves. But that is very much the exception.
Consequently, we’re largely ignorant about the true architecture of any specific planetary system. We can of course fill in some of the blanks using our knowledge of orbital dynamics and our theories of planet formation. In some cases that has allowed us to even predict and then confirm the presence of initially unseen worlds. But for the most part we are hopelessly in the dark.
Nonetheless, if a system is interesting enough we can throw everything we have at it. Our nearest stellar neighbor, Proxima Centauri, is a good example. In 2016 radial velocity measurements of the star’s subtle wobbling motions revealed the presence of a planet of at least 1.3 Earth masses in a 11-day orbit. Since Proxima is 0.17% the luminosity of our Sun this tiny orbit actually places the planet, Proxima Cen b, in a zone where it could conceivably sustain a temperate environment (modulo many unknown factors).
But what about other planets in the system? Even in 2016 the data of stellar wobbles hinted at the possibility of further worlds, whose gravitational pull would perturb the star. Later, in 2017, data from the Atacama Large Millimeter/Submillimeter Array (ALMA) hinted at both a radio ‘blip’ and dust belts in the system that could have been sculpted by other worlds – suggesting that maybe a 2nd planet, Proxima Cen c, was lurking there. By 2018 there was data from the stellar astrometry mission Gaia also suggesting that the star was shifting in the sky in a way that could be explained if there was another, more massive planet orbiting farther out in the system.
This year, in January of 2020, over 15 years’ worth of radial velocity measurements were analyzed and used to claim the signal of Proxima Cen c; a world of at least 5.8 times the mass of Earth, in an orbit that takes some 5.2 years to circle the star. And now, incorporating those estimates into new analyses of Gaia and Hubble Space Telescope star guidance data (both probing the apparent motion of the star on the sky) other researchers have proposed both a confirmation of Proxima Cen c and suggestions of a somewhat larger mass for this world, spanning a range of ‘best bets’ between about 12 times the mass of Earth and 18 times the mass of Earth.
Intriguingly, the astrometric data also help constrain the orbital plane of planet c. If that plane is aligned with the orbital plane of the inner world (Proxima Cen b) it helps further constrain that planet’s mass, putting it somewhere in the 2 to 3 times Earth mass range.
Exactly what the Proxima Centauri planetary system looks like in the end is still up in the air. There might be further planets, and the odds of planet ‘b’ harboring a temperate surface environment might increase or decrease. But, like persistent house-shoppers, we’re managing to gradually look beyond the front door to see the full architecture of our nearest neighbor.