The discovery of the TRAPPIST-1 exo-planetary system is a truly remarkable event in science. It is an exemplar of how an armada of telescopes – some located on the ground and some in space, and collectively owned by a league of nations – can work together to chart new frontiers in the cosmos. Seven Earth-sized planets have been found in resonant orbits around a faint star much smaller than the Sun. A planetary system has been revealed that closer in size to that of the Galilean satellites orbiting Jupiter than that of the terrestrial planets orbiting the Sun. Simply a stunning result!
This diagram compares the sizes of the planets around the faint red star TRAPPIST-1 with the Galilean moons of Jupiter and the inner Solar System. All the new planets found are of similar size to the Earth. Credit: ESO/O. Furtak.
Comparison of the orbits of the planets around TRAPPIST-1 with the Galilean moons of Jupiter and the inner Solar System. All the new planets orbit much closer to their host star than Mercury does to the Sun. However, as their host star is far fainter, they are exposed to similar levels of irradiation as Venus, Earth and Mars receive in our Solar System. The temperature may be right for liquid water to exist on the surface of some of them. Credit: ESO/O. Furtak
It is quite remarkable, and completely unexplained, how similar in size all the planets are. And while they are all very close to their host star, so faint is it in comparison to the Sun, that it is quite possible that water could exist in liquid form on several of their surfaces. Whether it does, however, is currently unknown. It is a matter for speculation. We simply know that the irradiation is about right to ensure that temperature range could be between freezing and boiling on the planetary surface(s). Many other factors will determine whether liquid water does actually exist. For instance, volcanic activity driven by tidal forces, with the planets locked in orbits with one side permanently facing the host star, and the other away from it, as with Io orbiting Jupiter, might render the surface uninhabitable. On the other hand, tidal forces might be sufficient to melt ice, but not rock, resulting in surfaces covered with floating ice on oceans, as with Europa. We do not yet know, though you can be sure that TRAPPIST-1 will now be the subject of intense scrutiny with new telescopes and new instruments trained on it as they come online.
The decoding of the suite of observations made over several years of TRAPPIST-1 to reveal its planetary system is worthy of special note in the methodology of science. The tiny, and seemingly erratic variations in the brightness of the light being measured (see figure below) by the telescopes has been unscrambled to yield a consistent picture of the planets and their orbits and sizes. A triumph of human deduction and reasoning!
This diagram shows the changing brightness of TRAPPIST-1 over a period of 20 days, as measured by NASA’s Spitzer Space Telescope and many other telescopes on the ground. On many occasions the brightness of the star drops for a short period and then returns to normal. These events, called transits, are due to one or more of the star’s seven planets passing in front of the star and blocking some of its light. The lower part of the diagram shows which of the system’s planets are responsible for the transits. Credit: ESO/M. Gillon et al.
On one remarkable day in December 2015 three of the planets were transiting the disc of TRAPPIST-1 at the same time. The small dips and rises in the light curve, as they successively passed across and out of the disc, tightly constrain the sizes and orbital properties of the planets. How exhilarating it must have been for the astronomers when they first saw this data and realized what it was telling them. A true Eureka moment of discovery in science! This is what scientists live for!
This plot shows the varying brightness TRAPPIST-1 during an unusual triple transit event on 11 December 2015. As the star was monitored on ESO’s Very Large Telescope three planets passed across the disc of the star, each causing some of its light to be blocked. This light curve shows for the first time three temperate Earth-sized planets, two of them in the habitable zone, passing in front of their star – a truly remarkable event! Credit: ESO/M. Gillon et al.
We are all excited by the discovery. With the possibility that liquid water might exist on the surfaces on some of the planets, interest in the possibility of life existing there too will be intense. However, for now this is just speculation. Let us enjoy the moment of discovery of a fascinating new solar system, the lessons it provides about the cosmos and the journey of discovery that is now going to commence. This is science in action!
Article written by: Professor Michael Burton, Director of Armagh Observatory and Planetarium