Space update special: the 7-exoplanet system

On Wednesday, February 22nd, US space agency NASA, working with a team of European astronomers, confirmed no fewer than seven extra-solar planets are orbiting a star some 39 light years away – with three of them within the so-called “Goldilocks zone” of habitability.

The star in question is TRAPPIST-1, named for the instruments used in its discovery, the Transiting Planets and Planetesimals Small Telescope (TRAPPIST),  and more formally known as 2MASS J23062928-0502285. Regular readers of my Space Sunday column might remember that I referred to the system back in November 2016, whilst discussing the James Webb Space Telescope and the hunt of exoplanets. The NASA announcement, which coincides with the publication of a new paper by the TRAPPIST team, adds dramatic new information to the distant star system.

The first two of the planets orbiting the star were located in May 2016, after the TRAPPIST team had studied the results of a continuous series of observations of the star between September and December 2015 using the telescope, located at the European Southern Observatory’s (ESO) La Silla Observatory in Chile.

What was intriguing about the two world was that not only were both within the so-called “Goldilocks zone” of their parent planet, where conditions might be “just right” for life to start, but both were roughly comparable to Earth in size, and therefore likely solid bodies, and spectral analysis suggested both have atmospheres.

A third planet, TRAPPIST-1d was also discovered the the same time, but it was behaving oddly. This prompted a further extended period of observation between September and October 2016, using both the ESO’s ground-based Very Large Telescope, and the Spitzer Space Telescope. This work revealed at “TRAPPIST-1d” was not one, but three worlds, again, all roughly in the Earth-sized category. Spitzer’s data additionally revealed two more planets of roughly the same size, taking the total to seven. Following this, Hubble turned its attention on the planets, looking for signs of hydrogen and helium – the chemical signatures that would indicate if any of them might be gas giants. It found none, further confirming they are likely rocky in nature.

The size, mass and density of these telluric worlds were obtained by measuring the periodic dips in TRAPPIST-1’s luminosity as a result of each of the planets passing in front of it. This allowed the international team studying the system to further assess whether each world was rocky, icy, or gaseous and determine which might be habitable.

TRAPPIST-1 is an ultra-cool red dwarf star only slightly larger than the planet Jupiter, and about 2,000 times dimmer than the sun.

Such stars, designated Class M, are the most frequent type of star in the Universe – making up an estimated 70% of stars in our galaxy alone. However, they do not radiate energy like our own sun, instead they are very volatile; all activity within them is entirely convective in nature, giving rise to massive stellar flares.

Given TRAPPIST-1 is so small, all of its planets orbit in very close proximity to it – closer than Mercury is to the Sun (the nearest orbits its parent star once every 1.5 terrestrial days, and the outermost, about once every 20 terrestrial days). This makes them very vulnerable to violent outbursts by the star, and could affect their surface conditions and their ability to retain an atmosphere.

This close proximity also means all of the planets are tidally locked – they always have the same side facing their sun. Thus they all are likely to have extremes of temperature, and those with an atmosphere are likely to have quite extreme weather as well. However – and conversely – it also means they could have the potential for liquid water to exist on their surfaces.

The innermost of the three planets in the habitable zone, TRAPPIST-1e, is very close in size to Earth, and receives about the same amount of light as Earth does, and may well have similar day time temperatures. The middle planet of the three, TRAPPIST-1f, meanwhile, might be a water rich world, also roughly the same size as Earth. It has a 9-day orbit, and receives about the same amount of light from its sun as Mars does from our own.

The outermost of the habitable zone planets is TRAPPIST-1g. With a radius that is 13% larger than that of Earth, it is the largest planet in the system, and receives about the same amount of light as a body positioned between Mars and the Asteroid Belt would. Between these three exoplanets, and the four others in the system, astronomers now have a multiple candidates within the same star system to study what potentially habitable worlds might look like.

One further aspect of the planets being in close proximity to their parent sun is that they would make one another’s night skies interesting, as when above the horizon, they would all appear roughly the size of the Moon as seen from Earth. Another interesting aspect is that the six inner planets are in near-resonance, meaning their orbital periods are related to each other by a ratio of two small integers. This arrangement suggests that the worlds formed farther out in the system, where water was likely plentiful, and then migrated to their current positions.

“The seven wonders of TRAPPIST-1 are the first Earth-size planets that have been found orbiting this kind of star,” said Michael Gillon, lead author of the paper announcing the findings of the ESO team. Based at the University of Liege, Belgium, his is the principal investigator for the TRAPPIST exoplanet survey. “It is also the best target yet for studying the atmospheres of potentially habitable, Earth-size worlds,” he added.

“This is the most exciting result I have seen in the fourteen years of Spitzer operations,” Sean Carey, manager of NASA’s Spitzer Science Centre at Caltech/IPAC in Pasadena, California, added. “Spitzer will follow-up in the fall to further refine our understanding of these planets.”

As well as Spitzer, NASA is already using the Kepler space telescope to study the system, conducting measurements of minuscule changes in the star’s brightness due to transiting planets, and thus gathering further information about them. The system is also now a prime target for observation by the James Webb Space Telescope (JWST) when that enters service in 2019. JWST will be able to detect the chemical fingerprints of water, methane, oxygen, ozone, and other components of a planet’s atmosphere, and will analyse the planets’ temperatures and surface pressures – key factors in assessing their habitability.