A Treasure Trove of Planets

NASA has just announced that another four planets have been discovered in the TRAPPIST-1 solar system, putting its total number planets at seven. TRAPPIST-1 isn’t the first star known to have multiple planets around it (our own Sun is case in point). But what makes this discovery special is the fact that all seven exoplanets seem to be Earth-sized, and likely to possess rocky surfaces.

These discoveries were made by a series of telescopes and teams. The first three planets were found in 2016 by the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile. Follow-up observations were conducted by the European Southern Observatory’s Very Large Telescope and the Spitzer Space Telescope, among others, which led to the discovery of the remaining four planets in the system. This discovery was published in the journal Nature following NASA’s press conference on 22 February 2017.

Finding evidence of life elsewhere in the universe is a holy grail quest in space exploration. The rocky surfaces and Earth-like sizes calculated using the Spitzer Space Telescope data raises many questions about whether these planets can support life.

At the moment, it is far too early to speculate on the habitability of TRAPPIST-1, however there are some promising features. At just 40 light-years away from the Earth, the system is close enough that we may be able to measure details of the planets’ atmospheres.

The Hubble Space Telescope has already made a start on this, looking for signs of hydrogen and helium gas, which are typically found in the atmospheres of gas giant planets. Two of the TRAPPIST-1 planets were analysed with the Wide Field Camera 3 on the telescope, focusing on the near-infrared light to study the chemical makeup of their atmospheres. Hubble found no significant levels of hydrogen or helium gas, which supports the idea that these planets are rocky.

The discovery of these new exoplanets sets the record for the largest number of Earth-sized planets found around a single star. And with three of the seven planets firmly within the so-called habitable zone, it also breaks the record for the largest number of habitable-zone planets found around a star. The habitable zone is often referred to as the ‘Goldilocks zone’ and is the region around a star where the temperature on the planet allows liquid water to exist. It is neither too hot or too cold. Here on Earth liquid water is vital for life to survive, so we naturally seek its presence on other worlds as we search for life.

We must reiterate that this discovery doesn’t confirm the presence of alien life or even liquid water, but that the conditions on the TRAPPIST-1 worlds make this possible. Intriguingly, the TRAPPIST-1 star is a rather small, relatively cool star called an ultra-cool dwarf. This has led some to speculate that perhaps liquid water could exist on all seven of its planets. Until further investigations are done, we won’t know for sure.

Dr. Matt Burleigh based at the University of Leicester was co-author on the paper detailing the discovery of additional planets around TRAPPIST-1. Alongside his PhD student, Alex Chaushev, they observed the system in 2016 from a telescope in South Africa. At the time they were attempting to measure the transit of one of the known planets in the system. However, they noticed unusually that the transit did not occur as predicted, which triggered the decision to investigate the system further.

Dr Burleigh said ‘Thanks to a whole programme of observations with different telescopes and the NASA Spitzer satellite over the last few months, we now know that there are at least seven planets in the system, all roughly the size of Earth. The reason we didn’t see our expected transit from South Africa, was because we were confusing different planets with each other, and getting their orbits all wrong.’

He added, ‘Planets which are likely made of rock have also been found before of course, including in their stars’ habitable zones, like the planet around our nearest star, Proxima Centauri b. The excitement with TRAPPIST-1 is the sheer number of rocky planets, with three in the habitable zone.’