New research using Nasa's powerful JWST telescope has identified a planet 41 light years away which may have an atmosphere. The planet is within the "habitable zone" , the region around a star where temperatures make it possible for liquid water to exist on the surface of a rocky world. This is important because water is a key ingredient that supports the existence of life.
Authors
- Hannah Wakeford
Associate professor, University of Bristol
- Ryan MacDonald
Lecturer in Extrasolar Planets, University of St Andrews
If confirmed by further observations, this would be the first rocky, habitable zone planet that's also known to host an atmosphere. The findings come from two new studies published in the journal Astrophysical Journal Letters.
The habitable zone is partly defined by the range of temperatures generated by heat from the star. The zone is located at a distance from its star where temperatures are neither too hot nor too cold (leading to it occasionally being nicknamed "the Goldilocks zone").
But exoplanets (worlds orbiting stars outside our solar system) capable of hosting liquid water often also need an atmosphere with a sufficient greenhouse effect . The greenhouse effect generates additional heating due to absorption and emission from gases in the atmosphere and will help prevent evaporation of water into space.
Together with an international team of colleagues, we trained the largest telescope in space, Nasa's JWST , on a planet called Trappist-1 e . We wanted to determine whether this rocky world, which lies in its star's habitable zone, hosts an atmosphere. The planet is one of seven rocky worlds known to orbit a small, cool "red dwarf" star called Trappist-1.
Rocky exoplanets are everywhere in our galaxy. The discovery of abundant rocky planets in the 2010s by the Kepler and Tess space telescopes has profound implications for our place in the Universe.
Most of the rocky exoplanets we've found so far orbit red dwarf stars , which are much cooler than the Sun (typically 2500°C/4,500°F, compared to the Sun's 5,600°C/10,000°F). This isn't because planets around Sun-like stars are rare, there are just technical reasons why it is easier to find and study planets orbiting smaller stars.
Red dwarfs also offer many advantages when we seek to measure the properties of their planets. Because the stars are cooler, their habitable zones, where temperatures are favourable to liquid water, are located much closer in comparison with our solar system, because the Sun is much hotter. As such, a year for a rocky planet with the temperature of Earth that orbits a red dwarf star can be just a few days to a week compared to Earth's 365 days.
Transit method
One way to detect exoplanets is to measure the slight dimming of light when the planet transits , or passes in front of, its star. Because planets orbiting red dwarfs take less time to complete an orbit, astronomers can observe more transits in a shorter space of time, making it easier to gather data.
During a transit, astronomers can measure absorption from gases in the planet's atmosphere (if it has one). Absorption refers to the process whereby certain gases absorb light at different wavelengths, preventing it from passing through. This provides scientists with a way of detecting which gases are present in an atmosphere.
Crucially, the smaller the star, the greater the fraction of its light is blocked by a planet's atmosphere during transit. So red dwarf stars are one of the best places for us to look for the atmospheres of rocky exoplanets.
Located at a relatively close distance of 41 light years from Earth, the Trappist-1 system has attracted significant attention since its discovery in 2016. Three of the planets, Trappist-1d, Trappist-1e, and Trappist-1f (the third, fourth, and fifth planets from the star) lie within the habitable zone.
JWST has been conducting a systematic search for atmospheres on the Trappist-1 planets since 2022. The results for the three innermost planets, Trappist-1b, Trappist-1c and Trappist-1d, point to these worlds most likely being bare rocks with thin atmospheres at best. But the planets further out, which are bombarded with less radiation and energetic flares from the star, could still potentially possess atmospheres.
We observed Trappist-1e, the planet in the centre of the star's habitable zone, with JWST on four separate occasions from June-October 2023 . We immediately noticed that our data was strongly affected by what's known as "stellar contamination" from hot and cold active regions (similar to sunspots) on Trappist-1. This required a careful analysis to deal with. In the end, it took our team over a year to sift through the data and distinguish the signal coming from the star from that of the planet.
We are seeing two possible explanations for what's going on at Trappist-1e. The most exciting possibility is that the planet has a so-called secondary atmosphere containing heavy molecules such as nitrogen and methane. But the four observations we obtained aren't yet precise enough to rule out the alternative explanation of the planet being a bare rock with no atmosphere.
Should Trappist-1e indeed have an atmosphere, it will be the first time we have found an atmosphere on a rocky planet in the habitable zone of another star.
Since Trappist-1e lies firmly in the habitable zone, a thick atmosphere with a sufficient greenhouse effect could allow for liquid water on the planet's surface. To establish whether or not Trappist-1e is habitable, we will need to measure the concentrations of greenhouse gases like carbon dioxide and methane. These initial observations are an important step in that direction, but more observations with JWST will be needed to be sure if Trappist-1e has an atmosphere and, if so, to measure the concentrations of these gases.
As we speak, an additional 15 transits of Trappist-1e are underway and should be complete by the end of 2025. Our follow-up observations use a different observing strategy where we target consecutive transits of Trappist-1b (which is a bare rock) and Trappist-1e. This will allow us to use the bare rock to better "trace out" the hot and cold active regions on the star. Any excess absorption of gases seen only during Trappist-1e's transits will be uniquely caused by the planet's atmosphere.
So within the next two years, we should have a much better picture of how Trappist-1e compares to the rocky planets in our solar system.
Hannah Wakeford receives funding from UK Research and Innovation (UKRI) framework under the UK government's Horizon Europe funding guarantee for an ERC Starter Grant (grant number EP/Y006313/1).
Ryan MacDonald has recieved funding from NASA through the NASA Hubble Fellowship grant HST-HF2-51513.001, awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555.
