Space Sunday: exoplanets, exocomets and Titan’s craters

K2-18b as it might appear in orbit around its red dwarf parent star and with the other known planet in the system – K2-18c – also visible. Credit: ESA/Hubble, M. Kornmesser

I first wrote about K2-18, a red dwarf star some 11 light-years from Earth, and its two companion planets in December 2017. At that time, the outermost of the two planets, called K2-18b or EPIC 201912552 b and discovered in 2015, was the subject of a study to determine its mass in an attempt to better understand the planet’s possible atmospheric properties and bulk composition. This was of particular interest to scientists as K2-18b lay within its parent star’s habitable zone – where liquid water might exist on the planet’s surface.

That study ultimately revealed K2-18b has a mass of around 8 times that of Earth, putting it in the “super-Earth” category of rocky worlds, with a diameter roughly 2.3 times greater than Earth’s (see: Space Sunday: Exoplanets Update). Since then, K2-18b has continued to be the subject of study – and it has now become the first exoplanet thus far discovered confirmed to have water vapour, mostly likely liquid water clouds, within its atmosphere.

The news came via two independent studies that have been carried out using the data gathered by the Hubble Space Telescope (HST). The first study, written by the team who originally gathered the data, appeared on September 10th, 2019 on, but has not been peer-reviewed. The second study – which has been peer-reviewed – appeared in the September 11th edition of Nature Astronomy.

The team responsible for gathering the data – led by Björn Benneke, a professor at the Institute for Research on Exoplanets,  Université de Montréal – did so after applying to use Hubble to observe K2-18b shortly after its discovery. They were ultimately granted telescope time in in 2016 and 2017, using Hubble to gather data in the light from the red dwarf star, and how that light changed under the influence of any atmosphere surrounding K2-18b as it transited in front of the star. Spectrographic analysis of the data confirmed the planet has a fairly dense atmosphere rich in hydrogen and helium – and which also contains the molecular signature of water.

Researchers gathered data on K2-18b’s atmosphere by using the Hubble Space Telescope to observe changes in the light from its parent star as it passed through the planet’s atmosphere during transits. Credit: NASA / ESA simulation

After gathering the data, Benneke’s team wanted time to carry out further observations to both confirm what they had found and make additional discoveries. In the meantime, their findings were available for others to study – which is exactly what a team led by Dr. Angelos Tsiaras based at the University College London (UCL), UK did.

Using independent means of analysing the data, both teams reached the same overall conclusions concerning the major finds within K2-18b’s atmosphere – although they come to different conclusions as to the planet’s likely form. The UCL specify K2-18b as a rocky planet with a dense atmosphere, between 0.01% and 50% of which is water vapour. By comparison, the amount of water vapour in our atmosphere is put at between 0.1% and 4% – so, K2-18b could have anything from a comparable amount of water vapour in its atmosphere to Earth through to being a completely flooded world.

By contrast, Benneke’s team believe the planet is more of a “mini-Neptune”: a planet with a small, solid core surrounded with a thick atmosphere that is predominantly hydrogen / helium in nature, with only trace amounts of water vapour – albeit enough to create liquid water clouds, and possibly even rain. However, the idea that the planet is a mini-Neptune is somewhat at odds with other findings about the planet – such as the December 2017 study.

There is also some tension between the two teams. While Benneke acknowledges his team’s research was open to others to use, he is somewhat aggrieved the UCL team did not bother to contact him or his team concerning their work or their intentions. However, he also sees the results of the UCL’s work as positive in respect to understanding the nature of K2-18b.

The presence of liquid water in the planet’s atmosphere doesn’t automatically mean it is home to life. There are some significant issues around this. For one thing, while the plant is within the habitable zone, the precise surface temperature has yet to be determined, and could range from -73ºC  to +47ºC (-100ºF and +116ºF), meaning it could be colder or hotter than the coldest / hottest places on Earth.

There’s also the fact that the planet is so close to its parent, orbiting once every 33 days, that it is likely tidally-locked with its star. This means one side of the planet will be in perpetual sunlight, and the other in perpetual darkness – something that could well give rise to extreme weather conditions. Finally, there’s the fact that K2-18 is a red dwarf star. These, as I’ve noted before, can be exceptionally violent, and flares and coronal mass ejections from the star are likely to both expose the planet to high levels of radiation and could strip away its atmosphere over time, although it is possible K2-18b’s atmosphere might be dense enough to help it withstand at least some of this stripping away.

Finding water on a potentially habitable world other than Earth is incredibly exciting. K2-18b is not ‘Earth 2.0’ as it is significantly heavier and has a different atmospheric composition. However, it brings us closer to answering the fundamental question: Is the Earth unique?

– Dr. Angelos Tsiaras (UCL Centre for Space Exochemistry Data)
co-author of the UCL study on K2-18b

The next phases in studying K2-18b will likely come in the mid-to-late 2020s. Benneke and his team are already planning to continue their work using NASA’s James Webb telescope, due to be launched in 2021, while Giovanna Tinetti, a member of the UCL team studying K2-18b also happens to be the Principal Investigator for Europe’s Atmospheric Remote-sensing Infra-red Exoplanet Large-survey (ARIEL). She has already indicated the planet will be target for study by that mission when it launches in 2028.

Extra-Solar Comet Imaged

In 2017, astronomers were fascinated by the identification of an extra-solar object dubbed `Oumuamua (officially referred to as 1I/2017 U1), as it swept around the Sun before heading back out of the solar system. Estimated to be 400 metres (1312 ft) in length and approximately 40-50 metres (130-162.5 ft) across, the object was believed to be an extra-solar asteroid or cometary fragment – although some actually suggested it may be of intelligent design.

Study of `Oumuamua, about which you can read more here (October 2017), here (November 2017) and here and here (both November 2018), revealed – among other things – that such extra-solar visitors might not be that uncommon. In proof of this, astronomers have now captured an image of an active extra-solar comet as it approaches the Sun.

Gemini Observatory two-colour composite image of C/2019 Q4 (2I/Borisov), the first interstellar comet ever identified. Credit: Travis Rector / Gemini Observatory

The object – denoted C/2019 Q4 (or 2I/Borisov, the “i” meaning “interstellar”) – was first observed by Russian amateur astronomer Gennady Borisov on August 30th, 2019. His observations were quickly confirmed, and data on the object revealed it to be travelling with a hyperbolic excess velocity of about 34 km per second (21.25 miles per second – or London to New York in under 3 minutes), clearly making it extra-terrestrial in origin. The fact that it has yet to pass around the Sun means that astronomers will have more time to observe its passage through the inner solar system than was the case with `Oumuamua, making it a prime target for study.

The first image of the object to be obtained came from the Gemini Multi-Object Spectrograph on the Gemini North Telescope on Hawaii’s Mauna Kea, it reveals C/2019 Q4 (2I/Borisov) to be a classic cometary object, outgassing a corona of material around itself as it is heated by the Sun during its approach. It marks first time an interstellar visitor to our Solar System has clearly shown a tail due to such out-gassing – while changes in `Oumuamua’s velocity away from the Sun were indicative of such activity, none was actually imaged.

C/2019 Q4’s progress through the inner solar system. Credit: NASA/JPL

C/2019 Q4’s trajectory shows it to have come from the direction of Perseus and Cassiopeia, and very close to the galactic plane. Currently, it is a northern hemisphere object but as it continues towards and through perihelion – which will be around 2 AU from the Sun in early December 2019 (and also the closest it will get to Earth) – it will become a southern hemisphere object, and will continue to be so as it heads back out of the solar system.

All told, C/2019 Q4 should remain observable through many telescopes through until April 2020, after which it will only be observable with larger professional telescopes until around October 2020. Currently, observatories around the world have been turning their attention towards the object in order to gather as much information on it as possible, including the size of the nucleus and its rotation. Initial observations put C/2019 Q4 at somewhere between 2 and 16km (1.6 and 10 mi) in diameter.

Titan’s Lakes: Explosion Craters?

Titan is the only planetary body in our solar system outside of Earth that has liquid lakes and rivers on its surface. However, rather than being filled with water, Titan’s liquid bodies are filled with methane and ethane kept in a liquid form thanks to Titan’s frigid climate.

These lakes fall broadly into two categories: very large bodies that form massive depressions in Titan’s surface, and much smaller bodies – in the order of tens of kilometres across – that have rims that rise well above the surrounding plains. The very large lakes demonstrate all the signs of having been formed by karstification. In short, liquid methane in the atmosphere acts as a very weak acid, dissolving the moon’s bedrock of ice and solid organic compounds to form depressions and / or possibly collapsing the surface over subterranean caverns, which then fill with the liquid methane / ethane.

An artist’s impression of one of Titan’s larger lakes as it might appear under almost normal lighting conditions on the moon. These large lakes were most likely the result of karstification. Credit: NASA

However, the smaller lakes with their high rims do not fit this model of formation, as karst lakes cannot have raised rims. Instead, it had been suggested that they were the result of impact craters being flooded over time. However, a study published Nature Geosciences suggests an alternative model.

Established models of Titan’s development suggest that the moon’s atmosphere has fluctuated over the aeons between “cold” and “warm” periods (both being relatively subjective given Titan’s “warm” periods are massively cold by Earth standards). During the “cold” periods, nitrogen dominates the atmosphere, falling as “snow” to pool in depressions that over time become covered by an ice/dust crust. During the “warmer” periods, methane dominates the atmosphere, acting as a greenhouse gas.

Using these models as a baseline and working with data gathered by the Cassini mission’s final passage by Titan shortly before that mission came to an end, the new study reveals that the “methane warming” is sufficient enough to cause subsurface pools of liquid nitrogen to explosively vaporise.  The result is a crater that subsequently fills with liquid methane.

This is a completely different explanation for the steep rims around those small lakes, which has been a tremendous puzzle. As scientists continue to mine the treasure trove of Cassini data, we’ll keep putting more and more pieces of the puzzle together. Over the next decades, we will come to understand the Saturn system better and better.

– Cassini Project Scientist Linda Spilker, JPL

Space Pictures of the Week

A Shadow on Jupiter

It might look like a still captured towards the end of 2010: The Year We Make Contact, but this is not alien monoliths multiplying across the face of Jupiter; it is the shadow of the Galilean Moon Io captured by NASA’s Juno mission during Perijove 22 on September 12th, 2019. It is an approximate true colour/contrast image. Credit: NASA/JPL / Björn Jónsson

Cassini’s Last Full View of Saturn

This colour composite shows Cassini’s final full view of Saturn prior to the end of that mission. It comprises 11 colour composite images of Saturn taken by the probe using its visible light filters on September 13th, 2017. Two days later, its fuel reserves expended after a 13-year mission, the vehicle entered Saturn’s upper atmosphere and burned up. Credit: NASA/JPL via Jason Major


2 thoughts on “Space Sunday: exoplanets, exocomets and Titan’s craters

  1. “mass of around 8 times that of Earth”

    Imagine going on holiday on that planet. Just about 2 weeks should be plenty enough to build some serious muscles and lose fat and return fit as a pair of sneakers. 😉


    1. A human standing on its surface (if it has a surface) would feel around 35% heavier than they do on Earth, because the planet’s mass is offset by its much larger diameter (2.3 times that of Earth). The real problem for any talk to walk on the planet’s surface (again, assuming it has one) would be atmospheric pressure: it is probably great enough to immediately crush any human setting foot on K2-18b.


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