Newly Discovered 'Sub-Neptune' Exoplanet is Ripe for Investigation

By Robert LeaJun 10 2021

Orbiting a red dwarf in our galactic neighborhood, the small and cool exoplanet TOI-1231 b could offer astronomers a unique target for investigations of alien worlds. 

Astronomy has truly entered the era of extra-solar planet  —  or exoplanet  —  investigation. Since discovering the first exoplanet orbiting a Sun-like star in 1995, our catalog of such objects has grown exponentially. 

To date, astronomers have spotted and confirmed over 4,000 planets outside the solar system, with 1000s more candidates awaiting final verification. But, even with so many of these objects on the books, we are still discovering remarkable worlds, some with qualities that make them ripe for deeper investigation.

An international team of astronomers, including researchers from the University of New Mexico and NASA's Jet Propulsion Laboratory (JPL), have discovered such a world in orbit around a relatively nearby small red dwarf star  —  NLTT 24399. 

The exoplanet, which has been designated TOI-1231 b, is a roughly Neptune-sized planet that has been determined as having a cool atmosphere, which completes an orbit of its parent star in just 24 days.

Even though TOI 1231b is eight times closer to its star than the Earth is to the Sun, its temperature is similar to that of Earth, thanks to its cooler and less bright host star. However, the planet itself is actually larger than earth and a little bit smaller than Neptune — we could call it a sub-Neptune.

Diana Dragomir, Assistant Professor, Department of Physics and Astronomy, UNM

Perhaps the most exciting thing about this find is the opportunity it offers for further investigation. The astronomers point out that what makes TOI-1231 b a prime target for future research is the fact that it possesses a substantial atmosphere, orbits a small star, and is moving away from Earth at considerable speed. 

The team's findings will be published in a forthcoming edition of The Astronomical Journal but are already available in a pre-print paper published online¹. 

How did Astronomers spot TOI-1231 b? 

The astronomers spotted the planet in photometric data collected by NASA's Transiting Exoplanet Survey Satellite (TESS), an all-sky survey project with the primary mission of detecting exoplanets.

This initial sighting was followed up with observations made with the Planet Finder Spectrograph (PFS) on the Magellan Clay telescope located at the Las Campanas Observatory, Chile. PFS can observe exoplanets via the gravitational effect they have upon their parent star. 

This is because planets and their host stars actually orbit a mutual barycentre, which — because the stars are tremendously more massive than their planets  — is usually found much closer to the stars, or even beneath its surface. 

While this has the net effect of making the planet appear as if it is orbiting its star, it creates a tiny wobble in the star's motion which can be seen in its velocity profile. And it's this tiny effect that PFS hones in on.

TESS uses a slightly different technique to spot exoplanets. The satellite divides the Northern and Southern hemisphere into 13 sectors and surveys each of these for around 4 weeks. What TESS is searching for are planets that pass between the face of their star and its viewpoint during their orbit  —  so-called transiting exoplanets.

This transit causes a momentary 'dip' in the light output of the star, and despite being another tiny effect, it's one that has allowed TESS to spot a cornucopia of exoplanets ranging in size from massive gas giants like Jupiter to rocky super-earths.

The technique works best around stars that are similar in size to the Sun, and those that are somewhat smaller  —  low mass M dwarf stars or red dwarfs. This is because these stars have low-luminosities and thus, a change in this light output caused by a transiting exoplanet is much more extreme and is, therefore, easier to spot.

"When the star is smaller and less massive, it makes detection methods work better because the planet suddenly plays a bigger role as it stands out more easily in relation to the star," Dragomir says. "Like the shadow cast on the star. The smaller the star, the less massive the star, the more the effect of the planet can be detected."

Fortunately, this dependence isn't too much of a hindrance for TESS as this class of stars also happens to be the most common in our galaxy, accounting for around 70% of all stellar bodies in the Milky Way.

The technique used by TESS also depends on planets crossing their host star's face twice during each observation period of 28 days; this means that the satellite is much better at glimpsing planets closer to their stars as these bodies tend to have shorter orbital periods. 

As a result, most of the planets spotted by TESS thus far have orbital periods of around just 14 days. That means TOI-1231 b's longer 24-day orbit of the red dwarf NLTT 24399 makes it a special find.

Fortunately, spotting this sub-Neptune exoplanet isn't the extent of these divergent methods' investigative abilities. These techniques have also allowed the team of astronomers to determine many of the exoplanet's characteristics.

What do we know about TOI-1231 b so far?

The fact that this exoplanet is much closer in mass to its host star doesn't just make it easier to spot. The ratio in masses between the two has also enabled the team to make some important determination about its characteristics.

Alongside Jennifer Burt, a NASA JPL scientist, Dragomir measured both the mass and the radius of TOI-1231 b. Other researchers should be able to determine details about the exoplanet's atmosphere from future exoplanet observing missions such as the James Webb Space Telescope (JWST) set to launch later this year.

Staring into the atmosphere of the exoplanet should reveal its chemical composition, and TOI-1231 b stands apart from other exoplanets for which this kind of investigation has been suggested in the past as it is one of the coolest and smallest planets viable for such detailed studies yet discovered.  

This investigation could help solve a mystery that currently exists in exoplanet science. Until recently the scientific consensus had been that cool planets such as TOI-1231 b have clouds high in their atmospheres which make determining what gases swirl beneath them more tricky.

Much to the surprise of astronomers, this belief was challenged by observations of another small, cool world  —  K2–18 b  —  which showed evidence of water in its atmosphere.

As TOI-1231 b is of a similar size with similar temperatures, a deep investigation of its atmosphere could help researchers determine how common it is for planets of this nature to possess water clouds. 

One thing that the astronomers don't yet know about TOI-1231 b is just how extensive its atmosphere is. 

"TOI1231b could have a large hydrogen or hydrogen-helium atmosphere, or a denser water vapor atmosphere," Dragomir points out. "Each of these would point to a different origin, allowing astronomers to understand whether and how planets form differently around M dwarfs when compared to the planets around our Sun."

The astronomer points to study with the JWST and upcoming Hubble Space Telescope (HST) observations as the key to answering these questions. Researchers could also learn more about the exoplanet's atmosphere by closely studying light from its host star.

If an envelope of hydrogen and helium extends around the planet, it should be visible using spectroscopy. This would normally be impossible for hydrogen as the Universe's lightest element is usually obscured by interstellar gases. In this case, however, the speed at which TOI-1231 b is moving away from us doppler shifts hydrogen's characteristic absorption and emission lines enough to make them clear.

And the research team predicts that this is by far the last we've heard about TOI-1231 b. Dragomir concludes, "This planet joins the ranks of just two or three other nearby small exoplanets that will be scrutinized with every chance we get and using a wide range of telescopes, for years to come so keep an eye out for new TOI1231b developments!"

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