When looking for exoplanets, size plays a role, but so does weight. The larger and heavier the planet, the more likely they will be discovered by current telescopes. Both the techniques for finding exoplanets and the telescopes that use these techniques are aimed at larger, heavier planets. So if even the current telescopes manage to find one that is about half the mass of Venus, that is cause for celebration. The same size is the planet that a team from the Very Large Telescope at the European Southern Observatory found orbiting a star named L98-59.
Known as L98-59b, it is not the smallest exoplanet ever discovered. This title seems to be held by Kepler-37b, whose size is roughly between the Moon and Mercury. But Kepler-37b was detected using a different technique than L98-50b, the lightest planet so far detected using the “radial velocity” technique of exoplanet detection.
NASA Goddard video describing the L98-59 system.
The radial velocity technique is based on the fact that a planet pulls on its star and the star pulls on its planet. A star “wobbles” when a world pulls it in one direction or the other. Modern telescopes can detect this wobble for most large, massive planets that have a greater influence of gravity on their host star.
On the other hand, relatively light planets like L98-59b don’t have as much gravitational pull on their host star, making them more difficult to spot using this method. Also, if their low mass matches their small size, they may be difficult to detect using the “transit” method – another popular exoplanet detection technique that looks for star dropouts that a planet might cause -B.
Infographic comparing the L 98-59 system with our own inner solar system.
Credit – ESO / L. Sidewalk / M. Kornmesser (thanks: O. Demangeon)
This technique was used by TESS as early as 2019 to discover three planets in the L98-59 system. This new radial velocity research found an additional fourth planet. L98-59b was actually one of the three planets first found by TESS, but it is difficult to determine the masses of the exoplanets found via the transit method.
This is where ESPRESSO comes in. ESPRESSO is the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations – an instrument at the VLT. It is not only a specialist in radial transits, but can also estimate the water content on an exoplanet. The first three planets of L 98-59 were good candidates for this practice as ESPRESSO found that the first two (including L 98-59b) had some water in the atmosphere, while the third contained water which was 30% of its mass . Therefore, the third planet is classified as the “ocean world”.
ESO video of the L98-59 system.
Credit – ESO’s YouTube Channel
But that’s not the only surprise from ESPRESSO’s view of the star system. It also found a fifth planetary candidate that happens to be in the star’s habitable zones. The researchers were unable to confirm the existence of the planet in their data set, but other scientists will no doubt adjust to the system soon. Even if the planetary candidate turns out not to exist, you will get some observation time on one of the easiest planets known to date. Hopefully they’ll get a lot of new telescopes soon.
Learn more:
ESO – New ESO observations show that the rocky exoplanet is only half the mass of Venus
Astronomy & Astrophysics – Warm terrestrial planet with half the mass of Venus in transit of a nearby star
Astronomy & Astrophysics – Characterization of the multiplanet system L 98-59 with HARPS
EarthSky – an inner solar system similar to ours, 35 light years away
Mission statement:
Artist’s impression of L 95-59.
Credit – ESO / M. Grain Knife
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