Researchers announce smallest exoplanet discovered to date

Most of the exoplanets we have discovered have been identified by large surveys such as the Kepler mission or the Transiting Exoplanet Survey Satellite (TESS). While these projects are great for spotting stars that host planets, they mostly tell us that the planets are there. Understanding an exosolar system and its planets requires multiple tracking observations and the telescope time that goes with it. Here, the phenomenal success of the surveys has given us much more to observe than we can conveniently obtain.

But follow-ups can provide critical insight, as a study released this week clearly shows. In it, the researchers describe the observations of a three-planet system discovered by TESS. Using the additional observations, the researchers find that there are likely two other planets that TESS was unable to see and that the one it spotted is the least massive exoplanet described to date.

Follow-up

The system is called L 98-59 and has a few properties that make it an excellent candidate for follow-up observations. One advantage is that it is quite close, at least in galactic terms, being only about 35 light years from Earth. It is also located where it will spend a significant amount of time in the James Webb Space Telescope’s field of view, should this mission succeed in reaching operational orbit. This means that it will be relatively convenient to get enough imagery to study the atmospheres of the planets in the system.

TESS locates the planets by monitoring the drop in light that occurs as the planets pass between their host star and Earth. So others have followed this study with observations using radial velocity measurements, which monitor changes in the star’s light that occur when the gravity of its planets pulls it in different directions. Radial velocity can give us the probable mass of a planet; combined with the size of the planet provided by TESS, this can tell us its density and thus offer clues about its composition.

Unfortunately, the telescope’s time was not sufficient to significantly reduce the uncertainties. The new article presents many additional observations that were made using an instrument connected to the Very Large Telescope at the European Southern Observatory in Chile.

The standard way to analyze this kind of data is to identify the strongest periodic signal caused by a planet and remove it from the data, then continue until you run out of statistically significant signals. Oddly enough, when this analysis got to the point where the three planets identified by TESS had been removed, there were still signals left. The signals indicated the clear presence of a fourth planet and the possible presence of a fifth (models with four and five planets were also consistent with the data).

What is in L 98-59?

One obvious thing to examine is to see if the other planets were apparent, but their signals had not been recognized. To verify this, the researchers relied on software called (and I’m not making this up) the Bad-Ass Transit Model cAlculatioN, or BATMAN. But in the transit data, the planets just aren’t there.

It is not a major problem. The transits rely on careful alignment of the exosolar system, where planets orbit in a plane that intersects Earth. If all the planets do not orbit precisely in this plane, they might not be transiting from the point of view of Earth. Nonetheless, this means that we either need additional data or careful analysis to look for gravitational interactions between planets, which can influence the timing of their transits.

If the fifth planet exists, it’s about two and a half times the mass of Earth, so it’s probably a rocky planet. It would also be in the middle of the habitable zone of L 98-59, where liquid water could potentially exist on the surface of the planet. However, since L 98-59 is a small, faint star, the habitable zone is so close that the planet would only take 23 days to complete an orbit.

Because sensing radial velocity tells us the mass of a planet and looking at the planet in transit gives us its size, we now know the density of the three planets detected by both methods. Two are a little less dense than Earth, which suggests that they have a similar structure, except for having a smaller iron core. In contrast, the third is only about half the density of the Earth, suggesting that it could contain up to 30% water – an ocean world.

The other striking finding is the mass of one of the Earth-like planets. That’s less than half that of Earth, leaving the planet noticeably more massive than Mercury and Mars but less than half the mass of Venus. It is therefore the smallest confirmed exoplanet discovered to date. Based on the solar system, we would expect there to be a good number of planets this size or smaller, so it’s encouraging to finally detect them.

At this point, there are thousands of applicants for the planets we haven’t looked at. Many similar surprises could await us. In the meantime, surveys like TESS will continue to add to the list of items we need to check more carefully.

Astronomy & Astrophysics, 2021. DOI: Not yet available.