Benchmarks for a ‘Second Venus’

The latest find from TESS, the Transiting Exoplanet Survey Satellite, is a reminder of how interesting, and useful, a planetary system can be even if we find no Earth-like worlds there. This seems obvious, but so much of the public attention to exoplanets has to do with finding a clone of our own world that we can forget the power of a ‘second Jupiter’ or, in this case, a ‘second Venus.’ For at L 98-59 we have not one but three planets that may fit this description.

One Venus, hellish as it is, would seem to be enough. But learning about planets with varying kinds of atmospheres that are in orbits that produce runaway greenhouse effects can help us place our own system’s evolution in context. To be sure, we don’t yet know what kind of atmospheres these planets have, or if they have atmospheres at all, but the encouraging thing is that tight orbits around relatively bright stars are what we need as we look toward future tools like the James Webb Space Telescope.

Astrophysicist Joshua Schlieder (NASA GSFC) is a co-author of the paper on this work, which was led by colleague Veselin Kostov:

“If we viewed the Sun from L 98-59, transits by Earth and Venus would lead us to think the planets are almost identical, but we know they’re not. We still have many questions about why Earth became habitable and Venus did not. If we can find and study similar examples around other stars, like L 98-59, we can potentially unlock some of those secrets.”

Image: The three planets discovered in the L98-59 system by NASA’s Transiting Exoplanet Survey Satellite (TESS) are compared to Mars and Earth in order of increasing size in this illustration. Credit: NASA’s Goddard Space Flight Center.

What we have at L 98-59 could conceivably become a primer in atmospheric transformation. The primary is an M-dwarf about a third the mass of the Sun, found 35 light years away in the constellation Volans. The planets include L 98-59b, about 80 percent the size of the Earth, and the smallest planet yet discovered by TESS. Here we have a 2.25 day orbit receiving 22 times the amount of insolation as the Earth. Moving outward, we find L 98-59c, about 1.4 times the size of Earth, in a 3.7 day orbit with 11 times the amount of energy Earth receives. The furthest planet found so far is L 98-59d, about 1.6 times Earth’s size, in a 7.5 day orbit with 4 times Earth’s radiant energy, possibly Venus-like or conceivably a hot Neptune.

This may not exhaust the possibilities, for there is the prospect of further discovery here, says GSFC’s Jonathan Brande, likewise a co-author of the paper:

“If you have more than one planet orbiting in a system, they can gravitationally interact with each other. TESS will observe L 98-59 in enough sectors that it may be able to detect planets with orbits around 100 days. But if we get really lucky, we might see the gravitational effects of undiscovered planets on the ones we currently know.”

The paper points out that these worlds are too small to retain atmospheres rich in hydrogen, so the focus will be on secondary atmospheres that are the result of volcanic activity, and infalling volatiles from the rest of the system via comets. The authors calculate that all three planets are in range for JWST to produce a transmission spectrum showing atmospheric features. The expected signal-to-noise ratio compares to another nearby red dwarf planet, GJ 1132b.

Understanding why Earth is habitable and Venus is not will depend upon our analysis of planets that have evolved through the greenhouse phase. In this regard, the L 98-59 planets stand out, particularly since other Venus analogs thus far discovered orbit fainter stars. From the paper:

The L 98-59 planets receive significantly more energy than the Earth receives from the Sun (a factor of between 4–22 more than Earth’s insolation) and fall into the region that Kane et al. (2014) dubbed the Venus Zone. This is a region where the atmosphere of a planet like Earth would likely have been forced into a runaway greenhouse, producing conditions similar to those found on Venus. The range of incident fluxes within the Venus Zone corresponds to insolations of between 1–25 times that received by the Earth. Planets in the Venus Zone that can be spectroscopically characterized will become increasingly important in the realm of comparative planetology that aims to characterize the conditions for planetary habitability. In that respect, and considering the potential for atmospheric characterization…, L 98-59 could become a benchmark system.

What we know of these worlds will be refined by future TESS observations, possibly uncovering other planets here and monitoring activity on the host star. The paper is Kostov et al., “The L 98-59 System: Three Transiting, Terrestrial-size Planets Orbiting a Nearby M Dwarf,” The Astronomical Journal Vol. 158, No. 1 (27 July 2019). Abstract / Preprint.