From AAS NOVA: “Two Planets Straddling the Gap”



19 June 2019
Susanna Kohler

We’ve observed a diverse range of planets larger than Earth (a few are shown in these illustrations), but they tend to fall into two size categories: super-Earth or mini-Neptune. [NASA Ames/JPL-Caltech]

As of last week, the count of confirmed exoplanets officially exceeds 4,000 — and while we’ve learned a lot about planet formation from this wealth of data, it’s also prompted new questions. Could the recent detection of two intriguing new planets shed light on one of these open puzzles?

In 2017, a team of scientists led by B.J. Fulton identified a gap in the distribution of radii of small Kepler-discovered planets. [NASA/Ames/Caltech/University of Hawaii (B. J. Fulton]

Mind the Gap

Our growing exoplanet statistics recently revealed a curious trait: there’s a gap in the radius distribution of planets slightly larger than Earth. Rocky super-Earth planets of up to ~1.5 Earth radii are relatively common, as are gaseous mini-Neptunes in the range of ~2–4 Earth radii. But we’ve detected very few planets in between these sizes.

What’s the cause of this odd deficit? One theory is that high-energy radiation emitted by stars early in their lifetimes erodes the atmospheres of planets that are too close in, stripping them of their expansive shells of gas and leaving behind only their dense, rocky cores. Planets that lie further out or start with a thicker shell may be spared this fate, retaining some of their gas for a significantly larger, fluffier construction.

Folded light curves for HD 15337 showing the transits of planets b (top) and c (bottom). [Gandolfi et al. 2019]

Disentangling Factors

This theory can be difficult to test, however, due to the large number of intertwined variables. The super-Earths and mini-Neptunes we’ve observed lie at varying distances from their host stars — but they also orbit around different types of stars with very different radiation histories. It’s hard to tell what role these various factors play in the planets’ evolution.

But a recent discovery from the Transiting Exoplanet Survey Satellite (TESS) may help simplify this picture. With more than 750 planet-candidate detections so far, TESS is rapidly adding to our exoplanet statistics — and two TESS-discovered planets around HD 15337 may be especially useful for better understanding the radius gap.

NASA/MIT TESS replaced Kepler in search for exoplanets

A Non-Identical Pair

In a publication led by Davide Gandolfi (University of Turin, Italy), a team of scientists carefully analyzes the TESS light curves for HD 15337, as well as archival spectroscopic data from the High Accuracy Radial velocity Planet Searcher. They show that there is evidence for the presence of two planets — HD 15337 b and c — that have similar masses: ~7.5 and ~8.1 Earth masses, respectively.

But while HD 15337 b appears to be a close-in (period of 4.8 days), rocky super-Earth with radius of 1.6 Earth radii and density of 9.3 g/cm3, HD 15337 c lies further out (period of 17.2 days) and is a fluffy mini-Neptune, with a radius of 2.4 Earth radii and a density of 3.2 g/cm3.

Since these two planets orbit the same star, it seems likely that their different orbital radii are what led to their places on either side of the radius gap. Using a planet atmospheric evolution algorithm, Gandolfi and collaborators show that the properties of the two planets can be produced by high-energy radiation from HD 15337 early in the system’s lifetime.

As our observational statistics for exoplanets continue to grow, it’s exciting to see how these continued discoveries can both raise and address new questions of planet formation and evolution. Who knows what else we’ll learn as detections continue to pile up!


“The Transiting Multi-planet System HD15337: Two Nearly Equal-mass Planets Straddling the Radius Gap,” Davide Gandolfi et al 2019 ApJL 876 L24.

See the full article here .


Please help promote STEM in your local schools.

Stem Education Coalition


AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009