Title: TESS spots a hot Jupiter with an inner-transiting Neptune

Authors: Chelsea X. Huang, et al.

First Author’s Institution: Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology

Status: open access on arXiv

For centuries, humankind has wondered if other planets exist outside of our own solar system, or if we are in fact unique. The first recorded attempts at observing other planets dates to around the 19th century – although they have been speculated since the 16th century – but we did not have the technology to make the detailed measurements required to detect exoplanets until the last few decades. The first detected exoplanet, 51 Pegasi b, was discovered in 1995, and since then we have learned that exoplanets are actually more of the rule than the exception. Some of the most common exoplanets that we are able to detect are called hot Jupiters – large gas giants like our Jupiter, but so close to their host stars that their orbital periods are on the order of 10 days or less – and mini-Neptunes, similar in composition to our Neptune, but smaller.

Figure 1: The radial velocity curve of TOI-1130 c. The radial velocity method is based on the slight circular or elliptical movements of a star due to the gravitational effects of its planet(s), and their resulting Doppler shifts. The orange line indicates the best fit of the curve, while the blue error bars account for systematic and astrophysical unknowns. This is Figure 3 in the paper.

In this paper, the authors discuss a unique system called TOI-1130 which contains both a hot Jupiter and a mini-Neptune. The hot Jupiter, TOI-1130 c, has been confirmed by radial velocity measurements (see Figure 1) and is roughly 0.974 M Jup with an orbital period of 8.4 days. Less is known about the Neptune, TOI-1130 b, since there are no radial velocity detections of it, but the authors are able to put an upper limit of 40 times the mass of the Earth on its mass. They do this by fitting the radial velocity data based on the assumption that there are two planets and determining what the largest mass for the Neptune could be based on the known mass of the hot Jupiter. But why is this system unique? TOI-1130 one of only three known systems in which a hot Jupiter-type exoplanet has another planet within its orbit around the host star, the other two being WASP-47 and Kepler-730. It is thought to be a strange occurrence both because of the small sample size, and because current migration models indicate the hot Jupiter would kick smaller planets out of its way as it settled into its current orbit, like a schoolyard bully.

Despite being so common, the way hot Jupiters are formed is still a hot research topic, and systems such as these three could help shed more light on the formation problem. The three main theories for hot Jupiter formation mentioned in this paper are:

Migration – the hot Jupiter formed further out in the protoplanetary disk and migrated inward due to various reasons In situ formation – the hot Jupiter formed where it is now Planet-planet scattering – planets close enough to each other will gravitationally interact and push each other into different orbits