You’ve Got a Friend in Me: A Hot Jupiter with a Unique Companion


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Title: TESS spots a hot Jupiter with an inner-transiting Neptune
Authors: Chelsea X. Huang, et al.
First Author’s Institution: Massachusetts Institute of Technology
Status: Published in ApJL

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 to observe other planets date to around the 19th century — although exoplanets have been speculated since the 16th century — but we did not have the technology to make the detailed measurements required to detect planets around other stars 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.

radial velocity curve

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. [Adapted from Huang et al. 2020]

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 MJup with an orbital period of 8.4 days. Less is known about the mini 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 are 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 the prevalence of hot Jupiters, the way they 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:

  1. Migration: the hot Jupiter formed further out in the protoplanetary disk and migrated inward due to various potential processes
  2. In situ formation: the hot Jupiter formed where it is now, very close to its host star
  3. Planet-planet scattering: planets that pass close to each other gravitationally interact and push each other onto new, different orbits

Artist’s impression of TESS observing planets orbiting a dwarf star. [NASA Goddard SFC]

In systems such as TOI-1130, the first theory, migration, is likely ruled out, due to the aforementioned lack of bullying of the mini Neptunes. This indicates that different formation mechanisms could be at work for different hot Jupiters (physics must keep things interesting, so we don’t get bored). There are several current and upcoming instruments capable of large exoplanet surveys, like the Transiting Exoplanet Survey Satellite (TESS), the James Webb Space Telescope (JWST, if it ever gets off the ground), and the Wide Field Infrared Survey Telescope (WFIRST). If a larger sample of these systems could be discovered with instruments such as these we could likely learn more about the formation mechanisms of hot Jupiters, as well as their lesser-known cousins, warm Jupiters (10 days < Porb < 100 days). Additionally, since the hot Jupiter in TOI-1130 has a longer orbital period than those of WASP-47 or Kepler-130, the authors believe that learning more about it could shed light on the differences between the formations of short-period and long-period giant exoplanets specifically, since its period is close to the 10-day limit for planets are considered to be hot Jupiters. TOI-1130 also has the benefit of being a much brighter host star than either WASP-47 or Kepler-730, which makes it easier to observe changes to the stellar shape and spectra caused by the exoplanets. By learning more about these strange systems, we can hopefully get a better idea of how these and other planetary systems form and what sort of systems we can expect to find in the future!

About the author, Ali Crisp:

I’m a second year grad student at Louisiana State University. I study both hot Jupiter exoplanets and binary star systems in the bulge of the Milky Way. I am originally from Tennessee and attended undergrad at Christian Brothers University, where I studied physics and history. In my “free time,” I enjoy cooking, hiking, and photography.