Where Do ‘Hot Neptunes’ Come From?

Learning about the orbital tilt of a distant exoplanet may help us understand how young planets evolve, and especially how they interact with both their star and other nearby planets. Thus the question of ‘hot Neptunes’ and the mechanisms that put them in place.The issue has been under study since 2004. Are we looking at planets laden with frozen ices that have somehow migrated to the inner system, or are these worlds that formed in place, so that their heavy elements are highly refractory materials that can withstand high disk temperatures?

Among the exoplanets that can give us guidance here is DS Tuc Ab, discovered in 2019 in data from the TESS mission (Transiting Exoplanet Survey Satellite). Here we have a young world whose host is conveniently part of the 45 million year old Tucana-Horologium moving group (allowing us to establish its age), a planet within a binary system in the constellation Tucana. The binary stars are a G-class and K-class star, with DS Tuc Ab orbiting the G-class primary.

A team at the Center for Astrophysics | Harvard & Smithsonian has developed a new modeling tool that is described in a paper to be published in The Astrophysical Journal Letters, one that allows them to measure the orbital tilt of DS Tuc Ab, the first time the tilt of a planet this young has been determined. Systems evolve over billions of years, making analysis of the formation and orbital configuration of their planets difficult. It’s clear from the team’s work that DS Tuc Ab did not, in the words of the CfA’s George Zhou, “get flung into its star system. That opens up many other possibilities for other, similar young exoplanets…”

The work was complicated by the fact that the host star, DS Tuc A, was covered up to 40% in star spots. David Latham (CfA) describes the situation:

“We had to infer how many spots there were, their size, and their color. Each time we’d add a star spot, we’d check its consistency with everything we already knew about the planet. As TESS finds more young stars like DS Tuc A, where the shadow of a transiting planet is hidden by variations due to star spots, this new technique for uncovering the signal of the planet will lead to a better understanding of the early history of planets in their infancy.”

The work proceeded using the Planet Finder Spectrograph on the Magellan Clay Telescope at Las Campanas Observatory in Chile, with the goal of finding out whether this newly formed world had experienced chaotic interactions in its past that could account for its current orbital position. The analysis involved modeling how the planet blocked light across the surface of the star, folding in the team’s projections of how the star spots changed the stellar light emitted. A well-aligned orbit would block an equal amount of light as the planet passed across the star’s surface. The method should aid the study of other young ‘hot Neptunes.’

Image: Animation courtesy of George Zhou, CfA. Top right illustration of DS Tuc AB by M Weiss, CfA.

Benjamin Montet (University of New South Wales) is lead author of a companion paper on which the CfA team were co-authors (citation below). Montet’s team used a different technique called the Rossiter-McLauglin effect to study the planet. Here the researchers measure slight blueshifts and redshifts in the star’s spectrum while simultaneously modeling both transit and stellar activity signals. Says Montet;

“DS Tuc Ab is at an interesting age. We can see the planet, but we thought it was still too young for the orbit of other distant stars to manipulate its path.”

What the combined work suggests is that DS Tuc Ab, because of its youth, probably did not form further out and migrate in. Its flat orbital tilt also indicates that the second star in the binary did not produce interactions that pulled it into its current position. The authors of the Montet paper consider this work “a first data point” in an analysis that may eventually confirm or rule out the hypothesis that wide binary companions can tilt protoplanetary disks to produce high inclination orbits in the planets that form within them.

A good deal of work is ahead to understand such young systems, but the methods the Montet team used are promising, for the Rossier-McLaughlin (R-M) effect proves to be potent. From the already published companion paper:

DS Tuc Ab is one of a small number of planets to be confirmed by a detection of its R-M signal rather than its spectroscopic orbit. This approach may be the optimal strategy for future confirmation of young planets orbiting rapidly-rotating stars. While the RV [radial velocity] of the star varies on rotational period timescales at the 300 m s−1 level, it does so relatively smoothly over transit timescales, enabling us to cleanly disentangle the stellar and planetary signals. While this planet would require a dedicated series of many spectra and a detailed data-driven analysis to measure a spectroscopic orbit, the R-M signal is visible by eye in observations from a single night. For certain systems, in addition to a more amenable noise profile, the amplitude of the R-M signal can be larger than the Doppler amplitude. Similar observations to these should be achievable for more young planets as they are discovered, which will shed light onto the end states of planet formation in protoplanetary disks.

So now we’re seeing two complementary methods for studying young planetary systems, both of which have turned in useful data on how one ‘hot Neptune’ must have formed.

Results of the CfA study will be published in The Astrophysical Journal Letters. The companion study is Montet et al., “The Young Planet DS Tuc Ab Has a Low Obliquity,” The Astronomical Journal Vol. 159, No. 3 (20 February 2020). Abstract / Preprint.