Gliese 3470's surface motions can rival signals from its Neptune-size planet

The Red dwarf Gliese 3470’s changing surface can create spectral effects as strong as signals from its Neptune-size planet, Gliese 3470 b. Simulations and observations show that these stellar effects complicate transit measurements, while current water-vapor line data also remain imperfect. A method for removing stellar patterns detected the planet without evident bias, but only in a narrow range of infrared light and for one orbital configuration.

Star system: Gliese 3470 | Distance from the Sun: 95.9 light years

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Gliese 3470's surface motions can rival signals from its Neptune-size planet

Star system: Gliese 3470 | Distance from the Sun: 95.9 light years

Surface motions on the Red dwarf Gliese 3470 can produce changes in starlight that rival the signal of its Neptune-size planet, Gliese 3470 b. These changes matter because researchers use a planet’s passage across its star to study the planet’s atmosphere. If the star’s own light varies during that passage, it can be difficult to separate the planet’s signature from the stellar effects.

Three-dimensional simulations of moving gas in Red dwarf atmospheres were used to create model spectra, or patterns of light at different wavelengths. These were compared with observations of Gliese 3470. The analysis also modeled a high-resolution sequence of the planet crossing the star, including changes in the star’s appearance from its center to its edge. It accounted for the Rossiter-McLaughlin effect, an apparent shift in the star’s light when a planet blocks parts of its rotating surface.

In the infrared K band examined, the simulated and observed water-vapor lines agreed only modestly in their strength and positions. This points to remaining inaccuracies in the line lists, which are reference data describing where these lines should appear and how strong they should be. During the transit, the combined effects of changing stellar appearance and the Rossiter-McLaughlin effect were comparable in size to the planet’s signal.

A processing method called principal component analysis can remove patterns shared across a sequence of spectra. It could reduce the stellar effects, but its performance depends strongly on whether signals from Earth’s atmosphere and the planet are also present. The planet was detected without an evident bias from leftover stellar effects in this analysis. That result applies only to a narrow spectral range and one specific orbital configuration, so it does not establish that the same approach will work in every case. Simulations of stellar convection can help model these changing patterns for future observations with extremely large telescopes.

Key findings
Gliese 3470’s surface-related spectral changes during transit can be comparable in size to the signal from Gliese 3470 b.
Models and observations showed only modest agreement for water-vapor line strengths and positions in the examined infrared range.
Principal component analysis can reduce stellar effects, but its performance depends on signals from Earth’s atmosphere and the planet.
The planet was detected without evident bias from remaining stellar effects, in a narrow spectral range and one orbital configuration.
Three-dimensional simulations can help model convection-driven changes in Red dwarf spectra for future observations.

Scientific study

The impact of M dwarf stellar convection on exoplanetary atmospheric signatures. A 3D radiation-hydrodynamics simulation analysis for high-resolution spectroscopy (external link)