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When Giant Planets Orbit Evolved Stars – Sky & Telescope

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What occurs to planets orbiting round stars that age previous the principle sequence and evolve into pink giants? These monumental reddened stars — our Solar’s destiny billions of years from now — might have vital impacts on the planet populations round them. A brand new examine explores these impacts by trying on the orbits of gasoline giants intently circling developed stars.

Artist’s impression of the gas-giant planet K2-132b intently orbiting its host, a red-giant star.
Karen Teramura, UH IfA

An Unexplored Inhabitants

The present measurement of the Solar as a main-sequence star (yellow circle) in comparison with the scale the Solar shall be as a pink big (pink circle). How do the inflated sizes of developed stars have an effect on their planets?
Oona Räisänen

The wealth of knowledge from the Kepler/K2 mission continues to be coming in — regardless of the spacecraft being nearly out of fuel! — and we’ve found hundreds of exoplanets with its assist. From these knowledge, we are able to begin to discover totally different populations of exoplanets: from small rocky worlds orbiting tiny dwarf stars, to gasoline giants round main-sequence stars like our Solar.

One exoplanet class we’re solely simply beginning to find out about is that of planets orbiting developed stars — stars which have left the principle sequence and grown into monumental pink giants. What occurs to close-in planets when their star ages and inflates? What destiny awaits us when this occurs to our personal Solar billions of years from now? And the way did these planets arrive of their orbits within the first place?

Latest discoveries of gas-giant planets round developed stars have allowed us to begin exploring these planets as a inhabitants. In a brand new examine, a crew of scientists led by Samuel Grunblatt (Institute for Astronomy, College of Hawaii) explored the properties of two such planets particularly, after which put them into context with different identified big planets round each big stars and non-evolved stars.

Radial velocity observations of Kepler-643 (high), K2-132 (middle) and K2-97 (backside), three programs the place close-in big planets orbit developed stars. The planets seem to observe a pattern, the place these on longer orbits are extra eccentric than these orbiting their host star extra intently.
Grunblatt et al. 2018

Eccentricities for Two Extra Giants

Grunblatt and collaborators used radial-velocity measurements of the large planets K2-97b and K2-132b, made with the HIRES spectrograph on the Keck-I telescope in Hawaii, to discover the orbits of those two planets round their red-giant host star. They then in contrast these planets’ orbital eccentricities to these of different identified big planets orbiting each big stars and dwarfs.

The authors discover that for close-in planets, these orbiting developed stars are likely to have extra eccentric orbits than equal planets round main-sequence stars. The distinction is slight — a median eccentricity of ~zero.152 vs. ~zero.056 for close-in giants orbiting big stars vs. dwarfs — nevertheless it’s sufficient to point a statistical distinction between the 2 populations.

Witnessing a Demise Spiral

Orbital interval vs. eccentricity for large planets orbiting big and dwarf stars. Shut-in big planets orbiting big stars have barely totally different eccentricity distributions than these orbiting dwarfs.
Grunblatt et al. 2018

Why would possibly this be? Grunblatt and collaborators suggest that as these planets evolve, their orbits go right into a transient, reasonably eccentric section. Because of fast tidal interactions with their developed hosts — which have a lot bigger stellar radii and convective envelopes than main-sequence stars — the planets’ orbits shrink sooner than they circularize. If that is so, we’re observing these planets as they decay to their eventual fiery loss of life.

How can we verify this image sooner or later? The K2 mission could also be almost over, however the NASA Transiting Exoplanet Survey Satellite tv for pc (TESS) has solely simply launched, and it’s anticipated to watch greater than 100 occasions as many developed stars as Kepler/K2. With TESS knowledge on the horizon, we are able to hope that we’ll quickly have the statistics wanted to construct the image of planet evolution round big stars.


Samuel Okay. Grunblatt et al 2018 ApJL 861 L5. doi:10.3847/2041-8213/aacc67

Associated Journal Articles

This submit initially appeared on AAS Nova, which options analysis highlights from the journals of the American Astronomical Society.

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