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Exoplanet that is too big for its star



  Hot Jupiter

I Tau b is a paradoxical planet, but new research on the warm Jupiter's mass, brightness and carbon monoxide in the atmosphere begins to answer questions about how a planet could have formed so great around a star that only is two million years old.

For decades, most astronomers believed giant planets such as Jupiter and Saturn formed far from their stars for periods of 10 million years or more. But the discovery of dozens of "hot jupiter" led to new theoretical models describing how such planets can form.

At today's meeting of American astronomical society, astronomers Christopher Johns-Krull from Rice University and Lisa Prato from Lowell Observatory presented results from a four-year near-infrared spectroscopic analysis of light from CI Tau b, a close-fitting giant exoplanet or " hot Jupiter "in a nine-day orbit around his parent star about 450 light-years from Earth in the constellation Taurus.

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"The exciting thing is that we are able to discover light directly from the planet, and this is the first time that has been done for a nearby planet around a star this young" said Johns-Krull, professor of physics and astronomy, and co-author of a study slated for publication in the AAS Astrophysical Journal Letters. "The most valuable way to learn how planets form is to study planets, such as CI Tau b, which is either still formed or just formed."

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Johns-Krull said that CI Tau bs age made it the perfect candidate for observation with Immersion Grating Infrared Spectrograph (IGRINS), a unique high resolution instrument used during CI observations. Tau b from McDonald Observatory 2.7 meters Harlan J. Smith Telescope and Lowell Observatory's 4.3 meter Discovery Channel Telescope.

Since each atomic element and molecule in a star emits light from a unique set of wavelengths, astronomers can search for specific signatures or spectral lines to see if an element is present in a distant star or planets. Spectral lines can also reveal the temperature and density of a star and how fast it moves.

Prato said the research group used the spectral lines from carbon monoxide to distinguish the light emitted by the planet from the light from the nearby star.

"Many of the spectral lines that are on the planet are also in the star," Prato said. "If both the planet and the star were stationary, their spectral lines would all be mixed together, and we would not be able to tell what was from the star and what was from the planet. But because the planet rapidly circles the star, its lines shift back and forth dramatically. We can subtract the star's lines and only see the lines from the planet, and from them we can determine how bright the planet is in relation to the star, which tells us something about how it formed. "

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"Direct observation of the mass and brightness of CI Tau b is particularly useful because we also know that it orbits a very young star," says Rice Ph.D. student Laura Flagg, the lead author of the upcoming study. "Most of the hot Jupiters we've found are circumferential in the Middle Ages stars. CI Tau's age gives a strict limit to testing models: Can they produce a luminous and massive planet in such little time?"

Flag's analysis of spectral lines from carbon monoxide showed that CI Tau b has a mass of 11.6 Jupiter and is about 134 times weaker than its parent. Prato said it provides strong evidence that it formed through a "hot start", a theoretical model that describes how gravity instabilities can form giant planets faster than traditional models.

Prato said that the new study provides a unique empirical yardstick by which to measure competing theories. "About 2 million years old, CI Tau b is by far the youngest hot Jupiter directly discovered," she said. "We now have a lot and brightness for it – the only directly measured mass and brightness of a young, warm Jupiter – and it gives very powerful tests for planet formation models."

IGRINS, designed by study co-author Daniel Jaffe of the University of Texas at Austin, uses a silicon-based diffraction grating to improve both the resolution and the number of near-infrared spectral bands that can be observed from distant objects such as CI Tau b and its parent's. IGRINS was moved from McDonald to Lowell midway through the investigation.

Daily Galaxy via Rice University

Top of page image: ATG MEDIALAB, ESA


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