Water vapor found on an exoplanet

Newswand: Astronomers using the NASA/ESA Hubble Space Telescope observed the smallest exoplanet where water vapour has been detected in its atmosphere. At only approximately twice Earth’s diameter, the planet GJ 9827d could be an example of potential planets with water-rich atmospheres elsewhere in our galaxy.

“This would be the first time that we can directly show through an atmospheric detection that these planets with water-rich atmospheres can actually exist around other stars,” said team member Björn Benneke of the Université de Montréal. “This is an important step toward determining the prevalence and diversity of atmospheres on rocky planets.”

However, it remains too early to tell whether Hubble spectroscopically measured a small amount of water vapour in a puffy hydrogen-rich atmosphere, or if the planet’s atmosphere is mostly made of water, left behind after a primaeval hydrogen/helium atmosphere evapourated under stellar radiation.

“Our observing programme was designed specifically with the goal of not only detecting the molecules in the planet’s atmosphere, but of actually looking specifically for water vapour. Either result would be exciting, whether water vapour is dominant or just a tiny species in a hydrogen-dominant atmosphere,” said the science paper’s lead author, Pierre-Alexis Roy of the Université de Montréal.

Because the planet is as hot as Venus at roughly 425 degrees Celcius, it definitely would be an inhospitable, steamy world if the atmosphere were predominantly water vapour.

At present the team is left with two possibilities. The planet is still clinging to a hydrogen-rich envelope laced with water, making it a mini-Neptune. Alternatively, it could be a warmer version of Jupiter’s moon Europa, which has twice as much water as Earth beneath its crust. “The planet GJ 9827d could be half water, half rock. And there would be a lot of water vapour on top of some smaller rocky body,” said Björn.

If the planet has a residual water-rich atmosphere, then it must have formed farther away from its host star, where the temperature is cold and water is available in the form of ice, than its present location. In this scenario, the planet would have then migrated closer to the star and received more radiation. The hydrogen was then heated and escaped, or is still in the process of escaping, the planet’s weak gravity. The alternative theory is that the planet formed close to the hot star, with a trace of water in its atmosphere.

The Hubble programme observed the planet during 11 transits – events in which the planet crossed in front of its star – that were spaced out over three years. During transits, starlight is filtered through the planet’s atmosphere and carries the spectral fingerprint of water molecules. If there are clouds on the planet, they are low enough in the atmosphere that they don’t completely hide Hubble’s view of the atmosphere, and Hubble is able to probe water vapour above the clouds.

GJ 9827d was discovered by NASA’s Kepler Space Telescope in 2017. It completes an orbit around a red dwarf star every 6.2 days. The star, GJ 9827, lies 97 light-years from Earth in the constellation Pisces.

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Researchers found interesting planetary system

Newswand: An international team led by a young researcher from the Institut d’astrophysique de Paris announced the discovery of an interesting planetary system around a star other than Sun. It includes a planet with radius and mass between those of the Earth and Neptune, possibly transiting in front of its bright host star each 146 days, as well as an outer, more massive companion. Such rare systems are particularly interesting to better understand planetary formation and evolution.

TESS satellite: Credit: NASA’s Goddard Space Flight Cente

A collaboration of worldwide researchers announced that the new planetary system discovered around the star HD88986. The team includes researchers at the Institut d’astrophysique de Paris (IAP) and 30 other institutes from nine countries. It is led by Neda Heidari, an Iranian postdoctoral fellow at the IAP.

First, the planetary system includes a cold sub-Neptune, HD88986b. This planet has the longest orbital period (146 days) among known exoplanets smaller than Neptune or Uranus with precise mass measurements. It was detected using the SOPHIE high-precision spectrograph at the Haute-Provence Observatory, France. SOPHIE does detect and characterize exoplanets using the so-called radial-velocity method. It consists in measuring tiny motion variations of the star induced by planets orbiting it.

Those SOPHIE observations revealed the planet and allowed the team to estimate its mass to approximately 17 times that of the Earth. Complementary observations obtained with NASA’s TESS (Transiting Exoplanet Survey Satellite, NASA) and ESA’s space telescope CHEOPS (CHaracterising ExOPlanet Satellite, ESA) indicate that the planet probably “transits” in front of it host star: this occurs when its orbit passes on the line of sight between the Earth and the star, partially occulting the star hence causing a decrease in its brightness that can be observed and quantified. These observations by both satellites allowed the team to directly estimate the diameter of the planet as about twice that of the Earth. Moreover, with an atmosphere temperature of 190 Celsius degrees only, HD88986b provides a rare opportunity for studying the composition of the so-called “cold” atmospheres, as most of the detected atmospheres for exoplanets are above 1000 Celsius degrees.

The whole analysis uses more than 25 years of observations, and also includes data from the Gaia satellite Gaia and the Keck Telescope in Hawaii. It also revealed a second, outer companion around the central star. That companion is particularly massive (more than 100 times the mass of Jupiter) and its orbit has a period of several tens of years.

One important aspect of this new planetary system is the wide orbit of the sub-Neptune HD88986b (as large as 60% of the Earth-Sun distance). As a result, HD88986b probably underwent rare interactions with other planets that may exist in the planetary system, and weak mass loss from the strong ultraviolet radiation of the central star. It may therefore have retained its original chemical composition, allowing scientists to explore the possible scenarios for the formation and evolution of this planetary system, which must take into account the presence of the massive companion planet.

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Earth-sized planet discovered in ‘our solar backyard’

Newswand: A team of astronomers have discovered a planet closer and younger than any other Earth-sized world yet identified. It’s a remarkably hot world whose proximity to our own planet and to a star like our Sun marks it as a unique opportunity to study how planets evolve.

The new planet was described in a new study published this week by The Astronomical Journal. Melinda Soares-Furtado, a NASA Hubble Fellow at the University of Wisconsin–Madison, and UW–Madison graduate Benjamin Capistrant co-led the study with co-authors from around the world.

“It’s a useful planet because it may be like an early Earth,” says Soares-Furtado. The planet is known as HD 63433d and it’s the third planet found in orbit around a star called HD 63433. HD 63433d is so close to its star, it completes a trip all the way around every 4.2 days. “Even though it’s really close-orbiting, we can use follow-up data to search for evidence of outgassing and atmospheric loss that could be important constraints on how terrestrial worlds evolve,” Soares-Furtado says. “But that’s where the similarities end — and end dramatically.”

Based on its orbit, the astronomers are relatively certain HD 63433d is tidally locked, which means one side is perpetually facing its star. That side can reach a brutal 2,300 degrees Fahrenheit and may flow with lava, while the opposite side is forever dark.

HD 63433 is roughly the same size and star type as our sun, but (at about 400 million years old) it’s not even one-tenth our sun’s age. The star is about 73 light years away from our own Sun and part of the group of stars moving together that make up the constellation Ursa Major, which includes the Big Dipper. “On a dark night in Madison,” Soares-Furtado says, “You could see [HD 63433] through a good pair of binoculars.”

The study’s authors are collaborating on a planet-hunting project called THYME. In 2020, they used data from NASA’s Transiting Exoplanet Survey Satellite to identify two mini-Neptune-sized planets orbiting HD 63433. Since then, TESS took four more looks at the star, compiling enough data for the researchers to detect HD 63433d crossing between the star and the satellite.

The researchers, including UW–Madison study co-authors graduate student Andrew C. Nine, undergraduate Alyssa Jankowski and Juliette Becker, a UW–Madison astronomy professor, think there is plenty to learn from HD 63433d.

The planet is uniquely situated for further study. Its peppy young star is visible from both the Northern and Southern hemispheres, increasing the number of instruments, like the South African Large Telescope or WIYN Observatory in Arizona (both of which UW–Madison helped design and build) that can be trained on the system. And the star is orders of magnitude closer than many Soares-Furtado has studied, possibly affording opportunities to develop new methods to study gasses escaping from the planet’s interior or measure its magnetic field.

“This is our solar backyard, and that’s kind of exciting,” Soares-Furtado says. “What sort of information can a star this close, with such a crowded system around it, give away? How will it help us as we move on to look for planets among the maybe 100 other, similar stars in this young group it’s part of?”

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