Using the Hubble Space Telescope, scientists have found that a planet about 350 light-years away the size of a cotton-candy Jupiter is shrinking and could be on its way to reaching the size of a “sub-Neptune” or “super-Earth” world.
The extrasolar planet or “exoplanet” called V1298 Tau b is not only one of the lightest planets ever discovered, but also one of the youngest planets ever discovered as it “traverses” the surface of its star.
V1298 Tau b orbits a star about 23 million years old, which is an infant compared to older stars such as our middle-aged star, the Sun, which is 4.6 billion years old. It is accompanied in this system by at least three other planets, V1298 Tau c, d and e. This means that this system offers astronomers a unique opportunity to study the atmospheres of newly formed and evolving planets.
To do this, the team used the Hubble Wide Field Camera 3 (WFC3) instrument to observe V1298 Tau b as it passed the surface of its young parent star, which is classified as a T Tauri star, meaning it a very young star with a mass similar to that of the Sun.
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“V1298 Tau b is the size of Jupiter. However, we found that the mass of this planet is comparable to or even less than that of Neptune,” research team leader Saugata Barat of the University of Amsterdam told Space.com. “Therefore, at this age it is likely that it is a Neptune or sub-Neptune progenitor. Depending on how this planet evolves, it could lose much of its original atmosphere and end up as a sub-Neptune or possibly even a super-Earth-like planet.”
Since Jupiter is approximately 20 times the mass of Neptune, the fact that V1298 Tau b has the width of the former with the mass of the latter means that this exoplanet is incredibly “inflated” as astronomers currently see it.
“V1298 Tau b is one of the lowest density planets we have discovered to date. Its density of 0.1 grams per cubic centimeter is comparable to that of cotton candy,” Barat added.
Decoding the shrinking atmosphere of V1298 Tau b
Using its transits, the team conducted an atmospheric survey of V1298 Tau b and discovered a large and clear atmosphere spanning about 621 miles (1,000 kilometers). This is much larger than the atmospheres of Solar System bodies such as Saturn’s largest moon Titan, whose atmospheres extend between 31 and 62 miles (50 to 100 kilometers) beyond their main bodies.
V1298 Tau b is only about 16 million miles from its highly active, hot young parent star T Tauri, meaning it completes one orbit in just 24 Earth days. Because of this proximity, the planet is also bombarded with high doses of ultraviolet and X-ray radiation from its star, stripping it of its atmosphere.
Additionally, the team found that the interior of this planet is very hot, but is expected to cool as it matures, also leading to the loss of the atmosphere.
“A combination of these processes will likely result in significant mass loss and contraction of this planet,” Barat said. “A comparison between the nature and composition of V1298 Tau b with mature sub-Neptunes shows significant differences. Therefore, it is possible that the atmospheric composition and chemistry of V1298 Tau b will also change as it evolves.”
As a result, V1298 Tau b tells astronomers that when Neptune and sub-Neptune are first born in a cloud of gas and dust around a young star called a “protoplanetary disk,” they are in a state other than when they appear mature Age. The research also suggests that Neptune and sub-Neptune planets could shift positions in their planetary systems.
“They are born with very large primordial shells that were probably accreted in a protoplanetary disk during their formation,” Barat said. “By analyzing the atmospheric composition of V1298 Tau b, we believe that this planet may have formed close to its present location, close to its parent star.”
Because elements and chemical compounds absorb and emit light at characteristic wavelengths, analyzing the light passing through the planet’s atmosphere as it passes the surface of its star can reveal its composition using a process called spectroscopy.
For V1298 Tau b, this study revealed something unusual about the atmosphere of this exoplanet. The proportion of “metals” is surprisingly low. That’s the term astronomers use for elements that are heavier than the two lightest elements in the universe, hydrogen and helium.
“We were surprised that no methane was detected. The temperature of this planet is ideal for producing a large amount of methane,” Barat said. “However, its absence suggests chemical processes in its atmosphere, such as strong vertical mixing, that can remove the methane from the observable atmosphere.”
The University of Amsterdam scientist further explained that methane can only be removed if the interior of a planet is very hot. The lack of methane in V1298 Tau b’s atmosphere led them to hypothesize that temperatures in the planet’s atmosphere are 80 degrees Fahrenheit (27 degrees Celsius) or more. In comparison, the average temperature of Jupiter’s atmosphere is minus 110 degrees Celsius.
Barat said the current observations only observed water vapor in the atmosphere of V1298 Tau b. This means that in the future they intend to measure the abundance of other molecules such as carbon dioxide, carbon monoxide and sulfur dioxide to complete an inventory of the chemicals present in this exoplanet’s atmosphere.
“It is important to narrow down this frequency because it is directly related to where this planet formed,” Barat added. “It is also important to study the influence of the host star on the planet’s atmospheric chemistry.”
Barat and colleagues have obtained observations of V1298 Tau b using the James Webb Space Telescope (JWST), which they are currently analyzing.
“The JWST is sensitive to a variety of molecules such as water, methane, carbon dioxide, carbon monoxide and sulfur dioxide. Therefore, we will be able to answer these questions soon,” Barat concluded.
The team’s research results were published May 9 in the journal Nature Astronomy.