Some stars not far from the Sun are orbited by planets with a tail similar to that of comets. Identified by astronomers over the past 13 years, these planets are gas giants the size of Jupiter, the largest planet in our solar system. The main difference is that, in general, they are very close to their stars, which erode the planetary atmosphere. As this happens, the planets release a trail of gas and dust into space. Two Brazilian astronomers— specialists in the chemistry of the interstellar medium—decided to investigate the reactions that might occur in the tail of some of these planets. They found that, under certain conditions, simple molecules essential to life—such as water—could form.
Astronomer Heloísa Boechat-Roberty and her doctoral student, Rafael Pinotti, came to this conclusion by simulating the environment in the tail of the planet Osiris. A gas giant similar to Jupiter, orbiting the star HD 209458, it is located in the direction of the constellation Pegasus, 154 light-years from our solar system. The characteristics of HD 209458 are very similar to those of our Sun. But Osiris is so close to it that HD 209458 heats and erodes the planet’s atmosphere, forming a gigantic tail of gas and dust in space.
Osiris was discovered in 1999 and has been observed by NASA’s Hubble and Spitzer space telescopes. It is a planet like Jupiter that is closer to its star than Mercury is to the Sun. In 2003, astronomers identified Osiris’s tail, a stream of more than 10,000 tons of gas escaping from the planet’s atmosphere every second, at speeds of up to 130 kilometers per second. “We thought it would be interesting to investigate the chemical reactions that could be taking place there,” says Boechat-Roberty, a professor and researcher at the Valongo Observatory at the Federal University of Rio de Janeiro (UFRJ).
Boechat-Roberty and Pinotti thought that the particles in the gas stream propelled into space could interact and generate stable molecules. Before them, other astronomers had already confirmed the presence of different molecules in Osiris’s atmosphere. Molecular hydrogen (H2) is there in abundance in gaseous form, together with simple carbon molecules and water vapor, in addition to silicon carbide, titanium oxide and vanadium molecules, which probably form grains of dust. But many thought that these molecules would be destroyed as they were dragged to Osiris’s tail.
One reason is that, just above the planet’s atmosphere, at the base of its tail, the gas temperature reaches 10,000 degrees Celsius. In addition to the high temperature, ultraviolet radiation from the star would be severe enough to break up any gas molecule leaving Osiris’s atmosphere. “Studies by other researchers have indicated that there would be no molecules there,” says Pinotti. “The radiation would be so strong that the gas would consist only of isolated atoms and ions.”
The researchers explained, however, that most previous theoretical studies focused on calculating the temperatures, velocities and densities of the gas in the tail at a relatively close distance from the planet. A physical model developed by French astrophysicist Vincent Bourrier, of the Geneva Observatory in Switzerland, caught the attention of Pinotti because it estimated densities and velocities in a region of the tail far from the planet. There, the temperature of the gas would be sufficiently low for atoms and ions to combine again and form molecules. Taking this model as a basis, Boechat-Roberty and Pinotti simulated 566 different chemical reactions involving 56 molecules and ions that could occur in Osiris’s tail.
Presented in February 2016 in the journal Planetary and Space Science, the results of this analysis indicate that if a fraction of molecular hydrogen in Osiris’s atmosphere survived the effects of radiation, water molecules could form in the tail of the planet. According to the duo’s calculations, space telescopes could prove this hypothesis if they looked in Osiris’s tail for signs of OH+ ions, the most abundant molecules there according to the simulations.
For now, the observations made using the space telescopes have only confirmed the existence of hydrogen, carbon and oxygen in the form of isolated atoms and ions in Osiris’s tail. But still-controversial observational evidence suggests that the tail of the planet might include some dust from Osiris, which could protect some hydrogen molecules from the effects of radiation.
“In the case of Osiris, the water molecules fall apart soon after forming, dissipating in the interplanetary medium as ions,” explains Pinotti. There are other planets with tails, but they are washed by milder doses of ultraviolet radiation than Osiris. One is Gliese 436b, a smaller gaseous planet, similar to Neptune, orbiting a red dwarf star 30 light-years from Earth, whose tail was discovered last year. “It’s pure speculation at the moment,” says the researcher, “but I think it is possible that water molecules, or even simple organic molecules, could survive and make a small interplanetary trip via the tail, ending up in the atmosphere of a planet further out, in the habitable zone of the star.”
PINOTTI, R. and BOECHAT-ROBERTY, H. M. Molecular formation along the atmospheric mass loss of HD 209458b and similar Hot Jupiters. Planetary and Space Science. V. 121, p. 83-93. 2016.