Large Exoplanet Could Have the Right Conditions for Life

Astronomers have found an exoplanet more than twice the size of Earth to be potentially habitable, opening the search for life to planets significantly larger than Earth but smaller than Neptune. A team from the University of Cambridge used the mass, radius, and atmospheric data of the exoplanet K2-18b and determined that it's possible for the planet to host liquid water at habitable conditions beneath its hydrogen-rich atmosphere. The results are reported in The Astrophysical Journal Letters. The exoplanet K2-18b, 124 light-years away, is 2.6 times the radius and 8.6 times the mass of Earth, and orbits its star within the habitable zone, where temperatures could allow liquid water to exist. The planet was the subject of significant media coverage in the autumn of 2019, as two different teams reported detection of water vapour in its hydrogen-rich atmosphere. However, the extent of the atmosphere and the conditions of the interior underneath remained unknown."Water vapour has been detected in the atmospheres of a number of exoplanets but, even if the planet is in the habitable zone, that doesn't necessarily mean there are habitable conditions on the surface," said Dr. Nikku Madhusudhan from Cambridge's Institute of Astronomy, who led the new research. "To establish the prospects for habitability, it is important to obtain a unified understanding of the interior and atmospheric conditions on the planet - in particular, whether liquid water can exist beneath the atmosphere." Given the large size of K2-18b, it has been suggested that it would be more like a smaller version of Neptune than a larger version of Earth. A 'mini-Neptune' is expected to have a significant hydrogen 'envelope' surrounding a layer of high-pressure water, with an inner core of rock and iron. If the hydrogen envelope is too thick, the temperature and pressure at the surface of the water layer beneath would be far too great to support life.

Now, Madhusudhan and his team have shown that despite the size of K2-18b, its hydrogen envelope is not necessarily too thick and the water layer could have the right conditions to support life. They used the existing observations of the atmosphere, as well as the mass and radius, to determine the composition and structure of both the atmosphere and interior using detailed numerical models and statistical methods to explain the data.

The researchers confirmed the atmosphere to be hydrogen-rich with a significant amount of water vapour. They also found that levels of other chemicals such as methane and ammonia were lower than expected for such an atmosphere. Whether these levels can be attributed to biological processes remains to be seen.

The team then used the atmospheric properties as boundary conditions for models of the planetary interior. They explored a wide range of models that could explain the atmospheric properties as well as the mass and radius of the planet. This allowed them to obtain the range of possible conditions in the interior, including the extent of the hydrogen envelope and the temperatures and pressures in the water layer.

"We wanted to know the thickness of the hydrogen envelope - how deep the hydrogen goes," said co-author Matthew Nixon, a PhD student at the Institute of Astronomy. "While this is a question with multiple solutions, we've shown that you don't need much hydrogen to explain all the observations together."

The researchers found that the maximum extent of the hydrogen envelope allowed by the data is around 6% of the planet's mass, though most of the solutions require much less. The minimum amount of hydrogen is about one-millionth by mass, similar to the mass fraction of the Earth's atmosphere. In particular, a number of scenarios allow for an ocean world, with liquid water below the atmosphere at pressures and temperatures similar to those found in Earth's oceans.

This study opens the search for habitable conditions and bio-signatures outside the solar system to exoplanets that are significantly larger than Earth, beyond Earth-like exoplanets.

Additionally, planets such as K2-18b are more accessible to atmospheric observations with current and future observational facilities. The atmospheric constraints obtained in this study can be refined using future observations with large facilities such as the upcoming James Webb Space Telescope.

Astronomers Uncover New Clues to the Star That Wouldn't Die

What happens when a star behaves like it exploded, but it's still there? About 170 years ago, astronomers witnessed a major outburst by Eta Carinae, one of the brightest known stars in the Milky Way galaxy. The blast unleashed almost as much energy as a standard supernova explosion. Yet Eta Carinae survived. An explanation for the eruption has eluded astrophysicists. They can't take a time machine back to the mid-1800s to observe the outburst with modern technology. However, astronomers can use nature's own "time machine," courtesy of the fact that light travels at a finite speed through space. Rather than heading straight toward Earth, some of the light from the outburst rebounded or "echoed" off of interstellar dust, and is just now arriving at Earth. This effect is called a light echo. The light is behaving like a postcard that got lost in the mail and is only arriving 170 years later. By performing modern astronomical forensics of the delayed light with ground-based telescopes, astronomers uncovered a surprise. The new measurements of the 19th-century eruption reveal material expanding with record-breaking speeds up to 20 times faster than astronomers expected. The observed velocities are more like the fastest material ejected by the blast wave in a supernova explosion, rather than the relatively slow and gentle winds expected from massive stars before they die.

Based on this data, researchers suggest that the 1840s eruption may have been triggered by a prolonged stellar brawl among three rowdy sibling stars, which destroyed one star and left the other two in a binary system. This tussle may have culminated with a violent explosion when Eta Carinae devoured one of its two companions, rocketing more than 10 times the mass of our Sun into space. The ejected mass created gigantic bipolar lobes resembling the dumbbell shape seen in present-day images.

The results are reported in a pair of papers by a team led by Nathan Smith of the University of Arizona in Tucson, Arizona, and Armin Rest of the Space Telescope Science Institute in Baltimore, Maryland.

The light echoes were detected in visible-light images obtained since 2003 with moderate-sized telescopes at the Cerro Tololo Inter-American Observatory in Chile. Using larger telescopes at the Magellan Observatory and the Gemini South Observatory, both also located in Chile, the team then used spectroscopy to dissect the light, allowing them to measure the ejecta's expansion speeds. They clocked material zipping along at more than 20 million miles per hour (fast enough to travel from Earth to Pluto in a few days).

The observations offer new clues to the mystery surrounding the titanic convulsion that, at the time, made Eta Carinae the second-brightest nighttime star seen in the sky from Earth between 1837 and 1858. The data hint at how it may have come to be the most luminous and massive star in the Milky Way galaxy.

"We see these really high velocities in a star that seems to have had a powerful explosion, but somehow the star survived," Smith explained. "The easiest way to do this is with a shock wave that exits the star and accelerates material to very high speeds."

Massive stars normally meet their final demise in shock-driven events when their cores collapse to make a neutron star or black hole. Astronomers see this phenomenon in supernova explosions where the star is obliterated. So how do you have a star explode with a shock-driven event, but it isn't enough to completely blow itself apart? Some violent event must have dumped just the right amount of energy onto the star, causing it to eject its outer layers. But the energy wasn't enough to completely annihilate the star.

One possibility for just such an event is a merger between two stars, but it has been hard to find a scenario that could work and match all the data on Eta Carinae.

The researchers suggest that the most straightforward way to explain a wide range of observed facts surrounding the eruption is with an interaction of three stars, where the objects exchange mass.

If that's the case, then the present-day remnant binary system must have started out as a triple system. "The reason why we suggest that members of a crazy triple system interact with each other is because this is the best explanation for how the present-day companion quickly lost its outer layers before its more massive sibling," Smith said.

In the team's proposed scenario, two hefty stars are orbiting closely and a third companion is orbiting farther away. When the most massive of the close binary stars nears the end of its life, it begins to expand and dumps most of its material onto its slightly smaller sibling.

The sibling has now bulked up to about 100 times the mass of our Sun and is extremely bright. The donor star, now only about 30 solar masses, has been stripped of its hydrogen layers, exposing its hot helium core.

Hot helium core stars are known to represent an advanced stage of evolution in the lives of massive stars. "From stellar evolution, there's a pretty firm understanding that more massive stars live their lives more quickly and less massive stars have longer lifetimes," Rest explained. "So the hot companion star seems to be further along in its evolution, even though it is now a much less massive star than the one it is orbiting. That doesn't make sense without a transfer of mass."

The mass transfer alters the gravitational balance of the system, and the helium-core star moves farther away from its monster sibling. The star travels so far away that it gravitationally interacts with the outermost third star, kicking it inward. After making a few close passes, the star merges with its heavyweight partner, producing an outflow of material.

In the merger's initial stages, the ejecta is dense and expanding relatively slowly as the two stars spiral closer and closer. Later, an explosive event occurs when the two inner stars finally join together, blasting off material moving 100 times faster. This material eventually catches up with the slow ejecta and rams into it like a snowplow, heating the material and making it glow. This glowing material is the light source of the main historical eruption seen by astronomers a century and a half ago.

Meanwhile, the smaller helium-core star settles into an elliptical orbit, passing through the giant star's outer layers every 5.5 years. This interaction generates X-ray emitting shock waves.

A better understanding of the physics of Eta Carinae's eruption may help to shed light on the complicated interactions of binary and multiple stars, which are critical for understanding the evolution and death of massive stars.

The Eta Carinae system resides 7,500 light-years away inside the Carina nebula, a vast star-forming region seen in the southern sky.

Nanodiamonds explain mysterious source of Milky Way microwaves

Astronomers have discovered microscopic gemstones surrounding three infant star systems in the Milky Way. Researchers believe tiny diamonds account for the shimmer of cosmic microwave light that has puzzled astronomers for 20 years. The shimmer is known as anomalous microwave emission, or AME. For decades, scientists have struggled to explain why the odd glow emanates from several of the galaxy's protoplanetary disks. Until now, scientists thought the most likely culprit was a type of carbon-based molecule called a polycyclic aromatic hydrocarbon, or PAH. The interstellar particles yield a faint infrared signature. Another possible culprit, hydrogenated nanodiamonds, produce a similar but slightly different infrared pattern.Using the National Science Foundation's Green Bank Telescope in West Virginia and the Australia Telescope Compact Array, astronomers were able to observe AME surrounding three young stars, V892 Tau, HD 97048 and MWC 297. Scientists found the AME emissions most directly matched the infrared pattern produced by nanodiamonds. "This is the first clear detection of anomalous microwave emission coming from protoplanetary disks," Green Bank astronomer David Frayer said in a news release.

Previous observations have shown other star systems produce the signature made by PAHs but show no signs of AME, suggesting nanodiamonds alone account for the faint shimmer.

Studies have previously suggested the presence of nanodiamonds, tiny particles of crystalline carbon, in the protoplanetary disks surrounding distant stars, but the latest findings -- published this week in the journal Nature Astronomy -- are the first to link the particles with AME.

Scientists believe cosmic nanodiamonds are formed when vaporized carbon atoms become superheated by young stars.

Nanodiamonds produce what's called a "dipole moment," yielding an electromagnetic radiation when they spin. Because they're so small, they can spin at tremendous speeds, emitting electromagnetic radiation in the microwave range.

"This is a cool and unexpected resolution to the puzzle of anomalous microwave radiation," said Jane Greaves, an astronomer at Cardiff University in Wales. "It's even more interesting that it was obtained by looking at protoplanetary disks, shedding light on the chemical features of early solar systems, including our own."