By Rafi Letzter - Staff Writer - Space 

What's the recipe for a living planet? A new paper suggests that the right number of impacts is a key ingredient.

A NASA illustration shows an asteroid striking a planet.

(Image: © NASA/Don Davis)

That's because planets almost certainly need "volatiles" in their atmospheres in order to sprout life, Wyatt told Live Science. Volatiles are chemicals, like water and carbon dioxide, that can boil at low temperatures. All life that we know of relies on water and carbon to sustain itself at a basic chemical level, and scientists believe that the properties of those chemicals make them necessary for life to arise anywhere in the universe.

But not all planets start off with the necessary concentrations of volatiles. Early in a star's lifetime, it's much brighter. And that extra shine is hot enough to bake all the loose dust in the region that will become the star's habitable zone — the not-too-hot, not-too-cold area — later on. Those hot early temperatures likely strip water and other volatiles from the dust that will eventually become habitable planets. So after planets form and the star cools down, these rocky orbs need to acquire their volatiles from somewhere else in the solar system. In other words, they've got to smash into a bunch of big stray objects.

The researchers found that the best candidates for delivering volatiles while not stripping the planet's atmosphere and sterilizing it are medium-size objects. Impacts from 60-foot-wide (20 meters) to 3,300-foot-wide (1 kilometer) asteroids and comets are very efficient at delivering volatiles and will tend to add more to the atmosphere than they subtract, the authors found. Bigger asteroids, between about 1 and 12 miles (2 and 20 km) across, will tend to strip more atmosphere than they add.

Giant impacts like the one that formed Earth's moon, the authors found, don't mess with that story as much as you might expect. Such events are pretty rare, and while they can change the composition of an atmosphere, they won't completely remove it.

One of the important lessons from this paper is that small "M class" stars — the most common category of stars, too dim to see with the naked eye, many of them red dwarfs — are likely bad candidates for life, the authors wrote. That's significant, because a great many potentially habitable exoplanets have turned up around those sorts of stars.

"For M stars, their low luminosity means that the habitable zone is much closer to the star than for a star like the sun," Wyatt said.

To get enough light, an Earth-like planet circling an M-class star might have to be as close to that star as Mercury is to our sun.

And it gets worse. Right up next to a small, low-mass star, asteroids and comets fly around at much higher speeds and crash more dramatically into planets.

"Higher-velocity impacts are much more efficient at stripping an atmosphere," Wyatt said.

That's bad news for life on M worlds. And it's not the only factor that makes M-world life unlikely.

"There are a number of reasons why habitable planets orbiting M dwarfs might not have an atmosphere, including stripping from stellar winds and the planets being much closer in to their host star," said Sarah Rugheimer, an expert in exoplanet atmospheres at the University of Oxford, who was not involved in this research.

So is there any hope for life on M worlds?

"I think, ultimately, we will answer this question observationally with [the James Webb Space Telescope] soon after it launches: Do habitable planets orbiting M dwarfs have atmospheres?" Rugheimer said. "We know that slightly hotter and bigger planets orbiting M dwarfs do have thick atmospheres. But this question still remains for habitable planets: Can they retain a thin enough atmosphere, something like Earth rather than Venus?"

The authors emphasized in the paper that many of their conclusions are based on uncertainties: Where does life form? How much do other star systems out there resemble our solar system?

Edwin Bergin, an expert in planet formation and water at the University of Michigan who was not involved in this research, agreed with the authors that there are what he called "significant complications" in the calculations behind this paper.

"But the general trends they present are quite interesting and could be important," he said.

He pointed to his own work, which has suggested that Earth started out with a thicker, nitrogen-rich atmosphere but lost much of it to impacts. The authors of this new paper suggested in their model that impacts from comets and asteroids might have shaped the atmospheres of Earth, Mars and Venus.

Down the road, the researchers said, there's more to learn about how this work can explain our own solar system, particularly the role of giant impacts here. This paper has not yet been published in a peer-reviewed journal and is available on the preprint server arXiv.

• Fallen Stars: A Gallery of Famous Meteorites
• The 12 Strangest Objects in the Universe
• Top 10 Ways to Destroy Earth

Originally published on Live Science.

{ https://www.livescience.com/ }

By Chelsea Gohd - Science & Astronomy 

Finding this hidden black hole could lead to more discoveries.

An artist's impression of a black hole. In a new study, scientists identify a black hole in a non-interacting binary with a star, a discovery that could support future black hole findings.

(Image: © Shutterstock)

In the study, scientists looked at a binary star system made up of the giant star 2MASS J05215658+4359220 and a companion, which, through a unique approach, they found is likely a low-mass black hole, lead author Todd Thompson, a professor in the Department of Astronomy at The Ohio State University, told Space.com.

Black hole in a haystack

To spot this dim, low-mass black hole, the research team "had to develop a new type of search," Thompson said. Spotting the elusive black hole was a "needle-in-the-haystack kind of search," he added. 

To find the hidden black hole, the team began by poring through data from APOGEE, a survey that uses spectroscopy data to scour the sky for stars. After observing over 100,000 stars, the team identified which stars had significant doppler shift, or redshift and blueshift. Redshift occurs when the wavelengths of light coming from objects moving away from Earth get longer, and the object appears red; blueshift occurs when the wavelengths of light coming from objects moving closer shorten, and the object appears blue. 

The team combined data on stars exhibiting doppler shift with data from the All-Sky Automated Survey for Supernovae (ASAS-SN) project, in which robotic telescopes worldwide monitor the night sky and collect data on millions of stars. With this data, scientists can see the brightness of stars over time. 

The researchers whittled down the list of stars to those "that showed the most extreme doppler shift, Thompson said. They then looked at the stars' light curves, or how the brightness of those stars changed over time in the ASAS-SN data.

The "combination of those two factors led us to this one particular star, this giant star in the outer galaxy," Thompson said about 2MASS J05215658+4359220.

The star's brightness kept changing, brightening and dimming back and forth. The team assumed, and later confirmed, that the brightness variations were in line with the orbital period of an object orbiting the star, Thompson said. 

When they studied the system further, the researchers determined that the object must have a mass of around 3.3 solar masses (though they did give a fairly wide margin of error, from 2.6 to 6.1 solar masses). This mass is consistent with a low-mass black hole. 

Uncovering the unseen

Identifying this black hole which, because it doesn't emit bright X-rays, had previously escaped detection by astronomers, is an important step forward in allowing researchers to more easily identify these "hidden" black holes, Thompson said. 

This black hole is approximately 3.3 times the mass of the sun. That makes it much less massive than black holes in bright, X-ray-emitting systems, which are usually five to six solar masses. It's also less massive than black holes that merge across the universe in gravitational waves, which are often 20 to 30 solar masses, Thompson said. "There are very few or possibly no systems that contain a black hole of around 3 or 3.3 [solar masses] … This is a very-low-mass system for a black hole," he said.

What's more, if this object is found to have the lowest mass deemed possible in its margin or error, it could turn out to be a neutron star — the collapsed core of a giant, dead star — at its maximum mass, Thompson said. Alternatively, if this object is closer to the upper range of its estimated mass, it would add to the definition of what qualifies as a black hole. 

If 3.3 solar masses holds up, it's going to be "quite important," Thompson said, adding that it "will show us that these types of black holes are out there." 

In other words, if the black hole is, in fact, 3.3 solar masses as these researchers think it likely is, it will provide evidence for the assumption that there are possibly plenty of black holes in the cosmos that don't emit X-rays. 

"This was our first candidate," Thompson said, adding that the team is "working on trying to find new candidates using the same search method."

The work was published today (Oct. 31) in the journal Science. 

• Black Hole Quiz: How Well Do You Know Nature's Weirdest Creations?
• The Strangest Black Holes in the Universe
• Stephen Hawking's Most Far-Out Ideas About Black Holes

Follow Chelsea Gohd on Twitter @chelsea_gohd. Follow us on Twitter @Spacedotcom and on Facebook.

{ https://www.space.com/ }

By Meghan Bartels - Spaceflight 

An artist's depiction of a Mars sample-return mission.

(Image: © NASA/JPL)

Arranging a mission to bring the samples back is a challenge, one that NASA intends to tackle in a partnership with the European Space Agency. "This process is remarkably interrelated, everything is connected to everything else in some way," Muirhead said. There are a host of spacecraft components: NASA's Mars 2020 rover to select the samples, a "fetch rover" to pick up those samples, a rocket to launch them off the Red Planet, a capsule to bring them back to Earth.

Mars 2020 will depart next summer and head toward Jezero crater for a February 2021 landing. It will be conducting its own investigations and stashing away intriguing rocks for its successor to bring home.

That mission could launch in 2026, Muirhead said. The mission's early days would be fairly typical for any Red Planet mission. One of the first key challenges of the mission is for spacecraft engineers to land much more weight on Mars than previous missions have done. That will stretch out the anxiety-inducing landing process that haunts Mars spacecraft personnel, as the vehicle risks crashing into the surface.

"Once at Mars, we're going to do entry, descent and landing, our famous seven minutes of terror," Muirhead said. "We're going to make that probably about eight or nine minutes of terror." But he said the team has found a way to deliver significantly more bulk to the surface safely than the techniques that NASA will use to land the Mars 2020 mission.

Once that spacecraft lands, it will deploy the fetch rover, which will drive out to collect the sample containers Mars 2020 has so carefully prepared. The fetch rover will be smaller than that machine and Curiosity — more like the size of Spirit and Opportunity, Muirhead said.

The fetch rover is inspired by the Rosalind Franklin rover that the European Space Agency is sending to Mars on a similar timeline to Mars 2020, but it will end up looking very different from its older sibling. "ExoMars is a serious science rover; this is a speedster," Muirhead said. "That's a fast-moving, highly autonomous rover," he said of its planned travel stats.

Next on the task list is to pack up the samples for the long journey to Earth. Scientists are designing a return capsule that can safely carry as many as 30 rock samples and two air samples. "All of that fits into Darth Vader's helmet," Muirhead said, referring to the shape of the container, which will then itself be packed into a capsule for the long voyage. At around this point of the mission, the sample container will be sealed and sterilized to make sure no loose Martian material sneaks free on Earth.

While plenty of spacecraft have journeyed out to the Red Planet, none have ever retraced their steps. "To fly to Mars, that one-way trip is fairly traditional," Muirhead said. "It's the return trip that's particularly challenging." The vehicle will rely on electric propulsion to make its way to Earth, where it will eject the sample assembly in 2031.

The team doesn't want to use parachutes, which Muirhead called "notoriously tricky"; instead, the capsule will navigate its own way through Earth's atmosphere. That means the vehicle will need to carefully slow down enough not to splatter on the surface, but not slow down so much that atmospheric friction burns it.

At the end of his presentation, Muirhead tackled several questions from the audience quibbling with specific aspects of the mission design under discussion. One audience member expressed dismay that the return capability would not test the sorts of vehicles that could bring humans home. Another thought the mission could be cheaper if it ignored the risk of letting Martian material loose in Earth's biosphere.

In both cases, Muirhead emphasized that a Mars sample-return mission would be about the scientific secrets held in those samples. "This mission is about bringing samples back," he said with regards to planetary protection. "From an architecture point of view and from ensuring the integrity of the science samples, I'm not sure there's a much simpler solution than the one we're considering."

• Ancient Mars Could Have Supported Life (Photos)
• 1st Drilling on Mars by NASA's Curiosity Rover (Photos)
• NASA Puts Mars 2020 Rover's Space Ride to the Test (Photo)

Email Meghan Bartels at mbartels@space.com or follow her @meghanbartels. Follow us on Twitter @Spacedotcom and on Facebook.

{ https://www.space.com/news }