"They're out there," goes a saying about extraterrestrials. It would seem more likely to be true in light of a new study on planetary axis tilts.

Astrophysicists at the Georgia Institute of Technology have modeled a theoretical twin of Earth into other star systems called binary systems because they have two stars. They concluded that 87% of exo-Earths one might find in binary systems should have axis tilts similarly steady to Earth's, an important ingredient for climate stability that favors the evolution of complex life.

"Multiple-star systems are common, and about 50% of stars have binary companion stars. So, this study can be applied to a large number of solar systems," said Gongjie Li, the study's co-investigator an assistant professor at Georgia Tech's School of Physics.

Single-star solar systems like our own with multiple planets appear to be rarer.

Alpha Centauri B? Wretched

The researchers started out contrasting how the Earth's axis tilt, also called obliquity, varies over time with the variation of Mars' axis tilt. Whereas our planet's mild obliquity variations have been great for a livable climate and evolution, the wild variations of Mars' axis tilt may have helped wreck its atmosphere, as explained in the section below.

Then the researchers modeled Earth into habitable, or Goldilocks, zones in Alpha Centauri AB - our solar system's nearest neighbor, a binary system with one star called "A" and the other "B." After that, they expanded the model to a more universal scope. {module INSIDE STORY}

"We simulated what it would be like around other binaries with multiple variations of the stars' masses, orbital qualities, and so on," said Billy Quarles, the study's principal investigator and a research scientist in Li's lab. "The overall message was positive but not for our nearest neighbor."

Alpha Centauri A didn't look bad, but the outlook for mild axis dynamics on an exo-Earth modeled around star B was wretched. This may douse some hopes because Alpha Centauri AB is four lightyears away, and a mission named Starshot with big-name backers plans to launch a space probe to look for signs of advanced life there.

The researchers are publishing their study, which was co-led by Jack Lissauer from NASA Ames Research Center, in Astrophysical Journal on November 19, 2019, under the title: "Obliquity Evolution of Circumstellar Planets in Sun-like Stellar Binaries." The research was funded by the NASA Exobiology Program.

No exoplanets have been confirmed around A or B; an exoplanet has been confirmed around the nearby red dwarf star Proxima Centauri, but it is very likely to be uninhabitable.

Earth? Just right

Even with its ice ages and hot phases, Earth's climatological framework has been calm for hundreds of millions of years - in part because of its mild orbital and axis-tilt dynamics - allowing evolution to take big strides. Wildly varying dynamics, and thus climate, like on Mars would stand to regularly kill off advanced life, stunting evolution.

Earth's orbit around the sun is on a slight incline that seesaws gently and very slowly through a slight precession, a kind of oscillation. As Earth revolves, it shifts position relative to the sun, circling it a little like a spirograph drawing. The orbit also precesses in shape between slightly more and slightly less oblong over 100,000-year periods.

Earth's axis tilt precesses between 22.1 and 24.5 degrees over 41,000 years. Our large moon stabilizes our tilt through its gravitational relationship with Earth, otherwise, bouncy gravitational interconnections with Mercury, Venus, Mars, and Jupiter would jolt our tilt with resonances.

"If we didn't have the moon, Earth's tilt could vary by about 60 degrees," Quarles said. "We'd look maybe like Mars, and the precession of its axis appears to have helped deplete its atmosphere."

Mars' axis precesses between 10 degrees and 60 degrees every 2 million years. At the 10-degree tilt, the atmosphere condenses at the poles, creating caps that lock up a lot of the atmosphere in ice. At 60 degrees, Mars could grow an ice belt around its equator.

Universe? Hopeful

In Alpha Centauri AB, star B, about the size of our sun, and the larger star, A, orbit one another at about the distance between Uranus and our sun, which is a very close for two stars in a binary system. The study modeled variations of an exo-Earth orbiting either star but concentrated on a modeled Earth orbit in the habitable zone centered around B, with A being the orbiting star. {module INSIDE STORY}

A's orbit is very elliptical, passing close by and then moving very far away from B and slinging powerful gravity, which, in the model, overpowered exo-Earth's dynamics. Its tilt and orbit varied widely; adding our moon to the model didn't help.

"Around Alpha Centauri B, if you don't have a moon, you have a more stable axis than if you do have a moon. If you have a moon, it's pretty much bad news," Quarles said.

Even without a moon and with mild axis variability, complex, Earthlike evolution would seem to have a hard time on the modeled exo-Earth around B.

"The biggest effect you would see is differences in the climate cycles related to how elongated the orbit is. Instead of having ice ages every 100,000 years like on Earth, they may come every 1 million years, be worse, and last much longer," Quarles said.

But a sliver of hope for Earthlike conditions turned up in the model: "Planetary orbit and spin need to precess just right relative to the binary orbit. There is this tiny sweet spot," Quarles said.

When the researchers expanded the model to binary systems in the universe, the probability of gentle obliquity variations ballooned.

"In general, the separation between the stars is larger in binary systems, and then the second star has less of an effect on the model of Earth. The planet's own motion dynamics dominate other influences, and obliquity usually has a smaller variation," Li said. "So, this is quite optimistic."

Does the flap of a butterfly's wings in Brazil set off a tornado in Texas?

Scientists have recently discovered the mechanism by which a minuscule change in 3 atoms in a protein molecule can affect immune signaling in cells. This 'butterfly effect' is used by the bacterium, Shigella flexneri, to survive within the host cells that it infects.

Ranabir Das' team at the National Centre for Biological Sciences (NCBS), Bangalore, has found that a tiny change in the protein UBC13, caused by a bacterial enzyme, creates a cascade of small atomic alterations that add up until they prevent UBC13 from binding to a partner protein, TRAF6. Without the UBC13-TRAF6 complex, the host cell is unable to begin signaling for an immune response against the bacteria. This study examines the mechanics of a subtle alteration--involving the loss of just 3 atoms--and investigates how it can have far-reaching consequences.

According to the chaos theory in mathematics, a minute change such as the 'flap of a butterfly's wing' could cause huge changes elsewhere. This seems to hold true at much smaller scales too--for example, within a cell. Scientists from the National Centre for Biological Sciences (NCBS), Bangalore, have now found how a minuscule atomic change in a protein molecule enables the bacterium to shut down its host's immune signaling system. {module INSIDE STORY}

Shigella flexneri is a sneaky and highly infective bacteria. The organisms, which cause diarrhea in humans, first attach to the cells lining the host's gut. Then using a needle-like apparatus, S. flexneri begins pumping in their secret weapon--an enzyme that alters a single amino acid in the host cell protein UBC13 (Ubiquitin Conjugating enzyme E2 13). This change renders UBC13 unable to bind to a partner protein TRAF6 (TumouR necrosis factor Associated Factor 6) and effectively blocks the host cell from signaling for an inflammatory response. Subsequently, Shigella penetrates into the host cells to multiply.

As of now, little was known about how such a subtle effect--a change in just 3 atoms in a protein molecule containing roughly 3000 atoms--could shut down a host's immune response.

A new study by Ranabir Das' team at NCBS, however, has shown that when an amine group (formed of one nitrogen and two hydrogen atoms) is removed from a single amino acid in the host protein UBC13, a series of relatively small atomic changes pile up to block a key step in the immune response pathway. The work, which received funding support from the Tata Institute of Fundamental Research and the Department of Biotechnology, India, has been published as a paper in the journal eLife.

"We used a combination of structural studies, computational modeling, and enzymatic experiments to find that the atomic changes in UBC13 do not alter its structure. Rather, these changes completely disrupt its ability to bind its partner protein, TRAF6," says Priyesh Mohanty, who is part of Das' team, and the first author in the paper that describes these results.

The 14th amino acid in UBC13, an arginine residue (Arginine14), is critical in forming a 'salt bridge'--a bond between oppositely charged ions--with TRAF6. This salt bridge between the surfaces of the two proteins is necessary to stabilize and maintain the UBC13-TRAF6 complex, which in turn, plays a pivotal role in immune response signaling. When the bacterial enzyme, named Ospl, deamidates a glutamine residue (Glutamine100), a neutral amino group is replaced by a negatively charged hydroxyl group. Now, this group literally steals away Arginine14 from the UBC13-TRAF6 inter-molecular salt bridge, to form an intramolecular salt bridge with itself. This, in turn, compromises the transient interactions between UBC13 and TRAF6, which are necessary for forming the final complex. Finally, the new negative charge on the UBC13 surface creates a transient repulsive force between the UBC13 and TRAF6 proteins.

The salt bridge and the surface attractive forces collectively create a force powerful enough to steady (or hold) the UBC13-TRAF6 complex. With the loss of these forces, the complex falls apart, and without the complex, the immune signal against the bacteria is blocked.

"This investigation has really helped us understand the role of individual amino acids in the associations between proteins and how proteins function within the cell," say Mohanty and Das. "To the best of our knowledge, our study is the first to provide a mechanism by which glutamine deamidation of a target protein hinders its function inside the host cell. Interestingly, this mechanism allows the bacteria to attenuate the inflammatory response and promotes its ability to survive in a human host," they add.

Currently, Shigella infections cause almost 0.2 million deaths due to diarrhea every year, of which one-third occur in infants. Several multidrug-resistant strains of these bacteria have also appeared, and yet there are no vaccines or drugs to prevent or limit the spread of Shigella outbreaks.

"It is, therefore, very important to study what mechanism Shigella uses to attenuate inflammatory responses in human hosts. Such studies may help researchers to identify new drugs that could control Shigella outbreaks in humans," say Mohanty and Das.