Astronomers who have been searching for extraterrestrial intelligence for decades are suddenly saying such an encounter might not be a happy one.

Aliens might destroy life on Earth or plan to eat or enslave humans if they sense our civilization was expanding too rapidly and could harm others, according to a latest study.

The scenario was brought up in a joint study by Seth Baum, Jacob Haqq-Misra and Shawn Domagal-Goldman.

Researchers say extraterrestrials might behave the way we humans have behaved whenever we have discovered other previously unknown intelligent beings on Earth, like unfamiliar humans or chimpanzees and gorillas.

Just as we did to those beings, the extraterrestrials might proceed to kill, infect, dissect, conquer, displace or enslave us, stuff us as specimens for their museums or pickle our skulls and use us for medical research, according to the study, which was published in the journal Acta Astronautica.

Why should we worry about aliens? The simple reason is that if they can find us, they would be more advanced than humans.

A core concern is that ETI will learn of our presence and quickly travel to Earth to eat or enslave us, the study says.

The authors speculate that extraterrestrials might try to spread their beliefs through evangelism or to use humans for entertainment.

Just because an ETI civilization holds universalist ethics does not mean that it would never seek our harm. This is because ETI may be quite different from us and could conclude that harming us would help maximize whatever they value intrinsically.

For example, if ETI place intrinsic value on lives, then perhaps they could bring about more lives by destroying us and using our resources more efficiently for other lives. Other forms of intrinsic value may cause a universalist ETI to seek our harm or destruction as long as more value is produced without us than with us.

Aliens also could harm or destroy us if they believe we are a threat to other civilizations. Rapidly expanding civilizations may have a tendency to destroy other civilizations in the process, just as humanity has already destroyed many species on Earth.

Though this scenario might seem unlikely given the likelihood of our technological inferiority relative to other civilizations, we would be at the receiving end if ET thinks that our resources could be used more efficiently to generate or retain other civilizations.

Perhaps ETI is observing rapid and destructive expansion on Earth and could become concerned at our trajectory.

ETI might prefer that our civilization change its ways to survive, but if it doubts that our course can be changed, it may seek to preemptively destroy us to protect other civilizations from us.

A preemptive strike would be particularly likely in the early phases of our expansion because a civilization may become increasingly difficult to destroy as it continues to expand.

Humanity may just now be entering the period in which its rapid civilizational expansion could be detected by an ETI because our expansion is changing the composition of Earth's atmosphere (e.g. via greenhouse gas emissions), which therefore changes the spectral signature of Earth, the study's authors say,

Human civilization affects ecosystems so strongly that some ecologists have begun calling this epoch of Earth's history the anthropocene, a new and unprecedented phase in the planet's history.

If the goal is to maximize ecosystem health, then perhaps it would be better if humanity did not exist, or at least if it existed in significantly reduced form. Since at least some humans believe so, invoking universalist ethical principles, then it is likely that ETI might agree.

But since we don't know what kind of aliens we will end up meeting, there are certain steps humans should take when making contact, the authors urge. Those steps include not sharing details of our biology and DNA structure, and not appearing as if we are rapidly expanding off the Earth.