Our expert voices conversation on "How to look for alien life."

Earth is our best guide as we look for new forms of life. Our own planet hosts life in amazing complexity, from single-celled organisms to humans.

Asteroids and comets delivered the amino acids that form proteins to Earth over 4 billion years ago. Based on what we know from the evolution of life thereafter, we are looking for a liquid, probably water. We start out searching for places where liquid water is stable, either in the past, as on the surface of Mars, or currently beneath the icy crust on Europa and Enceladus. We then look for life's building block molecules such as amino acids. It took several billion years for complex life to emerge on Earth, so in our solar system we search for microbes, or fossils of simple organisms like algae or bacteria, the earliest forms of life.

Outside of our solar system, the first step is confirming habitability by analyzing extrasolar planet atmospheres, looking for gases like carbon dioxide and methane that are associated with life here. To confirm life, we need to build large enough space-based telescopes to image extrasolar planets, hopefully only a few decades from now.

Given how quickly life evolved here on this planet after conditions had stabilized, I am optimistic that where we have similar conditions, we will find evidence of life — or past life. The hard part may be finding life and not realizing it, if the life forms are very different from those here on Earth.

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Zeta, now a Category 2 Hurricane, makes landfall on Louisiana coast

The probable path of Zeta, per the National Hurricane Center. Photo: NHC/NOAA

Zeta, classified as a "significant" Category 2 hurricane, made landfall along the southeastern coast of Louisiana on Wednesday evening, according to the National Hurricane Center.

The state of play: Louisiana Gov. John Bel Edwards (D) requested a pre-landfall Emergency Declaration in a letter to President Trump on Tuesday. The hurricane is producing 110-mph maximum sustained winds and stronger gusts. The core of Zeta — including its destructive eyewall — moved ashore near Cocodrie.

Supreme Court won't expedite Pennsylvania GOP's request to block mail-in ballot extension

Amy Coney Barrett being sworn in by Chief Justice John Roberts. Photo: Fred Schilling/Collection of the Supreme Court of the United States via Getty Images

The Supreme Court voted 5-3 on Wednesday to deny a bid from Pennsylvania Republicans to expedite their request to shorten the deadline for receiving mail-in ballots. Newly-confirmed Justice Amy Coney Barrett did not participate in the decision.

Why it matters: A lower court ruling allowing ballots to be counted until 5 p.m. on Nov. 6, as long as they are postmarked by Election Day, will remain in place for now.

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Europe faces "stronger and deadlier" coronavirus wave

Paris under curfew. Photo: Kiran Ridley/Getty Images

The coronavirus is still winning: Now even Germany is entering another national lockdown, joined by France.

Why it matters: France has been "overpowered by a second wave,” President Emmanuel Macron said in a nationally televised address today. Macron said the "new wave will be stronger and deadlier" than the first.