Astronomers Debate: How Many Habitable Planets Does Each Sun-Like Star Have?
“We know that there are between 200 billion and 400 billion stars in the Milky Way galaxy. About 20% of those stars are Sun-like, for about 40-to-80 billion Sun-like stars in our galaxy. There are very likely billions of Earth-sized worlds orbiting those stars with the potential for the right conditions to have liquid water on their surfaces and being otherwise Earth-like, but whether that’s 1 or 2 billion or 50 or 100 billion is still unknown. Future planet-finding and exploring missions will need better answers than we presently have today, and that’s all the more reason to keep looking with every tool in our arsenal.”
Most of the time, in science, the quality of our data drives the size of our uncertainties. When we have very little data and it’s only of poor quality, our uncertainties tend to be large; when we have lots of very good data, our uncertainties shrink. NASA’s Kepler mission has provided astronomers with an unprecedented suite of data on exoplanets, revealing thousands of new worlds beyond our Solar System. And yet, despite all it’s found, if you ask the simple question of “how many Earth-like planets orbit a typical Sun-like star,” answers disagree by a factor of 100: from about 1% of stars have them to there’s between 1 and 2 for each and every such star.
What’s the real story? Where do these uncertainties arise, and are they larger than they need to be? Come get the full story (and watch David Kipping’s video at the end) and find out!
Ask Ethan: Can Failed Stars Eventually Succeed?
“Will the orbit of these [brown dwarfs] over a long period of time, eventually become smaller and smaller from the loss of energy through gravitational waves? Will they then eventually end up merging? If so, what happens in a [brown dwarf] merger? Will they merge to become an actual star that goes through fusion? Or is it something else entirely?”
For every star that’s out there in the Universe, for every object that ignited hydrogen fusion in its core, there are many other objects out there that never got massive enough to do so. The largest failures, those that gathered between 13 and 80 Jupiter masses’ worth of material, are known as brown dwarfs. They achieve deuterium fusion in their core, but never cross that critical threshold to become true stars. Many brown dwarfs, just like many stars, though, come in binary systems. If you were to add up all the mass in some of these systems, it would, in fact, be enough to create a star out of. The closest brown dwarf binary to us, Luhman 16, has exactly the right conditions that it could create a star if a merger ever took place.
What are our prospects for this pair of failed stars, or any failed star, eventually succeeding after all? Find out on this week’s Ask Ethan!
Ask Ethan: Why Do Stars Come In Different Sizes?
“Why can suns grow to… many different sizes? That is, ranging from somewhat larger [than] Jupiter up to suns exceeding Jupiter’s orbit?”
“Bigger mass makes a bigger star,” you might be inclined to say. The smallest stars in size should be small because they have the least amount of material in them, while the largest ones of all are the largest because they’ve got the most material to make stars out of. And that’s a tempting explanation, but it doesn’t account for either the smallest stars or the largest ones in the Universe. As it turns out, neutron stars and white dwarfs are almost all larger in mass than our own Sun is, and yet the Sun is hundreds or even many thousands of times larger than they are. The most massive star known is only 30 times the physical size of our Sun, while the largest star of all is nearly 2,000 times our Sun’s size. As it turns out, there’s much, much more at play than mass alone.
Why do stars really come in different sizes, and how do we even know how big a star is at all? Find out on this week’s Ask Ethan!
It’s Dimming! Astronomers Jump At Opportunity To Solve The Mystery Of Tabby’s Star
“Perhaps, as many suggested, this was evidence of an alien megastructure being constructed?
But another astrophysical scenario could explain it: a recently devoured planet.
Gases would dim the star overall, while outbursts and flares create irregular flux dips.”
Earlier this decade, the Kepler mission became the most successful planet-finding endeavor of all time, turning up thousands of new worlds by measuring the transit data of some 150,000 stars. When planets passed in front of their parent star, they blocked a tiny fraction of their light, leaving behind an imprint of a periodic dimming signal. But one star dims differently from all the others. KIC 8462852, known as Tabby’s star, has irregular dips of up to 20% in brightness, equivalent to ten times the effect of all the Solar System’s planets combined. What could be causing this? While a few astronomers have proposed alien megastructures, another, simpler explanation might explain it all: a recently devoured planet.
As a new dimming event is underway, astronomers hope to collect enough quality observations to validate or disfavor all the ideas bouncing around. Find out more today!
When Will The First Star Go Dark?
“I’m sorry to disappoint you, but there aren’t any black dwarfs around today. The Universe is simply far too young for it. In fact, the coolest white dwarfs have, to the best of our estimates, lost less than 0.2% of their total heat since the very first ones were created in this Universe. For a white dwarf created at 20,000 K, that means its temperature is still at least 19,960 K, telling us we’ve got a terribly long way to go, if we’re waiting for a true dark star.”
Stars live for a variety of ages, from just a million or two years for some to tens of trillions of years for others. But even after a star has run out of its fuel and died, its stellar corpse continues to shine on. Neutron stars and white dwarfs are both extremely massive, but very small in volume compared to a star. As a result, they cool very slowly, so slow that a single one has not yet gone dark in all the Universe. So how long will it take, and who will get there first: neutron stars or white dwarfs? Believe it or not, there’s still enough uncertainty about how neutron stars cool, mostly due to uncertainties in neutrino physics, that we think we know the answer to be white dwarfs – and 10^14 or 10^15 years – but we’re not entirely sure!
Come find out what we know about finding the first truly dark star in the Universe today.
What’s The Largest Planet In The Universe?
“Above a certain mass, the atoms inside large planets will begin to compress so severely that adding more mass will actually shrink your planet.
This happens in our Solar System, explaining why Jupiter is three times Saturn’s mass, but only 20% physically larger.
But many solar systems have planets made out of much lighter elements, without large, rocky cores inside.”
You might think that Jupiter is the largest planet in the Solar System because it’s the most massive, but that’s not quite right. If you kept adding mass to Saturn, it would get larger in size, but if you kept adding mass to Jupiter, it would shrink! For a given set of elements that your planet is made out of, there’s a maximum size it can reach, that’s somewhere in between the mass of Saturn and Jupiter in general. Our Solar System is on the dense side of things, meaning that we’ve discovered a large number of exoplanets out there that are approximately twice the physical size of Jupiter without becoming brown dwarfs or hydrogen-fusing stars. For worlds like WASP-17b, where we’ve measured both the radius and mass, we find that they’re only about half the mass of Jupiter, despite being double the size.
Come get the full scientific story, and some very informative and illustrative images with no more than 200 words, on today’s Mostly Mute Monday!