Sun-like, Earth-Eating Stars

Some Sun-like stars are 'Earth-eaters.' During their development they ingest large amounts of the rocky material from which 'terrestrial' planets like Earth, Mars and Venus are made. Credit: Vanderbilt University

Some Sun-like stars are ‘Earth-eaters.’ During their development they ingest large amounts of the rocky material from which ‘terrestrial’ planets like Earth, Mars and Venus are made. Credit: Vanderbilt University

A new model is helping astronomers identify Sun-like stars that have ‘ingested’ the Earth-like planets that once orbited them.

Planets are created from disks of dust and debris that swirl around young stars after they form. Small chunks of material smash together and coalesce into larger and larger bodies, and sometimes these objects become large enough to form planets.

Where in the disk a planet appears is key to whether or not it could become Earth-like. The planet has to have a stable orbit at just the right distance from its star to receive the correct amount of heat and energy for liquid water and life.

If the planet’s orbit isn’t stable withinis jostled or pushed out of this ‘habitable zone,’ by other, larger planets orbiting the star, it could eventually be drawn in and gobbled up by its sun.

Such ‘earth-eating’ suns are the focus of a recent study published in the Astrophysical Journal. Working with a new model and observations from the MIKE instrument at the Magellan observatory, a team of astronomers has outlined how the chemical composition of a star’s atmosphere is altered when it starts dining on rocky planets.

Trey Mack, graduate student in astronomy at Vanderbilt University, developed the model for analyzing how Earth-like planets are ingested by Sun-like stars. Using what they learned from the model, the team began testing their theories with observations. The results show that a star’s chemical signature can be analyzed in very high detail – enough detail to see how the chemical signature changes when an Earth-like planet is consumed.

“After obtaining a high-resolution spectrum for a given star, we can actually detect that signature in detail, element by element,” said Keivan Stassun, Professor of Astronomy at Vanderbilt University in a recent press release.

The team examined a binary pair of Sun-like stars known to host planets, dubbed HD 20781 and HD 20782. Using the Sun itself as a comparison, they found a higher abundance of certain elements in these stars. Crucially, they found that the pattern of enhanced abundance by element matched their model, indicating that they the stars had indeed ingested Earth-like planets in the past.

Hungry Stars and the Search for Life

According to the study, astrobiologists hunting for habitable worlds may need to focus not just on how planets are created, but also on how rocky worlds survive after they are formed.

What if we could determine if a given star is likely to host a planetary system like our own by breaking down its light into a single high-resolution spectrum and analyzing it? A spectrum taken of the Sun is shown above. The dark bands result from specific chemical elements in the star’s outer layer, like hydrogen or iron, absorbing specific frequencies of light. By carefully measuring the width of each dark band, astronomers can determine just how much hydrogen, iron, calcium and other elements are present in a distant star. The new model suggests that a G-class star with levels of refractory elements like aluminum, silicon and iron significantly higher than those in the Sun may not have any Earth-like planets because it has swallowed them. Credit: N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF

What if we could determine if a given star is likely to host a planetary system like our own by breaking down its light into a single high-resolution spectrum and analyzing it? A spectrum taken of the Sun is shown above. The dark bands result from specific chemical elements in the star’s outer layer, like hydrogen or iron, absorbing specific frequencies of light. By carefully measuring the width of each dark band, astronomers can determine just how much hydrogen, iron, calcium and other elements are present in a distant star. The new model suggests that a G-class star with levels of refractory elements like aluminum, silicon and iron significantly higher than those in the Sun may not have any Earth-like planets because it has swallowed them. Credit: N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF

“What I think is truly new in our work is the demonstration that the detailed chemical abundance signals we see really are exactly what you should get if these stars are swallowing up lots and lots of Earths,” said Stassun in communications with Astrobiology Magazine. “An implication of this is that perhaps Earth-like worlds are manufactured in great numbers but relatively few of them survive the jostling of their heftier Jupiter-like planetary siblings, which dumps them into their stars.”

With the number of known extrasolar planets continually increasing, the task now is to narrow down the number of locations most likely to support habitable worlds. If a star’s chemical signature shows that it has been hungrily eating any rocky planets in orbit, it might not be the best star to focus on.

“This may point the way to more efficient searches for other Earths out there,” said Stassun. “We can simply measure the light spectrum of a star, and if the star appears to have already swallowed up its Earths, that may not be the first place to go looking for an Earth in orbit!”

A Time to Search for Life

Many astronomers are currently working on techniques to remotely detect biosignatures on extrasolar planets; for instance, by analyzing chemical signatures in a planet’s atmosphere.

Astrobiology Magazine posed the question: are these studies premature? Do we first need to narrow down the number of stars that are actually capable of supporting such worlds before we move to studying habitability?

“I think our work definitely suggests a new way of thinking about which stars might be the best hunting grounds for Earth-like worlds,” answered Stassun. “But I would not agree with the statement that developing techniques to look for biosignatures is premature. Quite the contrary, I think the time to develop those techniques — and indeed even to understand what biosignatures are best to look for — is now. Our understanding of planetary atmospheres, and biosignatures in general, is still very much in its infancy, and in my mind represents one of the frontiers that we must now make strong headway in if we are to have any hope of eventually detecting life in another world.”

The study was supported by grants AAG AST-1009810 and PAARE AST-0849736 from the National Science Foundation (NSF).


Astronomers identify signature of Earth-eating stars. Credit: Vanderbilt University (YouTube)